[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[SCM] gawk branch, master, updated. gawk-4.1.0-4608-g7388d79e
|
From: |
Arnold Robbins |
|
Subject: |
[SCM] gawk branch, master, updated. gawk-4.1.0-4608-g7388d79e |
|
Date: |
Wed, 5 Jan 2022 14:35:12 -0500 (EST) |
This is an automated email from the git hooks/post-receive script. It was
generated because a ref change was pushed to the repository containing
the project "gawk".
The branch, master has been updated
via 7388d79e02299193e991b6714c3b3c5314191154 (commit)
from 84107702c4afa003299cbbf3150b52a67a2a518c (commit)
Those revisions listed above that are new to this repository have
not appeared on any other notification email; so we list those
revisions in full, below.
- Log -----------------------------------------------------------------
http://git.sv.gnu.org/cgit/gawk.git/commit/?id=7388d79e02299193e991b6714c3b3c5314191154
commit 7388d79e02299193e991b6714c3b3c5314191154
Author: Arnold D. Robbins <arnold@skeeve.com>
Date: Wed Jan 5 21:34:50 2022 +0200
Continuing to fix master.
diff --git a/doc/gawk.info b/doc/gawk.info
new file mode 100644
index 00000000..e11d3b06
--- /dev/null
+++ b/doc/gawk.info
@@ -0,0 +1,39341 @@
+This is gawk.info, produced by makeinfo version 6.8 from gawk.texi.
+
+Copyright (C) 1989, 1991, 1992, 1993, 1996-2005, 2007, 2009-2021
+Free Software Foundation, Inc.
+
+
+ This is Edition 5.1 of 'GAWK: Effective AWK Programming: A User's
+Guide for GNU Awk', for the 5.1.1 (or later) version of the GNU
+implementation of AWK.
+
+ Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with the
+Invariant Sections being "GNU General Public License", with the
+Front-Cover Texts being "A GNU Manual", and with the Back-Cover Texts as
+in (a) below. A copy of the license is included in the section entitled
+"GNU Free Documentation License".
+
+ a. The FSF's Back-Cover Text is: "You have the freedom to copy and
+ modify this GNU manual."
+INFO-DIR-SECTION Text creation and manipulation
+START-INFO-DIR-ENTRY
+* Gawk: (gawk). A text scanning and processing language.
+END-INFO-DIR-ENTRY
+
+INFO-DIR-SECTION Individual utilities
+START-INFO-DIR-ENTRY
+* awk: (gawk)Invoking Gawk. Text scanning and processing.
+END-INFO-DIR-ENTRY
+
+
+File: gawk.info, Node: Top, Next: Foreword3, Up: (dir)
+
+General Introduction
+********************
+
+This file documents 'awk', a program that you can use to select
+particular records in a file and perform operations upon them.
+
+ Copyright (C) 1989, 1991, 1992, 1993, 1996-2005, 2007, 2009-2021
+Free Software Foundation, Inc.
+
+
+ This is Edition 5.1 of 'GAWK: Effective AWK Programming: A User's
+Guide for GNU Awk', for the 5.1.1 (or later) version of the GNU
+implementation of AWK.
+
+ Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with the
+Invariant Sections being "GNU General Public License", with the
+Front-Cover Texts being "A GNU Manual", and with the Back-Cover Texts as
+in (a) below. A copy of the license is included in the section entitled
+"GNU Free Documentation License".
+
+ a. The FSF's Back-Cover Text is: "You have the freedom to copy and
+ modify this GNU manual."
+
+* Menu:
+
+* Foreword3:: Some nice words about this
+ Info file.
+* Foreword4:: More nice words.
+* Preface:: What this Info file is about; brief
+ history and acknowledgments.
+* Getting Started:: A basic introduction to using
+ 'awk'. How to run an 'awk'
+ program. Command-line syntax.
+* Invoking Gawk:: How to run 'gawk'.
+* Regexp:: All about matching things using regular
+ expressions.
+* Reading Files:: How to read files and manipulate fields.
+* Printing:: How to print using 'awk'. Describes
+ the 'print' and 'printf'
+ statements. Also describes redirection of
+ output.
+* Expressions:: Expressions are the basic building blocks
+ of statements.
+* Patterns and Actions:: Overviews of patterns and actions.
+* Arrays:: The description and use of arrays. Also
+ includes array-oriented control statements.
+* Functions:: Built-in and user-defined functions.
+* Library Functions:: A Library of 'awk' Functions.
+* Sample Programs:: Many 'awk' programs with complete
+ explanations.
+* Advanced Features:: Stuff for advanced users, specific to
+ 'gawk'.
+* Internationalization:: Getting 'gawk' to speak your
+ language.
+* Debugger:: The 'gawk' debugger.
+* Namespaces:: How namespaces work in 'gawk'.
+* Arbitrary Precision Arithmetic:: Arbitrary precision arithmetic with
+ 'gawk'.
+* Dynamic Extensions:: Adding new built-in functions to
+ 'gawk'.
+* Language History:: The evolution of the 'awk'
+ language.
+* Installation:: Installing 'gawk' under various
+ operating systems.
+* Notes:: Notes about adding things to 'gawk'
+ and possible future work.
+* Basic Concepts:: A very quick introduction to programming
+ concepts.
+* Glossary:: An explanation of some unfamiliar terms.
+* Copying:: Your right to copy and distribute
+ 'gawk'.
+* GNU Free Documentation License:: The license for this Info file.
+* Index:: Concept and Variable Index.
+
+* History:: The history of 'gawk' and
+ 'awk'.
+* Names:: What name to use to find
+ 'awk'.
+* This Manual:: Using this Info file. Includes
+ sample input files that you can use.
+* Conventions:: Typographical Conventions.
+* Manual History:: Brief history of the GNU project and
+ this Info file.
+* How To Contribute:: Helping to save the world.
+* Acknowledgments:: Acknowledgments.
+* Running gawk:: How to run 'gawk' programs;
+ includes command-line syntax.
+* One-shot:: Running a short throwaway
+ 'awk' program.
+* Read Terminal:: Using no input files (input from the
+ keyboard instead).
+* Long:: Putting permanent 'awk'
+ programs in files.
+* Executable Scripts:: Making self-contained 'awk'
+ programs.
+* Comments:: Adding documentation to 'gawk'
+ programs.
+* Quoting:: More discussion of shell quoting
+ issues.
+* DOS Quoting:: Quoting in Windows Batch Files.
+* Sample Data Files:: Sample data files for use in the
+ 'awk' programs illustrated in
+ this Info file.
+* Very Simple:: A very simple example.
+* Two Rules:: A less simple one-line example using
+ two rules.
+* More Complex:: A more complex example.
+* Statements/Lines:: Subdividing or combining statements
+ into lines.
+* Other Features:: Other Features of 'awk'.
+* When:: When to use 'gawk' and when to
+ use other things.
+* Intro Summary:: Summary of the introduction.
+* Command Line:: How to run 'awk'.
+* Options:: Command-line options and their
+ meanings.
+* Other Arguments:: Input file names and variable
+ assignments.
+* Naming Standard Input:: How to specify standard input with
+ other files.
+* Environment Variables:: The environment variables
+ 'gawk' uses.
+* AWKPATH Variable:: Searching directories for
+ 'awk' programs.
+* AWKLIBPATH Variable:: Searching directories for
+ 'awk' shared libraries.
+* Other Environment Variables:: The environment variables.
+* Exit Status:: 'gawk''s exit status.
+* Include Files:: Including other files into your
+ program.
+* Loading Shared Libraries:: Loading shared libraries into your
+ program.
+* Obsolete:: Obsolete Options and/or features.
+* Undocumented:: Undocumented Options and Features.
+* Invoking Summary:: Invocation summary.
+* Regexp Usage:: How to Use Regular Expressions.
+* Escape Sequences:: How to write nonprinting characters.
+* Regexp Operators:: Regular Expression Operators.
+* Regexp Operator Details:: The actual details.
+* Interval Expressions:: Notes on interval expressions.
+* Bracket Expressions:: What can go between '[...]'.
+* Leftmost Longest:: How much text matches.
+* Computed Regexps:: Using Dynamic Regexps.
+* GNU Regexp Operators:: Operators specific to GNU software.
+* Case-sensitivity:: How to do case-insensitive matching.
+* Regexp Summary:: Regular expressions summary.
+* Records:: Controlling how data is split into
+ records.
+* awk split records:: How standard 'awk' splits
+ records.
+* gawk split records:: How 'gawk' splits records.
+* Fields:: An introduction to fields.
+* Nonconstant Fields:: Nonconstant Field Numbers.
+* Changing Fields:: Changing the Contents of a Field.
+* Field Separators:: The field separator and how to change
+ it.
+* Default Field Splitting:: How fields are normally separated.
+* Regexp Field Splitting:: Using regexps as the field separator.
+* Single Character Fields:: Making each character a separate
+ field.
+* Command Line Field Separator:: Setting 'FS' from the command
+ line.
+* Full Line Fields:: Making the full line be a single
+ field.
+* Field Splitting Summary:: Some final points and a summary table.
+* Constant Size:: Reading constant width data.
+* Fixed width data:: Processing fixed-width data.
+* Skipping intervening:: Skipping intervening fields.
+* Allowing trailing data:: Capturing optional trailing data.
+* Fields with fixed data:: Field values with fixed-width data.
+* Splitting By Content:: Defining Fields By Content
+* More CSV:: More on CSV files.
+* FS versus FPAT:: A subtle difference.
+* Testing field creation:: Checking how 'gawk' is
+ splitting records.
+* Multiple Line:: Reading multiline records.
+* Getline:: Reading files under explicit program
+ control using the 'getline'
+ function.
+* Plain Getline:: Using 'getline' with no
+ arguments.
+* Getline/Variable:: Using 'getline' into a variable.
+* Getline/File:: Using 'getline' from a file.
+* Getline/Variable/File:: Using 'getline' into a variable
+ from a file.
+* Getline/Pipe:: Using 'getline' from a pipe.
+* Getline/Variable/Pipe:: Using 'getline' into a variable
+ from a pipe.
+* Getline/Coprocess:: Using 'getline' from a coprocess.
+* Getline/Variable/Coprocess:: Using 'getline' into a variable
+ from a coprocess.
+* Getline Notes:: Important things to know about
+ 'getline'.
+* Getline Summary:: Summary of 'getline' Variants.
+* Read Timeout:: Reading input with a timeout.
+* Retrying Input:: Retrying input after certain errors.
+* Command-line directories:: What happens if you put a directory on
+ the command line.
+* Input Summary:: Input summary.
+* Input Exercises:: Exercises.
+* Print:: The 'print' statement.
+* Print Examples:: Simple examples of 'print'
+ statements.
+* Output Separators:: The output separators and how to
+ change them.
+* OFMT:: Controlling Numeric Output With
+ 'print'.
+* Printf:: The 'printf' statement.
+* Basic Printf:: Syntax of the 'printf' statement.
+* Control Letters:: Format-control letters.
+* Format Modifiers:: Format-specification modifiers.
+* Printf Examples:: Several examples.
+* Redirection:: How to redirect output to multiple
+ files and pipes.
+* Special FD:: Special files for I/O.
+* Special Files:: File name interpretation in
+ 'gawk'. 'gawk' allows
+ access to inherited file descriptors.
+* Other Inherited Files:: Accessing other open files with
+ 'gawk'.
+* Special Network:: Special files for network
+ communications.
+* Special Caveats:: Things to watch out for.
+* Close Files And Pipes:: Closing Input and Output Files and
+ Pipes.
+* Nonfatal:: Enabling Nonfatal Output.
+* Output Summary:: Output summary.
+* Output Exercises:: Exercises.
+* Values:: Constants, Variables, and Regular
+ Expressions.
+* Constants:: String, numeric and regexp constants.
+* Scalar Constants:: Numeric and string constants.
+* Nondecimal-numbers:: What are octal and hex numbers.
+* Regexp Constants:: Regular Expression constants.
+* Using Constant Regexps:: When and how to use a regexp constant.
+* Standard Regexp Constants:: Regexp constants in standard
+ 'awk'.
+* Strong Regexp Constants:: Strongly typed regexp constants.
+* Variables:: Variables give names to values for
+ later use.
+* Using Variables:: Using variables in your programs.
+* Assignment Options:: Setting variables on the command line
+ and a summary of command-line syntax.
+ This is an advanced method of input.
+* Conversion:: The conversion of strings to numbers
+ and vice versa.
+* Strings And Numbers:: How 'awk' Converts Between
+ Strings And Numbers.
+* Locale influences conversions:: How the locale may affect conversions.
+* All Operators:: 'gawk''s operators.
+* Arithmetic Ops:: Arithmetic operations ('+',
+ '-', etc.)
+* Concatenation:: Concatenating strings.
+* Assignment Ops:: Changing the value of a variable or a
+ field.
+* Increment Ops:: Incrementing the numeric value of a
+ variable.
+* Truth Values and Conditions:: Testing for true and false.
+* Truth Values:: What is "true" and what is
+ "false".
+* Typing and Comparison:: How variables acquire types and how
+ this affects comparison of numbers and
+ strings with '<', etc.
+* Variable Typing:: String type versus numeric type.
+* Comparison Operators:: The comparison operators.
+* POSIX String Comparison:: String comparison with POSIX rules.
+* Boolean Ops:: Combining comparison expressions using
+ boolean operators '||' ("or"),
+ '&&' ("and") and '!'
+ ("not").
+* Conditional Exp:: Conditional expressions select between
+ two subexpressions under control of a
+ third subexpression.
+* Function Calls:: A function call is an expression.
+* Precedence:: How various operators nest.
+* Locales:: How the locale affects things.
+* Expressions Summary:: Expressions summary.
+* Pattern Overview:: What goes into a pattern.
+* Regexp Patterns:: Using regexps as patterns.
+* Expression Patterns:: Any expression can be used as a
+ pattern.
+* Ranges:: Pairs of patterns specify record
+ ranges.
+* BEGIN/END:: Specifying initialization and cleanup
+ rules.
+* Using BEGIN/END:: How and why to use BEGIN/END rules.
+* I/O And BEGIN/END:: I/O issues in BEGIN/END rules.
+* BEGINFILE/ENDFILE:: Two special patterns for advanced
+ control.
+* Empty:: The empty pattern, which matches every
+ record.
+* Using Shell Variables:: How to use shell variables with
+ 'awk'.
+* Action Overview:: What goes into an action.
+* Statements:: Describes the various control
+ statements in detail.
+* If Statement:: Conditionally execute some
+ 'awk' statements.
+* While Statement:: Loop until some condition is
+ satisfied.
+* Do Statement:: Do specified action while looping
+ until some condition is satisfied.
+* For Statement:: Another looping statement, that
+ provides initialization and increment
+ clauses.
+* Switch Statement:: Switch/case evaluation for conditional
+ execution of statements based on a
+ value.
+* Break Statement:: Immediately exit the innermost
+ enclosing loop.
+* Continue Statement:: Skip to the end of the innermost
+ enclosing loop.
+* Next Statement:: Stop processing the current input
+ record.
+* Nextfile Statement:: Stop processing the current file.
+* Exit Statement:: Stop execution of 'awk'.
+* Built-in Variables:: Summarizes the predefined variables.
+* User-modified:: Built-in variables that you change to
+ control 'awk'.
+* Auto-set:: Built-in variables where 'awk'
+ gives you information.
+* ARGC and ARGV:: Ways to use 'ARGC' and
+ 'ARGV'.
+* Pattern Action Summary:: Patterns and Actions summary.
+* Array Basics:: The basics of arrays.
+* Array Intro:: Introduction to Arrays
+* Reference to Elements:: How to examine one element of an
+ array.
+* Assigning Elements:: How to change an element of an array.
+* Array Example:: Basic Example of an Array
+* Scanning an Array:: A variation of the 'for'
+ statement. It loops through the
+ indices of an array's existing
+ elements.
+* Controlling Scanning:: Controlling the order in which arrays
+ are scanned.
+* Numeric Array Subscripts:: How to use numbers as subscripts in
+ 'awk'.
+* Uninitialized Subscripts:: Using Uninitialized variables as
+ subscripts.
+* Delete:: The 'delete' statement removes an
+ element from an array.
+* Multidimensional:: Emulating multidimensional arrays in
+ 'awk'.
+* Multiscanning:: Scanning multidimensional arrays.
+* Arrays of Arrays:: True multidimensional arrays.
+* Arrays Summary:: Summary of arrays.
+* Built-in:: Summarizes the built-in functions.
+* Calling Built-in:: How to call built-in functions.
+* Boolean Functions:: A function that returns Boolean
+ values.
+* Numeric Functions:: Functions that work with numbers,
+ including 'int()', 'sin()'
+ and 'rand()'.
+* String Functions:: Functions for string manipulation,
+ such as 'split()', 'match()'
+ and 'sprintf()'.
+* Gory Details:: More than you want to know about
+ '\' and '&' with
+ 'sub()', 'gsub()', and
+ 'gensub()'.
+* I/O Functions:: Functions for files and shell
+ commands.
+* Time Functions:: Functions for dealing with timestamps.
+* Bitwise Functions:: Functions for bitwise operations.
+* Type Functions:: Functions for type information.
+* I18N Functions:: Functions for string translation.
+* User-defined:: Describes User-defined functions in
+ detail.
+* Definition Syntax:: How to write definitions and what they
+ mean.
+* Function Example:: An example function definition and
+ what it does.
+* Function Calling:: Calling user-defined functions.
+* Calling A Function:: Don't use spaces.
+* Variable Scope:: Controlling variable scope.
+* Pass By Value/Reference:: Passing parameters.
+* Function Caveats:: Other points to know about functions.
+* Return Statement:: Specifying the value a function
+ returns.
+* Dynamic Typing:: How variable types can change at
+ runtime.
+* Indirect Calls:: Choosing the function to call at
+ runtime.
+* Functions Summary:: Summary of functions.
+* Library Names:: How to best name private global
+ variables in library functions.
+* General Functions:: Functions that are of general use.
+* Strtonum Function:: A replacement for the built-in
+ 'strtonum()' function.
+* Assert Function:: A function for assertions in
+ 'awk' programs.
+* Round Function:: A function for rounding if
+ 'sprintf()' does not do it
+ correctly.
+* Cliff Random Function:: The Cliff Random Number Generator.
+* Ordinal Functions:: Functions for using characters as
+ numbers and vice versa.
+* Join Function:: A function to join an array into a
+ string.
+* Getlocaltime Function:: A function to get formatted times.
+* Readfile Function:: A function to read an entire file at
+ once.
+* Shell Quoting:: A function to quote strings for the
+ shell.
+* Isnumeric Function:: A function to test whether a value is
+ numeric.
+* Data File Management:: Functions for managing command-line
+ data files.
+* Filetrans Function:: A function for handling data file
+ transitions.
+* Rewind Function:: A function for rereading the current
+ file.
+* File Checking:: Checking that data files are readable.
+* Empty Files:: Checking for zero-length files.
+* Ignoring Assigns:: Treating assignments as file names.
+* Getopt Function:: A function for processing command-line
+ arguments.
+* Passwd Functions:: Functions for getting user
+ information.
+* Group Functions:: Functions for getting group
+ information.
+* Walking Arrays:: A function to walk arrays of arrays.
+* Library Functions Summary:: Summary of library functions.
+* Library Exercises:: Exercises.
+* Running Examples:: How to run these examples.
+* Clones:: Clones of common utilities.
+* Cut Program:: The 'cut' utility.
+* Egrep Program:: The 'egrep' utility.
+* Id Program:: The 'id' utility.
+* Split Program:: The 'split' utility.
+* Tee Program:: The 'tee' utility.
+* Uniq Program:: The 'uniq' utility.
+* Wc Program:: The 'wc' utility.
+* Bytes vs. Characters:: Modern character sets.
+* Using extensions:: A brief intro to extensions.
+* wc program:: Code for 'wc.awk'.
+* Miscellaneous Programs:: Some interesting 'awk'
+ programs.
+* Dupword Program:: Finding duplicated words in a
+ document.
+* Alarm Program:: An alarm clock.
+* Translate Program:: A program similar to the 'tr'
+ utility.
+* Labels Program:: Printing mailing labels.
+* Word Sorting:: A program to produce a word usage
+ count.
+* History Sorting:: Eliminating duplicate entries from a
+ history file.
+* Extract Program:: Pulling out programs from Texinfo
+ source files.
+* Simple Sed:: A Simple Stream Editor.
+* Igawk Program:: A wrapper for 'awk' that
+ includes files.
+* Anagram Program:: Finding anagrams from a dictionary.
+* Signature Program:: People do amazing things with too much
+ time on their hands.
+* Programs Summary:: Summary of programs.
+* Programs Exercises:: Exercises.
+* Nondecimal Data:: Allowing nondecimal input data.
+* Boolean Typed Values:: Values with 'number|bool' type.
+* Array Sorting:: Facilities for controlling array
+ traversal and sorting arrays.
+* Controlling Array Traversal:: How to use PROCINFO["sorted_in"].
+* Array Sorting Functions:: How to use 'asort()' and
+ 'asorti()'.
+* Two-way I/O:: Two-way communications with another
+ process.
+* TCP/IP Networking:: Using 'gawk' for network
+ programming.
+* Profiling:: Profiling your 'awk' programs.
+* Extension Philosophy:: What should be built-in and what
+ should not.
+* Advanced Features Summary:: Summary of advanced features.
+* I18N and L10N:: Internationalization and Localization.
+* Explaining gettext:: How GNU 'gettext' works.
+* Programmer i18n:: Features for the programmer.
+* Translator i18n:: Features for the translator.
+* String Extraction:: Extracting marked strings.
+* Printf Ordering:: Rearranging 'printf' arguments.
+* I18N Portability:: 'awk'-level portability
+ issues.
+* I18N Example:: A simple i18n example.
+* Gawk I18N:: 'gawk' is also
+ internationalized.
+* I18N Summary:: Summary of I18N stuff.
+* Debugging:: Introduction to 'gawk'
+ debugger.
+* Debugging Concepts:: Debugging in General.
+* Debugging Terms:: Additional Debugging Concepts.
+* Awk Debugging:: Awk Debugging.
+* Sample Debugging Session:: Sample debugging session.
+* Debugger Invocation:: How to Start the Debugger.
+* Finding The Bug:: Finding the Bug.
+* List of Debugger Commands:: Main debugger commands.
+* Breakpoint Control:: Control of Breakpoints.
+* Debugger Execution Control:: Control of Execution.
+* Viewing And Changing Data:: Viewing and Changing Data.
+* Execution Stack:: Dealing with the Stack.
+* Debugger Info:: Obtaining Information about the
+ Program and the Debugger State.
+* Miscellaneous Debugger Commands:: Miscellaneous Commands.
+* Readline Support:: Readline support.
+* Limitations:: Limitations and future plans.
+* Debugging Summary:: Debugging summary.
+* Global Namespace:: The global namespace in standard
+ 'awk'.
+* Qualified Names:: How to qualify names with a namespace.
+* Default Namespace:: The default namespace.
+* Changing The Namespace:: How to change the namespace.
+* Naming Rules:: Namespace and Component Naming Rules.
+* Internal Name Management:: How names are stored internally.
+* Namespace Example:: An example of code using a namespace.
+* Namespace And Features:: Namespaces and other 'gawk'
+ features.
+* Namespace Summary:: Summarizing namespaces.
+* Computer Arithmetic:: A quick intro to computer math.
+* Math Definitions:: Defining terms used.
+* MPFR features:: The MPFR features in 'gawk'.
+* FP Math Caution:: Things to know.
+* Inexactness of computations:: Floating point math is not exact.
+* Inexact representation:: Numbers are not exactly represented.
+* Comparing FP Values:: How to compare floating point values.
+* Errors accumulate:: Errors get bigger as they go.
+* Strange values:: A few words about infinities and NaNs.
+* Getting Accuracy:: Getting more accuracy takes some work.
+* Try To Round:: Add digits and round.
+* Setting precision:: How to set the precision.
+* Setting the rounding mode:: How to set the rounding mode.
+* Arbitrary Precision Integers:: Arbitrary Precision Integer Arithmetic
+ with 'gawk'.
+* Checking for MPFR:: How to check if MPFR is available.
+* POSIX Floating Point Problems:: Standards Versus Existing Practice.
+* Floating point summary:: Summary of floating point discussion.
+* Extension Intro:: What is an extension.
+* Plugin License:: A note about licensing.
+* Extension Mechanism Outline:: An outline of how it works.
+* Extension API Description:: A full description of the API.
+* Extension API Functions Introduction:: Introduction to the API functions.
+* General Data Types:: The data types.
+* Memory Allocation Functions:: Functions for allocating memory.
+* Constructor Functions:: Functions for creating values.
+* API Ownership of MPFR and GMP Values:: Managing MPFR and GMP Values.
+* Registration Functions:: Functions to register things with
+ 'gawk'.
+* Extension Functions:: Registering extension functions.
+* Exit Callback Functions:: Registering an exit callback.
+* Extension Version String:: Registering a version string.
+* Input Parsers:: Registering an input parser.
+* Output Wrappers:: Registering an output wrapper.
+* Two-way processors:: Registering a two-way processor.
+* Printing Messages:: Functions for printing messages.
+* Updating ERRNO:: Functions for updating 'ERRNO'.
+* Requesting Values:: How to get a value.
+* Accessing Parameters:: Functions for accessing parameters.
+* Symbol Table Access:: Functions for accessing global
+ variables.
+* Symbol table by name:: Accessing variables by name.
+* Symbol table by cookie:: Accessing variables by "cookie".
+* Cached values:: Creating and using cached values.
+* Array Manipulation:: Functions for working with arrays.
+* Array Data Types:: Data types for working with arrays.
+* Array Functions:: Functions for working with arrays.
+* Flattening Arrays:: How to flatten arrays.
+* Creating Arrays:: How to create and populate arrays.
+* Redirection API:: How to access and manipulate
+ redirections.
+* Extension API Variables:: Variables provided by the API.
+* Extension Versioning:: API Version information.
+* Extension GMP/MPFR Versioning:: Version information about GMP and
+ MPFR.
+* Extension API Informational Variables:: Variables providing information about
+ 'gawk''s invocation.
+* Extension API Boilerplate:: Boilerplate code for using the API.
+* Changes from API V1:: Changes from V1 of the API.
+* Finding Extensions:: How 'gawk' finds compiled
+ extensions.
+* Extension Example:: Example C code for an extension.
+* Internal File Description:: What the new functions will do.
+* Internal File Ops:: The code for internal file operations.
+* Using Internal File Ops:: How to use an external extension.
+* Extension Samples:: The sample extensions that ship with
+ 'gawk'.
+* Extension Sample File Functions:: The file functions sample.
+* Extension Sample Fnmatch:: An interface to 'fnmatch()'.
+* Extension Sample Fork:: An interface to 'fork()' and
+ other process functions.
+* Extension Sample Inplace:: Enabling in-place file editing.
+* Extension Sample Ord:: Character to value to character
+ conversions.
+* Extension Sample Readdir:: An interface to 'readdir()'.
+* Extension Sample Revout:: Reversing output sample output
+ wrapper.
+* Extension Sample Rev2way:: Reversing data sample two-way
+ processor.
+* Extension Sample Read write array:: Serializing an array to a file.
+* Extension Sample Readfile:: Reading an entire file into a string.
+* Extension Sample Time:: An interface to 'gettimeofday()'
+ and 'sleep()'.
+* Extension Sample API Tests:: Tests for the API.
+* gawkextlib:: The 'gawkextlib' project.
+* Extension summary:: Extension summary.
+* Extension Exercises:: Exercises.
+* V7/SVR3.1:: The major changes between V7 and
+ System V Release 3.1.
+* SVR4:: Minor changes between System V
+ Releases 3.1 and 4.
+* POSIX:: New features from the POSIX standard.
+* BTL:: New features from Brian Kernighan's
+ version of 'awk'.
+* POSIX/GNU:: The extensions in 'gawk' not
+ in POSIX 'awk'.
+* Feature History:: The history of the features in
+ 'gawk'.
+* Common Extensions:: Common Extensions Summary.
+* Ranges and Locales:: How locales used to affect regexp
+ ranges.
+* Contributors:: The major contributors to
+ 'gawk'.
+* History summary:: History summary.
+* Gawk Distribution:: What is in the 'gawk'
+ distribution.
+* Getting:: How to get the distribution.
+* Extracting:: How to extract the distribution.
+* Distribution contents:: What is in the distribution.
+* Unix Installation:: Installing 'gawk' under
+ various versions of Unix.
+* Quick Installation:: Compiling 'gawk' under Unix.
+* Compiling with MPFR:: Building with MPFR.
+* Shell Startup Files:: Shell convenience functions.
+* Additional Configuration Options:: Other compile-time options.
+* Configuration Philosophy:: How it's all supposed to work.
+* Compiling from Git:: Compiling from Git.
+* Building the Documentation:: Building the Documentation.
+* Non-Unix Installation:: Installation on Other Operating
+ Systems.
+* PC Installation:: Installing and Compiling
+ 'gawk' on Microsoft Windows.
+* PC Binary Installation:: Installing a prepared distribution.
+* PC Compiling:: Compiling 'gawk' for
+ Windows32.
+* PC Using:: Running 'gawk' on Windows32.
+* Cygwin:: Building and running 'gawk'
+ for Cygwin.
+* MSYS:: Using 'gawk' In The MSYS
+ Environment.
+* VMS Installation:: Installing 'gawk' on VMS.
+* VMS Compilation:: How to compile 'gawk' under
+ VMS.
+* VMS Dynamic Extensions:: Compiling 'gawk' dynamic
+ extensions on VMS.
+* VMS Installation Details:: How to install 'gawk' under
+ VMS.
+* VMS Running:: How to run 'gawk' under VMS.
+* VMS GNV:: The VMS GNV Project.
+* Bugs:: Reporting Problems and Bugs.
+* Bug definition:: Defining what is and is not a bug.
+* Bug address:: Where to send reports to.
+* Usenet:: Where not to send reports to.
+* Performance bugs:: What to do if you think there is a
+ performance issue.
+* Asking for help:: Dealing with non-bug questions.
+* Maintainers:: Maintainers of non-*nix ports.
+* Other Versions:: Other freely available 'awk'
+ implementations.
+* Installation summary:: Summary of installation.
+* Compatibility Mode:: How to disable certain 'gawk'
+ extensions.
+* Additions:: Making Additions To 'gawk'.
+* Accessing The Source:: Accessing the Git repository.
+* Adding Code:: Adding code to the main body of
+ 'gawk'.
+* New Ports:: Porting 'gawk' to a new
+ operating system.
+* Derived Files:: Why derived files are kept in the Git
+ repository.
+* Future Extensions:: New features that may be implemented
+ one day.
+* Implementation Limitations:: Some limitations of the
+ implementation.
+* Extension Design:: Design notes about the extension API.
+* Old Extension Problems:: Problems with the old mechanism.
+* Extension New Mechanism Goals:: Goals for the new mechanism.
+* Extension Other Design Decisions:: Some other design decisions.
+* Extension Future Growth:: Some room for future growth.
+* Notes summary:: Summary of implementation notes.
+* Basic High Level:: The high level view.
+* Basic Data Typing:: A very quick intro to data types.
+
+ To my parents, for their love, and for the wonderful example they set
+for me.
+
+ To my wife Miriam, for making me complete. Thank you for building
+your life together with me.
+
+ To our children Chana, Rivka, Nachum and Malka, for enrichening our
+lives in innumerable ways.
+
+
+File: gawk.info, Node: Foreword3, Next: Foreword4, Prev: Top, Up: Top
+
+Foreword to the Third Edition
+*****************************
+
+Arnold Robbins and I are good friends. We were introduced in 1990 by
+circumstances--and our favorite programming language, AWK. The
+circumstances started a couple of years earlier. I was working at a new
+job and noticed an unplugged Unix computer sitting in the corner. No
+one knew how to use it, and neither did I. However, a couple of days
+later, it was running, and I was 'root' and the one-and-only user. That
+day, I began the transition from statistician to Unix programmer.
+
+ On one of many trips to the library or bookstore in search of books
+on Unix, I found the gray AWK book, a.k.a. Alfred V. Aho, Brian W.
+Kernighan, and Peter J. Weinberger's 'The AWK Programming Language'
+(Addison-Wesley, 1988). 'awk''s simple programming paradigm--find a
+pattern in the input and then perform an action--often reduced complex
+or tedious data manipulations to a few lines of code. I was excited to
+try my hand at programming in AWK.
+
+ Alas, the 'awk' on my computer was a limited version of the language
+described in the gray book. I discovered that my computer had "old
+'awk'" and the book described "new 'awk'." I learned that this was
+typical; the old version refused to step aside or relinquish its name.
+If a system had a new 'awk', it was invariably called 'nawk', and few
+systems had it. The best way to get a new 'awk' was to 'ftp' the source
+code for 'gawk' from 'prep.ai.mit.edu'. 'gawk' was a version of new
+'awk' written by David Trueman and Arnold, and available under the GNU
+General Public License.
+
+ (Incidentally, it's no longer difficult to find a new 'awk'. 'gawk'
+ships with GNU/Linux, and you can download binaries or source code for
+almost any system; my wife uses 'gawk' on her VMS box.)
+
+ My Unix system started out unplugged from the wall; it certainly was
+not plugged into a network. So, oblivious to the existence of 'gawk'
+and the Unix community in general, and desiring a new 'awk', I wrote my
+own, called 'mawk'. Before I was finished, I knew about 'gawk', but it
+was too late to stop, so I eventually posted to a 'comp.sources'
+newsgroup.
+
+ A few days after my posting, I got a friendly email from Arnold
+introducing himself. He suggested we share design and algorithms and
+attached a draft of the POSIX standard so that I could update 'mawk' to
+support language extensions added after publication of 'The AWK
+Programming Language'.
+
+ Frankly, if our roles had been reversed, I would not have been so
+open and we probably would have never met. I'm glad we did meet. He is
+an AWK expert's AWK expert and a genuinely nice person. Arnold
+contributes significant amounts of his expertise and time to the Free
+Software Foundation.
+
+ This book is the 'gawk' reference manual, but at its core it is a
+book about AWK programming that will appeal to a wide audience. It is a
+definitive reference to the AWK language as defined by the 1987 Bell
+Laboratories release and codified in the 1992 POSIX Utilities standard.
+
+ On the other hand, the novice AWK programmer can study a wealth of
+practical programs that emphasize the power of AWK's basic idioms:
+data-driven control flow, pattern matching with regular expressions, and
+associative arrays. Those looking for something new can try out
+'gawk''s interface to network protocols via special '/inet' files.
+
+ The programs in this book make clear that an AWK program is typically
+much smaller and faster to develop than a counterpart written in C.
+Consequently, there is often a payoff to prototyping an algorithm or
+design in AWK to get it running quickly and expose problems early.
+Often, the interpreted performance is adequate and the AWK prototype
+becomes the product.
+
+ The new 'pgawk' (profiling 'gawk'), produces program execution
+counts. I recently experimented with an algorithm that for n lines of
+input, exhibited ~ C n^2 performance, while theory predicted ~ C n log n
+behavior. A few minutes poring over the 'awkprof.out' profile
+pinpointed the problem to a single line of code. 'pgawk' is a welcome
+addition to my programmer's toolbox.
+
+ Arnold has distilled over a decade of experience writing and using
+AWK programs, and developing 'gawk', into this book. If you use AWK or
+want to learn how, then read this book.
+
+ Michael Brennan
+ Author of 'mawk'
+ March 2001
+
+
+File: gawk.info, Node: Foreword4, Next: Preface, Prev: Foreword3, Up: Top
+
+Foreword to the Fourth Edition
+******************************
+
+Some things don't change. Thirteen years ago I wrote: "If you use AWK
+or want to learn how, then read this book." True then, and still true
+today.
+
+ Learning to use a programming language is about more than mastering
+the syntax. One needs to acquire an understanding of how to use the
+features of the language to solve practical programming problems. A
+focus of this book is many examples that show how to use AWK.
+
+ Some things do change. Our computers are much faster and have more
+memory. Consequently, speed and storage inefficiencies of a high-level
+language matter less. Prototyping in AWK and then rewriting in C for
+performance reasons happens less, because more often the prototype is
+fast enough.
+
+ Of course, there are computing operations that are best done in C or
+C++. With 'gawk' 4.1 and later, you do not have to choose between
+writing your program in AWK or in C/C++. You can write most of your
+program in AWK and the aspects that require C/C++ capabilities can be
+written in C/C++, and then the pieces glued together when the 'gawk'
+module loads the C/C++ module as a dynamic plug-in. *note Dynamic
+Extensions::, has all the details, and, as expected, many examples to
+help you learn the ins and outs.
+
+ I enjoy programming in AWK and had fun (re)reading this book. I
+think you will too.
+
+ Michael Brennan
+ Author of 'mawk'
+ October 2014
+
+
+File: gawk.info, Node: Preface, Next: Getting Started, Prev: Foreword4,
Up: Top
+
+Preface
+*******
+
+Several kinds of tasks occur repeatedly when working with text files.
+You might want to extract certain lines and discard the rest. Or you
+may need to make changes wherever certain patterns appear, but leave the
+rest of the file alone. Such jobs are often easy with 'awk'. The 'awk'
+utility interprets a special-purpose programming language that makes it
+easy to handle simple data-reformatting jobs.
+
+ The GNU implementation of 'awk' is called 'gawk'; if you invoke it
+with the proper options or environment variables, it is fully compatible
+with the POSIX(1) specification of the 'awk' language and with the Unix
+version of 'awk' maintained by Brian Kernighan. This means that all
+properly written 'awk' programs should work with 'gawk'. So most of the
+time, we don't distinguish between 'gawk' and other 'awk'
+implementations.
+
+ Using 'awk' you can:
+
+ * Manage small, personal databases
+
+ * Generate reports
+
+ * Validate data
+
+ * Produce indexes and perform other document-preparation tasks
+
+ * Experiment with algorithms that you can adapt later to other
+ computer languages
+
+ In addition, 'gawk' provides facilities that make it easy to:
+
+ * Extract bits and pieces of data for processing
+
+ * Sort data
+
+ * Perform simple network communications
+
+ * Profile and debug 'awk' programs
+
+ * Extend the language with functions written in C or C++
+
+ This Info file teaches you about the 'awk' language and how you can
+use it effectively. You should already be familiar with basic system
+commands, such as 'cat' and 'ls',(2) as well as basic shell facilities,
+such as input/output (I/O) redirection and pipes.
+
+ Implementations of the 'awk' language are available for many
+different computing environments. This Info file, while describing the
+'awk' language in general, also describes the particular implementation
+of 'awk' called 'gawk' (which stands for "GNU 'awk'"). 'gawk' runs on a
+broad range of Unix systems, ranging from Intel-architecture PC-based
+computers up through large-scale systems. 'gawk' has also been ported
+to Mac OS X, Microsoft Windows (all versions), and OpenVMS.(3)
+
+* Menu:
+
+* History:: The history of 'gawk' and
+ 'awk'.
+* Names:: What name to use to find 'awk'.
+* This Manual:: Using this Info file. Includes sample
+ input files that you can use.
+* Conventions:: Typographical Conventions.
+* Manual History:: Brief history of the GNU project and this
+ Info file.
+* How To Contribute:: Helping to save the world.
+* Acknowledgments:: Acknowledgments.
+
+ ---------- Footnotes ----------
+
+ (1) The 2018 POSIX standard is accessible online at
+<https://pubs.opengroup.org/onlinepubs/9699919799/>.
+
+ (2) These utilities are available on POSIX-compliant systems, as well
+as on traditional Unix-based systems. If you are using some other
+operating system, you still need to be familiar with the ideas of I/O
+redirection and pipes.
+
+ (3) Some other, obsolete systems to which 'gawk' was once ported are
+no longer supported and the code for those systems has been removed.
+
+
+File: gawk.info, Node: History, Next: Names, Up: Preface
+
+History of 'awk' and 'gawk'
+===========================
+
+ Recipe for a Programming Language
+
+ 1 part 'egrep' 1 part 'snobol'
+ 2 parts 'ed' 3 parts C
+
+ Blend all parts well using 'lex' and 'yacc'. Document minimally and
+release.
+
+ After eight years, add another part 'egrep' and two more parts C.
+Document very well and release.
+
+ The name 'awk' comes from the initials of its designers: Alfred V.
+Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version
+of 'awk' was written in 1977 at AT&T Bell Laboratories. In 1985, a new
+version made the programming language more powerful, introducing
+user-defined functions, multiple input streams, and computed regular
+expressions. This new version became widely available with Unix System
+V Release 3.1 (1987). The version in System V Release 4 (1989) added
+some new features and cleaned up the behavior in some of the "dark
+corners" of the language. The specification for 'awk' in the POSIX
+Command Language and Utilities standard further clarified the language.
+Both the 'gawk' designers and the original 'awk' designers at Bell
+Laboratories provided feedback for the POSIX specification.
+
+ Paul Rubin wrote 'gawk' in 1986. Jay Fenlason completed it, with
+advice from Richard Stallman. John Woods contributed parts of the code
+as well. In 1988 and 1989, David Trueman, with help from me, thoroughly
+reworked 'gawk' for compatibility with the newer 'awk'. Circa 1994, I
+became the primary maintainer. Current development focuses on bug
+fixes, performance improvements, standards compliance, and,
+occasionally, new features.
+
+ In May 1997, Jürgen Kahrs felt the need for network access from
+'awk', and with a little help from me, set about adding features to do
+this for 'gawk'. At that time, he also wrote the bulk of 'TCP/IP
+Internetworking with 'gawk'' (a separate document, available as part of
+the 'gawk' distribution). His code finally became part of the main
+'gawk' distribution with 'gawk' version 3.1.
+
+ John Haque rewrote the 'gawk' internals, in the process providing an
+'awk'-level debugger. This version became available as 'gawk' version
+4.0 in 2011.
+
+ *Note Contributors:: for a full list of those who have made important
+contributions to 'gawk'.
+
+
+File: gawk.info, Node: Names, Next: This Manual, Prev: History, Up: Preface
+
+A Rose by Any Other Name
+========================
+
+The 'awk' language has evolved over the years. Full details are
+provided in *note Language History::. The language described in this
+Info file is often referred to as "new 'awk'." By analogy, the original
+version of 'awk' is referred to as "old 'awk'."
+
+ On most current systems, when you run the 'awk' utility you get some
+version of new 'awk'.(1) If your system's standard 'awk' is the old
+one, you will see something like this if you try the following test
+program:
+
+ $ awk 1 /dev/null
+ error-> awk: syntax error near line 1
+ error-> awk: bailing out near line 1
+
+In this case, you should find a version of new 'awk', or just install
+'gawk'!
+
+ Throughout this Info file, whenever we refer to a language feature
+that should be available in any complete implementation of POSIX 'awk',
+we simply use the term 'awk'. When referring to a feature that is
+specific to the GNU implementation, we use the term 'gawk'.
+
+ ---------- Footnotes ----------
+
+ (1) Only Solaris systems still use an old 'awk' for the default 'awk'
+utility. A more modern 'awk' lives in '/usr/xpg6/bin' on these systems.
+
+
+File: gawk.info, Node: This Manual, Next: Conventions, Prev: Names, Up:
Preface
+
+Using This Book
+===============
+
+The term 'awk' refers to a particular program as well as to the language
+you use to tell this program what to do. When we need to be careful, we
+call the language "the 'awk' language," and the program "the 'awk'
+utility." This Info file explains both how to write programs in the
+'awk' language and how to run the 'awk' utility. The term "'awk'
+program" refers to a program written by you in the 'awk' programming
+language.
+
+ Primarily, this Info file explains the features of 'awk' as defined
+in the POSIX standard. It does so in the context of the 'gawk'
+implementation. While doing so, it also attempts to describe important
+differences between 'gawk' and other 'awk' implementations.(1) Finally,
+it notes any 'gawk' features that are not in the POSIX standard for
+'awk'.
+
+ There are sidebars scattered throughout the Info file. They add a
+more complete explanation of points that are relevant, but not likely to
+be of interest on first reading. All appear in the index, under the
+heading "sidebar."
+
+ Most of the time, the examples use complete 'awk' programs. Some of
+the more advanced minor nodes show only the part of the 'awk' program
+that illustrates the concept being described.
+
+ Although this Info file is aimed principally at people who have not
+been exposed to 'awk', there is a lot of information here that even the
+'awk' expert should find useful. In particular, the description of
+POSIX 'awk' and the example programs in *note Library Functions::, and
+in *note Sample Programs::, should be of interest.
+
+ This Info file is split into several parts, as follows:
+
+ * Part I describes the 'awk' language and the 'gawk' program in
+ detail. It starts with the basics, and continues through all of
+ the features of 'awk'. It contains the following chapters:
+
+ - *note Getting Started::, provides the essentials you need to
+ know to begin using 'awk'.
+
+ - *note Invoking Gawk::, describes how to run 'gawk', the
+ meaning of its command-line options, and how it finds 'awk'
+ program source files.
+
+ - *note Regexp::, introduces regular expressions in general, and
+ in particular the flavors supported by POSIX 'awk' and 'gawk'.
+
+ - *note Reading Files::, describes how 'awk' reads your data.
+ It introduces the concepts of records and fields, as well as
+ the 'getline' command. I/O redirection is first described
+ here. Network I/O is also briefly introduced here.
+
+ - *note Printing::, describes how 'awk' programs can produce
+ output with 'print' and 'printf'.
+
+ - *note Expressions::, describes expressions, which are the
+ basic building blocks for getting most things done in a
+ program.
+
+ - *note Patterns and Actions::, describes how to write patterns
+ for matching records, actions for doing something when a
+ record is matched, and the predefined variables 'awk' and
+ 'gawk' use.
+
+ - *note Arrays::, covers 'awk''s one-and-only data structure:
+ the associative array. Deleting array elements and whole
+ arrays is described, as well as sorting arrays in 'gawk'. The
+ major node also describes how 'gawk' provides arrays of
+ arrays.
+
+ - *note Functions::, describes the built-in functions 'awk' and
+ 'gawk' provide, as well as how to define your own functions.
+ It also discusses how 'gawk' lets you call functions
+ indirectly.
+
+ * Part II shows how to use 'awk' and 'gawk' for problem solving.
+ There is lots of code here for you to read and learn from. This
+ part contains the following chapters:
+
+ - *note Library Functions::, provides a number of functions
+ meant to be used from main 'awk' programs.
+
+ - *note Sample Programs::, provides many sample 'awk' programs.
+
+ Reading these two chapters allows you to see 'awk' solving real
+ problems.
+
+ * Part III focuses on features specific to 'gawk'. It contains the
+ following chapters:
+
+ - *note Advanced Features::, describes a number of advanced
+ features. Of particular note are the abilities to control the
+ order of array traversal, have two-way communications with
+ another process, perform TCP/IP networking, and profile your
+ 'awk' programs.
+
+ - *note Internationalization::, describes special features for
+ translating program messages into different languages at
+ runtime.
+
+ - *note Debugger::, describes the 'gawk' debugger.
+
+ - *note Namespaces::, describes how 'gawk' allows variables
+ and/or functions of the same name to be in different
+ namespaces.
+
+ - *note Arbitrary Precision Arithmetic::, describes advanced
+ arithmetic facilities.
+
+ - *note Dynamic Extensions::, describes how to add new variables
+ and functions to 'gawk' by writing extensions in C or C++.
+
+ * Part IV provides the appendices, the Glossary, and two licenses
+ that cover the 'gawk' source code and this Info file, respectively.
+ It contains the following appendices:
+
+ - *note Language History::, describes how the 'awk' language has
+ evolved since its first release to the present. It also
+ describes how 'gawk' has acquired features over time.
+
+ - *note Installation::, describes how to get 'gawk', how to
+ compile it on POSIX-compatible systems, and how to compile and
+ use it on different non-POSIX systems. It also describes how
+ to report bugs in 'gawk' and where to get other freely
+ available 'awk' implementations.
+
+ - *note Notes::, describes how to disable 'gawk''s extensions,
+ as well as how to contribute new code to 'gawk', and some
+ possible future directions for 'gawk' development.
+
+ - *note Basic Concepts::, provides some very cursory background
+ material for those who are completely unfamiliar with computer
+ programming.
+
+ - The *note Glossary::, defines most, if not all, of the
+ significant terms used throughout the Info file. If you find
+ terms that you aren't familiar with, try looking them up here.
+
+ - *note Copying::, and *note GNU Free Documentation License::,
+ present the licenses that cover the 'gawk' source code and
+ this Info file, respectively.
+
+ ---------- Footnotes ----------
+
+ (1) All such differences appear in the index under the entry
+"differences in 'awk' and 'gawk'."
+
+
+File: gawk.info, Node: Conventions, Next: Manual History, Prev: This
Manual, Up: Preface
+
+Typographical Conventions
+=========================
+
+This Info file is written in Texinfo
+(https://www.gnu.org/software/texinfo/), the GNU documentation
+formatting language. A single Texinfo source file is used to produce
+both the printed and online versions of the documentation. This minor
+node briefly documents the typographical conventions used in Texinfo.
+
+ Examples you would type at the command line are preceded by the
+common shell primary and secondary prompts, '$' and '>', respectively.
+Input that you type is shown 'like this'. Output from the command is
+preceded by the glyph "-|". This typically represents the command's
+standard output. Error messages and other output on the command's
+standard error are preceded by the glyph "error->". For example:
+
+ $ echo hi on stdout
+ -| hi on stdout
+ $ echo hello on stderr 1>&2
+ error-> hello on stderr
+
+ Characters that you type at the keyboard look 'like this'. In
+particular, there are special characters called "control characters."
+These are characters that you type by holding down both the 'CONTROL'
+key and another key, at the same time. For example, a 'Ctrl-d' is typed
+by first pressing and holding the 'CONTROL' key, next pressing the 'd'
+key, and finally releasing both keys.
+
+ For the sake of brevity, throughout this Info file, we refer to Brian
+Kernighan's version of 'awk' as "BWK 'awk'." (*Note Other Versions::
+for information on his and other versions.)
+
+Dark Corners
+------------
+
+ Dark corners are basically fractal--no matter how much you
+ illuminate, there's always a smaller but darker one.
+ -- _Brian Kernighan_
+
+ Until the POSIX standard (and 'GAWK: Effective AWK Programming'),
+many features of 'awk' were either poorly documented or not documented
+at all. Descriptions of such features (often called "dark corners") are
+noted in this Info file with "(d.c.)." They also appear in the index
+under the heading "dark corner."
+
+ But, as noted by the opening quote, any coverage of dark corners is
+by definition incomplete.
+
+ Extensions to the standard 'awk' language that are supported by more
+than one 'awk' implementation are marked "(c.e.)," and listed in the
+index under "common extensions" and "extensions, common."
+
+
+File: gawk.info, Node: Manual History, Next: How To Contribute, Prev:
Conventions, Up: Preface
+
+The GNU Project and This Book
+=============================
+
+The Free Software Foundation (FSF) is a nonprofit organization dedicated
+to the production and distribution of freely distributable software. It
+was founded by Richard M. Stallman, the author of the original Emacs
+editor. GNU Emacs is the most widely used version of Emacs today.
+
+ The GNU(1) Project is an ongoing effort on the part of the Free
+Software Foundation to create a complete, freely distributable,
+POSIX-compliant computing environment. The FSF uses the GNU General
+Public License (GPL) to ensure that its software's source code is always
+available to the end user. A copy of the GPL is included for your
+reference (*note Copying::). The GPL applies to the C language source
+code for 'gawk'. To find out more about the FSF and the GNU Project
+online, see the GNU Project's home page (https://www.gnu.org). This
+Info file may also be read from GNU's website
+(https://www.gnu.org/software/gawk/manual/).
+
+ A shell, an editor (Emacs), highly portable optimizing C, C++, and
+Objective-C compilers, a symbolic debugger and dozens of large and small
+utilities (such as 'gawk'), have all been completed and are freely
+available. The GNU operating system kernel (the HURD), has been
+released but remains in an early stage of development.
+
+ Until the GNU operating system is more fully developed, you should
+consider using GNU/Linux, a freely distributable, Unix-like operating
+system for Intel, Power Architecture, Sun SPARC, IBM S/390, and other
+systems.(2) Many GNU/Linux distributions are available for download
+from the Internet.
+
+ The Info file itself has gone through multiple previous editions.
+Paul Rubin wrote the very first draft of 'The GAWK Manual'; it was
+around 40 pages long. Diane Close and Richard Stallman improved it,
+yielding a version that was around 90 pages and barely described the
+original, "old" version of 'awk'.
+
+ I started working with that version in the fall of 1988. As work on
+it progressed, the FSF published several preliminary versions (numbered
+0.X). In 1996, edition 1.0 was released with 'gawk' 3.0.0. The FSF
+published the first two editions under the title 'The GNU Awk User's
+Guide'.
+
+ This edition maintains the basic structure of the previous editions.
+For FSF edition 4.0, the content was thoroughly reviewed and updated.
+All references to 'gawk' versions prior to 4.0 were removed. Of
+significant note for that edition was the addition of *note Debugger::.
+
+ For FSF edition 5.0, the content has been reorganized into parts, and
+the major new additions are *note Arbitrary Precision Arithmetic::, and
+*note Dynamic Extensions::.
+
+ This Info file will undoubtedly continue to evolve. If you find an
+error in the Info file, please report it! *Note Bugs:: for information
+on submitting problem reports electronically.
+
+ ---------- Footnotes ----------
+
+ (1) GNU stands for "GNU's Not Unix."
+
+ (2) The terminology "GNU/Linux" is explained in the *note Glossary::.
+
+
+File: gawk.info, Node: How To Contribute, Next: Acknowledgments, Prev:
Manual History, Up: Preface
+
+How to Contribute
+=================
+
+As the maintainer of GNU 'awk', I once thought that I would be able to
+manage a collection of publicly available 'awk' programs and I even
+solicited contributions. Making things available on the Internet helps
+keep the 'gawk' distribution down to manageable size.
+
+ The initial collection of material, such as it is, is still available
+at <ftp://ftp.freefriends.org/arnold/Awkstuff>.
+
+ In the hopes of doing something more broad, I acquired the
+'awklang.org' domain. Late in 2017, a volunteer took on the task of
+managing it.
+
+ If you have written an interesting 'awk' program, that you would like
+to share with the rest of the world, please see <http://www.awklang.org>
+and use the "Contact" link.
+
+ If you have written a 'gawk' extension, please see *note
+gawkextlib::.
+
+
+File: gawk.info, Node: Acknowledgments, Prev: How To Contribute, Up: Preface
+
+Acknowledgments
+===============
+
+The initial draft of 'The GAWK Manual' had the following
+acknowledgments:
+
+ Many people need to be thanked for their assistance in producing
+ this manual. Jay Fenlason contributed many ideas and sample
+ programs. Richard Mlynarik and Robert Chassell gave helpful
+ comments on drafts of this manual. The paper 'A Supplemental
+ Document for AWK' by John W. Pierce of the Chemistry Department at
+ UC San Diego, pinpointed several issues relevant both to 'awk'
+ implementation and to this manual, that would otherwise have
+ escaped us.
+
+ I would like to acknowledge Richard M. Stallman, for his vision of a
+better world and for his courage in founding the FSF and starting the
+GNU Project.
+
+ Earlier editions of this Info file had the following
+acknowledgements:
+
+ The following people (in alphabetical order) provided helpful
+ comments on various versions of this book: Rick Adams, Dr. Nelson
+ H.F. Beebe, Karl Berry, Dr. Michael Brennan, Rich Burridge, Claire
+ Cloutier, Diane Close, Scott Deifik, Christopher ("Topher") Eliot,
+ Jeffrey Friedl, Dr. Darrel Hankerson, Michal Jaegermann, Dr.
+ Richard J. LeBlanc, Michael Lijewski, Pat Rankin, Miriam Robbins,
+ Mary Sheehan, and Chuck Toporek.
+
+ Robert J. Chassell provided much valuable advice on the use of
+ Texinfo. He also deserves special thanks for convincing me _not_
+ to title this Info file 'How to Gawk Politely'. Karl Berry helped
+ significantly with the TeX part of Texinfo.
+
+ I would like to thank Marshall and Elaine Hartholz of Seattle and
+ Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet
+ vacation time in their homes, which allowed me to make significant
+ progress on this Info file and on 'gawk' itself.
+
+ Phil Hughes of SSC contributed in a very important way by loaning
+ me his laptop GNU/Linux system, not once, but twice, which allowed
+ me to do a lot of work while away from home.
+
+ David Trueman deserves special credit; he has done a yeoman job of
+ evolving 'gawk' so that it performs well and without bugs.
+ Although he is no longer involved with 'gawk', working with him on
+ this project was a significant pleasure.
+
+ The intrepid members of the GNITS mailing list, and most notably
+ Ulrich Drepper, provided invaluable help and feedback for the
+ design of the internationalization features.
+
+ Chuck Toporek, Mary Sheehan, and Claire Cloutier of O'Reilly &
+ Associates contributed significant editorial help for this Info
+ file for the 3.1 release of 'gawk'.
+
+ Dr. Nelson Beebe, Andreas Buening, Dr. Manuel Collado, Antonio
+Colombo, Stephen Davies, Scott Deifik, Akim Demaille, Daniel Richard G.,
+Juan Manuel Guerrero, Darrel Hankerson, Michal Jaegermann, Jürgen Kahrs,
+Stepan Kasal, John Malmberg, Chet Ramey, Pat Rankin, Andrew Schorr,
+Corinna Vinschen, and Eli Zaretskii (in alphabetical order) make up the
+current 'gawk' "crack portability team." Without their hard work and
+help, 'gawk' would not be nearly the robust, portable program it is
+today. It has been and continues to be a pleasure working with this
+team of fine people.
+
+ Notable code and documentation contributions were made by a number of
+people. *Note Contributors:: for the full list.
+
+ Thanks to Michael Brennan for the Forewords.
+
+ Thanks to Patrice Dumas for the new 'makeinfo' program. Thanks to
+Karl Berry for his past work on Texinfo, and to Gavin Smith, who
+continues to work to improve the Texinfo markup language.
+
+ Robert P.J. Day, Michael Brennan, and Brian Kernighan kindly acted as
+reviewers for the 2015 edition of this Info file. Their feedback helped
+improve the final work.
+
+ I would also like to thank Brian Kernighan for his invaluable
+assistance during the testing and debugging of 'gawk', and for his
+ongoing help and advice in clarifying numerous points about the
+language. We could not have done nearly as good a job on either 'gawk'
+or its documentation without his help.
+
+ Brian is in a class by himself as a programmer and technical author.
+I have to thank him (yet again) for his ongoing friendship and for being
+a role model to me for over 30 years! Having him as a reviewer is an
+exciting privilege. It has also been extremely humbling...
+
+ I must thank my wonderful wife, Miriam, for her patience through the
+many versions of this project, for her proofreading, and for sharing me
+with the computer. I would like to thank my parents for their love, and
+for the grace with which they raised and educated me. Finally, I also
+must acknowledge my gratitude to G-d, for the many opportunities He has
+sent my way, as well as for the gifts He has given me with which to take
+advantage of those opportunities.
+
+
+Arnold Robbins
+Nof Ayalon
+Israel
+March, 2020
+
+
+File: gawk.info, Node: Getting Started, Next: Invoking Gawk, Prev: Preface,
Up: Top
+
+1 Getting Started with 'awk'
+****************************
+
+The basic function of 'awk' is to search files for lines (or other units
+of text) that contain certain patterns. When a line matches one of the
+patterns, 'awk' performs specified actions on that line. 'awk'
+continues to process input lines in this way until it reaches the end of
+the input files.
+
+ Programs in 'awk' are different from programs in most other
+languages, because 'awk' programs are "data driven" (i.e., you describe
+the data you want to work with and then what to do when you find it).
+Most other languages are "procedural"; you have to describe, in great
+detail, every step the program should take. When working with
+procedural languages, it is usually much harder to clearly describe the
+data your program will process. For this reason, 'awk' programs are
+often refreshingly easy to read and write.
+
+ When you run 'awk', you specify an 'awk' "program" that tells 'awk'
+what to do. The program consists of a series of "rules" (it may also
+contain "function definitions", an advanced feature that we will ignore
+for now; *note User-defined::). Each rule specifies one pattern to
+search for and one action to perform upon finding the pattern.
+
+ Syntactically, a rule consists of a "pattern" followed by an
+"action". The action is enclosed in braces to separate it from the
+pattern. Newlines usually separate rules. Therefore, an 'awk' program
+looks like this:
+
+ PATTERN { ACTION }
+ PATTERN { ACTION }
+ ...
+
+* Menu:
+
+* Running gawk:: How to run 'gawk' programs; includes
+ command-line syntax.
+* Sample Data Files:: Sample data files for use in the 'awk'
+ programs illustrated in this Info file.
+* Very Simple:: A very simple example.
+* Two Rules:: A less simple one-line example using two
+ rules.
+* More Complex:: A more complex example.
+* Statements/Lines:: Subdividing or combining statements into
+ lines.
+* Other Features:: Other Features of 'awk'.
+* When:: When to use 'gawk' and when to use
+ other things.
+* Intro Summary:: Summary of the introduction.
+
+
+File: gawk.info, Node: Running gawk, Next: Sample Data Files, Up: Getting
Started
+
+1.1 How to Run 'awk' Programs
+=============================
+
+There are several ways to run an 'awk' program. If the program is
+short, it is easiest to include it in the command that runs 'awk', like
+this:
+
+ awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
+
+ When the program is long, it is usually more convenient to put it in
+a file and run it with a command like this:
+
+ awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ...
+
+ This minor node discusses both mechanisms, along with several
+variations of each.
+
+* Menu:
+
+* One-shot:: Running a short throwaway 'awk'
+ program.
+* Read Terminal:: Using no input files (input from the keyboard
+ instead).
+* Long:: Putting permanent 'awk' programs in
+ files.
+* Executable Scripts:: Making self-contained 'awk' programs.
+* Comments:: Adding documentation to 'gawk'
+ programs.
+* Quoting:: More discussion of shell quoting issues.
+
+
+File: gawk.info, Node: One-shot, Next: Read Terminal, Up: Running gawk
+
+1.1.1 One-Shot Throwaway 'awk' Programs
+---------------------------------------
+
+Once you are familiar with 'awk', you will often type in simple programs
+the moment you want to use them. Then you can write the program as the
+first argument of the 'awk' command, like this:
+
+ awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
+
+where PROGRAM consists of a series of patterns and actions, as described
+earlier.
+
+ This command format instructs the "shell", or command interpreter, to
+start 'awk' and use the PROGRAM to process records in the input file(s).
+There are single quotes around PROGRAM so the shell won't interpret any
+'awk' characters as special shell characters. The quotes also cause the
+shell to treat all of PROGRAM as a single argument for 'awk', and allow
+PROGRAM to be more than one line long.
+
+ This format is also useful for running short or medium-sized 'awk'
+programs from shell scripts, because it avoids the need for a separate
+file for the 'awk' program. A self-contained shell script is more
+reliable because there are no other files to misplace.
+
+ Later in this chapter, in *note Very Simple::, we'll see examples of
+several short, self-contained programs.
+
+
+File: gawk.info, Node: Read Terminal, Next: Long, Prev: One-shot, Up:
Running gawk
+
+1.1.2 Running 'awk' Without Input Files
+---------------------------------------
+
+You can also run 'awk' without any input files. If you type the
+following command line:
+
+ awk 'PROGRAM'
+
+'awk' applies the PROGRAM to the "standard input", which usually means
+whatever you type on the keyboard. This continues until you indicate
+end-of-file by typing 'Ctrl-d'. (On non-POSIX operating systems, the
+end-of-file character may be different.)
+
+ As an example, the following program prints a friendly piece of
+advice (from Douglas Adams's 'The Hitchhiker's Guide to the Galaxy'), to
+keep you from worrying about the complexities of computer programming:
+
+ $ awk 'BEGIN { print "Don\47t Panic!" }'
+ -| Don't Panic!
+
+ 'awk' executes statements associated with 'BEGIN' before reading any
+input. If there are no other statements in your program, as is the case
+here, 'awk' just stops, instead of trying to read input it doesn't know
+how to process. The '\47' is a magic way (explained later) of getting a
+single quote into the program, without having to engage in ugly shell
+quoting tricks.
+
+ NOTE: If you use Bash as your shell, you should execute the command
+ 'set +H' before running this program interactively, to disable the
+ C shell-style command history, which treats '!' as a special
+ character. We recommend putting this command into your personal
+ startup file.
+
+ This next simple 'awk' program emulates the 'cat' utility; it copies
+whatever you type on the keyboard to its standard output (why this works
+is explained shortly):
+
+ $ awk '{ print }'
+ Now is the time for all good men
+ -| Now is the time for all good men
+ to come to the aid of their country.
+ -| to come to the aid of their country.
+ Four score and seven years ago, ...
+ -| Four score and seven years ago, ...
+ What, me worry?
+ -| What, me worry?
+ Ctrl-d
+
+
+File: gawk.info, Node: Long, Next: Executable Scripts, Prev: Read Terminal,
Up: Running gawk
+
+1.1.3 Running Long Programs
+---------------------------
+
+Sometimes 'awk' programs are very long. In these cases, it is more
+convenient to put the program into a separate file. In order to tell
+'awk' to use that file for its program, you type:
+
+ awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ...
+
+ The '-f' instructs the 'awk' utility to get the 'awk' program from
+the file SOURCE-FILE (*note Options::). Any file name can be used for
+SOURCE-FILE. For example, you could put the program:
+
+ BEGIN { print "Don't Panic!" }
+
+into the file 'advice'. Then this command:
+
+ awk -f advice
+
+does the same thing as this one:
+
+ awk 'BEGIN { print "Don\47t Panic!" }'
+
+This was explained earlier (*note Read Terminal::). Note that you don't
+usually need single quotes around the file name that you specify with
+'-f', because most file names don't contain any of the shell's special
+characters. Notice that in 'advice', the 'awk' program did not have
+single quotes around it. The quotes are only needed for programs that
+are provided on the 'awk' command line. (Also, placing the program in a
+file allows us to use a literal single quote in the program text,
+instead of the magic '\47'.)
+
+ If you want to clearly identify an 'awk' program file as such, you
+can add the extension '.awk' to the file name. This doesn't affect the
+execution of the 'awk' program but it does make "housekeeping" easier.
+
+
+File: gawk.info, Node: Executable Scripts, Next: Comments, Prev: Long, Up:
Running gawk
+
+1.1.4 Executable 'awk' Programs
+-------------------------------
+
+Once you have learned 'awk', you may want to write self-contained 'awk'
+scripts, using the '#!' script mechanism. You can do this on many
+systems.(1) For example, you could update the file 'advice' to look
+like this:
+
+ #! /bin/awk -f
+
+ BEGIN { print "Don't Panic!" }
+
+After making this file executable (with the 'chmod' utility), simply
+type 'advice' at the shell and the system arranges to run 'awk' as if
+you had typed 'awk -f advice':
+
+ $ chmod +x advice
+ $ ./advice
+ -| Don't Panic!
+
+Self-contained 'awk' scripts are useful when you want to write a program
+that users can invoke without their having to know that the program is
+written in 'awk'.
+
+ Understanding '#!'
+
+ 'awk' is an "interpreted" language. This means that the 'awk'
+utility reads your program and then processes your data according to the
+instructions in your program. (This is different from a "compiled"
+language such as C, where your program is first compiled into machine
+code that is executed directly by your system's processor.) The 'awk'
+utility is thus termed an "interpreter". Many modern languages are
+interpreted.
+
+ The line beginning with '#!' lists the full file name of an
+interpreter to run and a single optional initial command-line argument
+to pass to that interpreter. The operating system then runs the
+interpreter with the given argument and the full argument list of the
+executed program. The first argument in the list is the full file name
+of the 'awk' program. The rest of the argument list contains either
+options to 'awk', or data files, or both. (Note that on many systems
+'awk' is found in '/usr/bin' instead of in '/bin'.)
+
+ Some systems limit the length of the interpreter name to 32
+characters. Often, this can be dealt with by using a symbolic link.
+
+ You should not put more than one argument on the '#!' line after the
+path to 'awk'. It does not work. The operating system treats the rest
+of the line as a single argument and passes it to 'awk'. Doing this
+leads to confusing behavior--most likely a usage diagnostic of some sort
+from 'awk'.
+
+ Finally, the value of 'ARGV[0]' (*note Built-in Variables::) varies
+depending upon your operating system. Some systems put 'awk' there,
+some put the full pathname of 'awk' (such as '/bin/awk'), and some put
+the name of your script ('advice'). (d.c.) Don't rely on the value of
+'ARGV[0]' to provide your script name.
+
+ ---------- Footnotes ----------
+
+ (1) The '#!' mechanism works on GNU/Linux systems, BSD-based systems,
+and commercial Unix systems.
+
+
+File: gawk.info, Node: Comments, Next: Quoting, Prev: Executable Scripts,
Up: Running gawk
+
+1.1.5 Comments in 'awk' Programs
+--------------------------------
+
+A "comment" is some text that is included in a program for the sake of
+human readers; it is not really an executable part of the program.
+Comments can explain what the program does and how it works. Nearly all
+programming languages have provisions for comments, as programs are
+typically hard to understand without them.
+
+ In the 'awk' language, a comment starts with the number sign
+character ('#') and continues to the end of the line. The '#' does not
+have to be the first character on the line. The 'awk' language ignores
+the rest of a line following a number sign. For example, we could have
+put the following into 'advice':
+
+ # This program prints a nice, friendly message. It helps
+ # keep novice users from being afraid of the computer.
+ BEGIN { print "Don't Panic!" }
+
+ You can put comment lines into keyboard-composed throwaway 'awk'
+programs, but this usually isn't very useful; the purpose of a comment
+is to help you or another person understand the program when reading it
+at a later time.
+
+ CAUTION: As mentioned in *note One-shot::, you can enclose short to
+ medium-sized programs in single quotes, in order to keep your shell
+ scripts self-contained. When doing so, _don't_ put an apostrophe
+ (i.e., a single quote) into a comment (or anywhere else in your
+ program). The shell interprets the quote as the closing quote for
+ the entire program. As a result, usually the shell prints a
+ message about mismatched quotes, and if 'awk' actually runs, it
+ will probably print strange messages about syntax errors. For
+ example, look at the following:
+
+ $ awk 'BEGIN { print "hello" } # let's be cute'
+ >
+
+ The shell sees that the first two quotes match, and that a new
+ quoted object begins at the end of the command line. It therefore
+ prompts with the secondary prompt, waiting for more input. With
+ Unix 'awk', closing the quoted string produces this result:
+
+ $ awk '{ print "hello" } # let's be cute'
+ > '
+ error-> awk: can't open file be
+ error-> source line number 1
+
+ Putting a backslash before the single quote in 'let's' wouldn't
+ help, because backslashes are not special inside single quotes.
+ The next node describes the shell's quoting rules.
+
+
+File: gawk.info, Node: Quoting, Prev: Comments, Up: Running gawk
+
+1.1.6 Shell Quoting Issues
+--------------------------
+
+* Menu:
+
+* DOS Quoting:: Quoting in Windows Batch Files.
+
+For short to medium-length 'awk' programs, it is most convenient to
+enter the program on the 'awk' command line. This is best done by
+enclosing the entire program in single quotes. This is true whether you
+are entering the program interactively at the shell prompt, or writing
+it as part of a larger shell script:
+
+ awk 'PROGRAM TEXT' INPUT-FILE1 INPUT-FILE2 ...
+
+ Once you are working with the shell, it is helpful to have a basic
+knowledge of shell quoting rules. The following rules apply only to
+POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again
+Shell). If you use the C shell, you're on your own.
+
+ Before diving into the rules, we introduce a concept that appears
+throughout this Info file, which is that of the "null", or empty,
+string.
+
+ The null string is character data that has no value. In other words,
+it is empty. It is written in 'awk' programs like this: '""'. In the
+shell, it can be written using single or double quotes: '""' or ''''.
+Although the null string has no characters in it, it does exist. For
+example, consider this command:
+
+ $ echo ""
+
+Here, the 'echo' utility receives a single argument, even though that
+argument has no characters in it. In the rest of this Info file, we use
+the terms "null string" and "empty string" interchangeably. Now, on to
+the quoting rules:
+
+ * Quoted items can be concatenated with nonquoted items as well as
+ with other quoted items. The shell turns everything into one
+ argument for the command.
+
+ * Preceding any single character with a backslash ('\') quotes that
+ character. The shell removes the backslash and passes the quoted
+ character on to the command.
+
+ * Single quotes protect everything between the opening and closing
+ quotes. The shell does no interpretation of the quoted text,
+ passing it on verbatim to the command. It is _impossible_ to embed
+ a single quote inside single-quoted text. Refer back to *note
+ Comments:: for an example of what happens if you try.
+
+ * Double quotes protect most things between the opening and closing
+ quotes. The shell does at least variable and command substitution
+ on the quoted text. Different shells may do additional kinds of
+ processing on double-quoted text.
+
+ Because certain characters within double-quoted text are processed
+ by the shell, they must be "escaped" within the text. Of note are
+ the characters '$', '`', '\', and '"', all of which must be
+ preceded by a backslash within double-quoted text if they are to be
+ passed on literally to the program. (The leading backslash is
+ stripped first.) Thus, the example seen in *note Read Terminal:::
+
+ awk 'BEGIN { print "Don\47t Panic!" }'
+
+ could instead be written this way:
+
+ $ awk "BEGIN { print \"Don't Panic!\" }"
+ -| Don't Panic!
+
+ Note that the single quote is not special within double quotes.
+
+ * Null strings are removed when they occur as part of a non-null
+ command-line argument, while explicit null objects are kept. For
+ example, to specify that the field separator 'FS' should be set to
+ the null string, use:
+
+ awk -F "" 'PROGRAM' FILES # correct
+
+ Don't use this:
+
+ awk -F"" 'PROGRAM' FILES # wrong!
+
+ In the second case, 'awk' attempts to use the text of the program
+ as the value of 'FS', and the first file name as the text of the
+ program! This results in syntax errors at best, and confusing
+ behavior at worst.
+
+ Mixing single and double quotes is difficult. You have to resort to
+shell quoting tricks, like this:
+
+ $ awk 'BEGIN { print "Here is a single quote <'"'"'>" }'
+ -| Here is a single quote <'>
+
+This program consists of three concatenated quoted strings. The first
+and the third are single-quoted, and the second is double-quoted.
+
+ This can be "simplified" to:
+
+ $ awk 'BEGIN { print "Here is a single quote <'\''>" }'
+ -| Here is a single quote <'>
+
+Judge for yourself which of these two is the more readable.
+
+ Another option is to use double quotes, escaping the embedded,
+'awk'-level double quotes:
+
+ $ awk "BEGIN { print \"Here is a single quote <'>\" }"
+ -| Here is a single quote <'>
+
+This option is also painful, because double quotes, backslashes, and
+dollar signs are very common in more advanced 'awk' programs.
+
+ A third option is to use the octal escape sequence equivalents (*note
+Escape Sequences::) for the single- and double-quote characters, like
+so:
+
+ $ awk 'BEGIN { print "Here is a single quote <\47>" }'
+ -| Here is a single quote <'>
+ $ awk 'BEGIN { print "Here is a double quote <\42>" }'
+ -| Here is a double quote <">
+
+This works nicely, but you should comment clearly what the escape
+sequences mean.
+
+ A fourth option is to use command-line variable assignment, like
+this:
+
+ $ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }'
+ -| Here is a single quote <'>
+
+ (Here, the two string constants and the value of 'sq' are
+concatenated into a single string that is printed by 'print'.)
+
+ If you really need both single and double quotes in your 'awk'
+program, it is probably best to move it into a separate file, where the
+shell won't be part of the picture and you can say what you mean.
+
+
+File: gawk.info, Node: DOS Quoting, Up: Quoting
+
+1.1.6.1 Quoting in MS-Windows Batch Files
+.........................................
+
+Although this Info file generally only worries about POSIX systems and
+the POSIX shell, the following issue arises often enough for many users
+that it is worth addressing.
+
+ The "shells" on Microsoft Windows systems use the double-quote
+character for quoting, and make it difficult or impossible to include an
+escaped double-quote character in a command-line script. The following
+example, courtesy of Jeroen Brink, shows how to escape the double quotes
+from this one liner script that prints all lines in a file surrounded by
+double quotes:
+
+ { print "\"" $0 "\"" }
+
+In an MS-Windows command-line the one-liner script above may be passed
+as follows:
+
+ gawk "{ print \"\042\" $0 \"\042\" }" FILE
+
+ In this example the '\042' is the octal code for a double-quote;
+'gawk' converts it into a real double-quote for output by the 'print'
+statement.
+
+ In MS-Windows escaping double-quotes is a little tricky because you
+use backslashes to escape double-quotes, but backslashes themselves are
+not escaped in the usual way; indeed they are either duplicated or not,
+depending upon whether there is a subsequent double-quote. The
+MS-Windows rule for double-quoting a string is the following:
+
+ 1. For each double quote in the original string, let N be the number
+ of backslash(es) before it, N might be zero. Replace these N
+ backslash(es) by 2*N+1 backslash(es)
+
+ 2. Let N be the number of backslash(es) tailing the original string, N
+ might be zero. Replace these N backslash(es) by 2*N backslash(es)
+
+ 3. Surround the resulting string by double-quotes.
+
+ So to double-quote the one-liner script '{ print "\"" $0 "\"" }' from
+the previous example you would do it this way:
+
+ gawk "{ print \"\\\"\" $0 \"\\\"\" }" FILE
+
+However, the use of '\042' instead of '\\\"' is also possible and easier
+to read, because backslashes that are not followed by a double-quote
+don't need duplication.
+
+
+File: gawk.info, Node: Sample Data Files, Next: Very Simple, Prev: Running
gawk, Up: Getting Started
+
+1.2 Data files for the Examples
+===============================
+
+Many of the examples in this Info file take their input from two sample
+data files. The first, 'mail-list', represents a list of peoples' names
+together with their email addresses and information about those people.
+The second data file, called 'inventory-shipped', contains information
+about monthly shipments. In both files, each line is considered to be
+one "record".
+
+ In 'mail-list', each record contains the name of a person, his/her
+phone number, his/her email address, and a code for his/her relationship
+with the author of the list. The columns are aligned using spaces. An
+'A' in the last column means that the person is an acquaintance. An 'F'
+in the last column means that the person is a friend. An 'R' means that
+the person is a relative:
+
+ Amelia 555-5553 amelia.zodiacusque@gmail.com F
+ Anthony 555-3412 anthony.asserturo@hotmail.com A
+ Becky 555-7685 becky.algebrarum@gmail.com A
+ Bill 555-1675 bill.drowning@hotmail.com A
+ Broderick 555-0542 broderick.aliquotiens@yahoo.com R
+ Camilla 555-2912 camilla.infusarum@skynet.be R
+ Fabius 555-1234 fabius.undevicesimus@ucb.edu F
+ Julie 555-6699 julie.perscrutabor@skeeve.com F
+ Martin 555-6480 martin.codicibus@hotmail.com A
+ Samuel 555-3430 samuel.lanceolis@shu.edu A
+ Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+
+ The data file 'inventory-shipped' represents information about
+shipments during the year. Each record contains the month, the number
+of green crates shipped, the number of red boxes shipped, the number of
+orange bags shipped, and the number of blue packages shipped,
+respectively. There are 16 entries, covering the 12 months of last year
+and the first four months of the current year. An empty line separates
+the data for the two years:
+
+ Jan 13 25 15 115
+ Feb 15 32 24 226
+ Mar 15 24 34 228
+ Apr 31 52 63 420
+ May 16 34 29 208
+ Jun 31 42 75 492
+ Jul 24 34 67 436
+ Aug 15 34 47 316
+ Sep 13 55 37 277
+ Oct 29 54 68 525
+ Nov 20 87 82 577
+ Dec 17 35 61 401
+
+ Jan 21 36 64 620
+ Feb 26 58 80 652
+ Mar 24 75 70 495
+ Apr 21 70 74 514
+
+ The sample files are included in the 'gawk' distribution, in the
+directory 'awklib/eg/data'.
+
+
+File: gawk.info, Node: Very Simple, Next: Two Rules, Prev: Sample Data
Files, Up: Getting Started
+
+1.3 Some Simple Examples
+========================
+
+The following command runs a simple 'awk' program that searches the
+input file 'mail-list' for the character string 'li' (a grouping of
+characters is usually called a "string"; the term "string" is based on
+similar usage in English, such as "a string of pearls" or "a string of
+cars in a train"):
+
+ awk '/li/ { print $0 }' mail-list
+
+When lines containing 'li' are found, they are printed because
+'print $0' means print the current line. (Just 'print' by itself means
+the same thing, so we could have written that instead.)
+
+ You will notice that slashes ('/') surround the string 'li' in the
+'awk' program. The slashes indicate that 'li' is the pattern to search
+for. This type of pattern is called a "regular expression", which is
+covered in more detail later (*note Regexp::). The pattern is allowed
+to match parts of words. There are single quotes around the 'awk'
+program so that the shell won't interpret any of it as special shell
+characters.
+
+ Here is what this program prints:
+
+ $ awk '/li/ { print $0 }' mail-list
+ -| Amelia 555-5553 amelia.zodiacusque@gmail.com F
+ -| Broderick 555-0542 broderick.aliquotiens@yahoo.com R
+ -| Julie 555-6699 julie.perscrutabor@skeeve.com F
+ -| Samuel 555-3430 samuel.lanceolis@shu.edu A
+
+ In an 'awk' rule, either the pattern or the action can be omitted,
+but not both. If the pattern is omitted, then the action is performed
+for _every_ input line. If the action is omitted, the default action is
+to print all lines that match the pattern.
+
+ Thus, we could leave out the action (the 'print' statement and the
+braces) in the previous example and the result would be the same: 'awk'
+prints all lines matching the pattern 'li'. By comparison, omitting the
+'print' statement but retaining the braces makes an empty action that
+does nothing (i.e., no lines are printed).
+
+ Many practical 'awk' programs are just a line or two long. Following
+is a collection of useful, short programs to get you started. Some of
+these programs contain constructs that haven't been covered yet. (The
+description of the program will give you a good idea of what is going
+on, but you'll need to read the rest of the Info file to become an 'awk'
+expert!) Most of the examples use a data file named 'data'. This is
+just a placeholder; if you use these programs yourself, substitute your
+own file names for 'data'.
+
+ Some of the following examples use the output of 'ls -l' as input.
+'ls' is a system command that gives you a listing of the files in a
+directory. With the '-l' option, this listing includes each file's size
+and the date the file was last modified. Its output looks like this:
+
+ -rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile
+ -rw-r--r-- 1 arnold user 10809 Nov 7 13:03 awk.h
+ -rw-r--r-- 1 arnold user 983 Apr 13 12:14 awk.tab.h
+ -rw-r--r-- 1 arnold user 31869 Jun 15 12:20 awkgram.y
+ -rw-r--r-- 1 arnold user 22414 Nov 7 13:03 awk1.c
+ -rw-r--r-- 1 arnold user 37455 Nov 7 13:03 awk2.c
+ -rw-r--r-- 1 arnold user 27511 Dec 9 13:07 awk3.c
+ -rw-r--r-- 1 arnold user 7989 Nov 7 13:03 awk4.c
+
+The first field contains read-write permissions, the second field
+contains the number of links to the file, and the third field identifies
+the file's owner. The fourth field identifies the file's group. The
+fifth field contains the file's size in bytes. The sixth, seventh, and
+eighth fields contain the month, day, and time, respectively, that the
+file was last modified. Finally, the ninth field contains the file
+name.
+
+ For future reference, note that there is often more than one way to
+do things in 'awk'. At some point, you may want to look back at these
+examples and see if you can come up with different ways to do the same
+things shown here:
+
+ * Print every line that is longer than 80 characters:
+
+ awk 'length($0) > 80' data
+
+ The sole rule has a relational expression as its pattern and has no
+ action--so it uses the default action, printing the record.
+
+ * Print the length of the longest input line:
+
+ awk '{ if (length($0) > max) max = length($0) }
+ END { print max }' data
+
+ The code associated with 'END' executes after all input has been
+ read; it's the other side of the coin to 'BEGIN'.
+
+ * Print the length of the longest line in 'data':
+
+ expand data | awk '{ if (x < length($0)) x = length($0) }
+ END { print "maximum line length is " x }'
+
+ This example differs slightly from the previous one: the input is
+ processed by the 'expand' utility to change TABs into spaces, so
+ the widths compared are actually the right-margin columns, as
+ opposed to the number of input characters on each line.
+
+ * Print every line that has at least one field:
+
+ awk 'NF > 0' data
+
+ This is an easy way to delete blank lines from a file (or rather,
+ to create a new file similar to the old file but from which the
+ blank lines have been removed).
+
+ * Print seven random numbers from 0 to 100, inclusive:
+
+ awk 'BEGIN { for (i = 1; i <= 7; i++)
+ print int(101 * rand()) }'
+
+ * Print the total number of bytes used by FILES:
+
+ ls -l FILES | awk '{ x += $5 }
+ END { print "total bytes: " x }'
+
+ * Print the total number of kilobytes used by FILES:
+
+ ls -l FILES | awk '{ x += $5 }
+ END { print "total K-bytes:", x / 1024 }'
+
+ * Print a sorted list of the login names of all users:
+
+ awk -F: '{ print $1 }' /etc/passwd | sort
+
+ * Count the lines in a file:
+
+ awk 'END { print NR }' data
+
+ * Print the even-numbered lines in the data file:
+
+ awk 'NR % 2 == 0' data
+
+ If you used the expression 'NR % 2 == 1' instead, the program would
+ print the odd-numbered lines.
+
+
+File: gawk.info, Node: Two Rules, Next: More Complex, Prev: Very Simple,
Up: Getting Started
+
+1.4 An Example with Two Rules
+=============================
+
+The 'awk' utility reads the input files one line at a time. For each
+line, 'awk' tries the patterns of each rule. If several patterns match,
+then several actions execute in the order in which they appear in the
+'awk' program. If no patterns match, then no actions run.
+
+ After processing all the rules that match the line (and perhaps there
+are none), 'awk' reads the next line. (However, *note Next Statement::
+and also *note Nextfile Statement::.) This continues until the program
+reaches the end of the file. For example, the following 'awk' program
+contains two rules:
+
+ /12/ { print $0 }
+ /21/ { print $0 }
+
+The first rule has the string '12' as the pattern and 'print $0' as the
+action. The second rule has the string '21' as the pattern and also has
+'print $0' as the action. Each rule's action is enclosed in its own
+pair of braces.
+
+ This program prints every line that contains the string '12' _or_ the
+string '21'. If a line contains both strings, it is printed twice, once
+by each rule.
+
+ This is what happens if we run this program on our two sample data
+files, 'mail-list' and 'inventory-shipped':
+
+ $ awk '/12/ { print $0 }
+ > /21/ { print $0 }' mail-list inventory-shipped
+ -| Anthony 555-3412 anthony.asserturo@hotmail.com A
+ -| Camilla 555-2912 camilla.infusarum@skynet.be R
+ -| Fabius 555-1234 fabius.undevicesimus@ucb.edu F
+ -| Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+ -| Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+ -| Jan 21 36 64 620
+ -| Apr 21 70 74 514
+
+Note how the line beginning with 'Jean-Paul' in 'mail-list' was printed
+twice, once for each rule.
+
+
+File: gawk.info, Node: More Complex, Next: Statements/Lines, Prev: Two
Rules, Up: Getting Started
+
+1.5 A More Complex Example
+==========================
+
+Now that we've mastered some simple tasks, let's look at what typical
+'awk' programs do. This example shows how 'awk' can be used to
+summarize, select, and rearrange the output of another utility. It uses
+features that haven't been covered yet, so don't worry if you don't
+understand all the details:
+
+ ls -l | awk '$6 == "Nov" { sum += $5 }
+ END { print sum }'
+
+ This command prints the total number of bytes in all the files in the
+current directory that were last modified in November (of any year).
+
+ As a reminder, the output of 'ls -l' gives you a listing of the files
+in a directory, including each file's size and the date the file was
+last modified. The first field contains read-write permissions, the
+second field contains the number of links to the file, and the third
+field identifies the file's owner. The fourth field identifies the
+file's group. The fifth field contains the file's size in bytes. The
+sixth, seventh, and eighth fields contain the month, day, and time,
+respectively, that the file was last modified. Finally, the ninth field
+contains the file name.
+
+ The '$6 == "Nov"' in our 'awk' program is an expression that tests
+whether the sixth field of the output from 'ls -l' matches the string
+'Nov'. Each time a line has the string 'Nov' for its sixth field, 'awk'
+performs the action 'sum += $5'. This adds the fifth field (the file's
+size) to the variable 'sum'. As a result, when 'awk' has finished
+reading all the input lines, 'sum' is the total of the sizes of the
+files whose lines matched the pattern. (This works because 'awk'
+variables are automatically initialized to zero.)
+
+ After the last line of output from 'ls' has been processed, the 'END'
+rule executes and prints the value of 'sum'. In this example, the value
+of 'sum' is 80600.
+
+ These more advanced 'awk' techniques are covered in later minor nodes
+(*note Action Overview::). Before you can move on to more advanced
+'awk' programming, you have to know how 'awk' interprets your input and
+displays your output. By manipulating fields and using 'print'
+statements, you can produce some very useful and impressive-looking
+reports.
+
+
+File: gawk.info, Node: Statements/Lines, Next: Other Features, Prev: More
Complex, Up: Getting Started
+
+1.6 'awk' Statements Versus Lines
+=================================
+
+Most often, each line in an 'awk' program is a separate statement or
+separate rule, like this:
+
+ awk '/12/ { print $0 }
+ /21/ { print $0 }' mail-list inventory-shipped
+
+ However, 'gawk' ignores newlines after any of the following symbols
+and keywords:
+
+ , { ? : || && do else
+
+A newline at any other point is considered the end of the statement.(1)
+
+ If you would like to split a single statement into two lines at a
+point where a newline would terminate it, you can "continue" it by
+ending the first line with a backslash character ('\'). The backslash
+must be the final character on the line in order to be recognized as a
+continuation character. A backslash followed by a newline is allowed
+anywhere in the statement, even in the middle of a string or regular
+expression. For example:
+
+ awk '/This regular expression is too long, so continue it\
+ on the next line/ { print $1 }'
+
+We have generally not used backslash continuation in our sample
+programs. 'gawk' places no limit on the length of a line, so backslash
+continuation is never strictly necessary; it just makes programs more
+readable. For this same reason, as well as for clarity, we have kept
+most statements short in the programs presented throughout the Info
+file.
+
+ Backslash continuation is most useful when your 'awk' program is in a
+separate source file instead of entered from the command line. You
+should also note that many 'awk' implementations are more particular
+about where you may use backslash continuation. For example, they may
+not allow you to split a string constant using backslash continuation.
+Thus, for maximum portability of your 'awk' programs, it is best not to
+split your lines in the middle of a regular expression or a string.
+
+ CAUTION: _Backslash continuation does not work as described with
+ the C shell._ It works for 'awk' programs in files and for
+ one-shot programs, _provided_ you are using a POSIX-compliant
+ shell, such as the Unix Bourne shell or Bash. But the C shell
+ behaves differently! There you must use two backslashes in a row,
+ followed by a newline. Note also that when using the C shell,
+ _every_ newline in your 'awk' program must be escaped with a
+ backslash. To illustrate:
+
+ % awk 'BEGIN { \
+ ? print \\
+ ? "hello, world" \
+ ? }'
+ -| hello, world
+
+ Here, the '%' and '?' are the C shell's primary and secondary
+ prompts, analogous to the standard shell's '$' and '>'.
+
+ Compare the previous example to how it is done with a
+ POSIX-compliant shell:
+
+ $ awk 'BEGIN {
+ > print \
+ > "hello, world"
+ > }'
+ -| hello, world
+
+ 'awk' is a line-oriented language. Each rule's action has to begin
+on the same line as the pattern. To have the pattern and action on
+separate lines, you _must_ use backslash continuation; there is no other
+option.
+
+ Another thing to keep in mind is that backslash continuation and
+comments do not mix. As soon as 'awk' sees the '#' that starts a
+comment, it ignores _everything_ on the rest of the line. For example:
+
+ $ gawk 'BEGIN { print "dont panic" # a friendly \
+ > BEGIN rule
+ > }'
+ error-> gawk: cmd. line:2: BEGIN rule
+ error-> gawk: cmd. line:2: ^ syntax error
+
+In this case, it looks like the backslash would continue the comment
+onto the next line. However, the backslash-newline combination is never
+even noticed because it is "hidden" inside the comment. Thus, the
+'BEGIN' is noted as a syntax error.
+
+ When 'awk' statements within one rule are short, you might want to
+put more than one of them on a line. This is accomplished by separating
+the statements with a semicolon (';'). This also applies to the rules
+themselves. Thus, the program shown at the start of this minor node
+could also be written this way:
+
+ /12/ { print $0 } ; /21/ { print $0 }
+
+ NOTE: The requirement that states that rules on the same line must
+ be separated with a semicolon was not in the original 'awk'
+ language; it was added for consistency with the treatment of
+ statements within an action.
+
+ ---------- Footnotes ----------
+
+ (1) The '?' and ':' referred to here is the three-operand conditional
+expression described in *note Conditional Exp::. Splitting lines after
+'?' and ':' is a minor 'gawk' extension; if '--posix' is specified
+(*note Options::), then this extension is disabled.
+
+
+File: gawk.info, Node: Other Features, Next: When, Prev: Statements/Lines,
Up: Getting Started
+
+1.7 Other Features of 'awk'
+===========================
+
+The 'awk' language provides a number of predefined, or "built-in",
+variables that your programs can use to get information from 'awk'.
+There are other variables your program can set as well to control how
+'awk' processes your data.
+
+ In addition, 'awk' provides a number of built-in functions for doing
+common computational and string-related operations. 'gawk' provides
+built-in functions for working with timestamps, performing bit
+manipulation, for runtime string translation (internationalization),
+determining the type of a variable, and array sorting.
+
+ As we develop our presentation of the 'awk' language, we will
+introduce most of the variables and many of the functions. They are
+described systematically in *note Built-in Variables:: and in *note
+Built-in::.
+
+
+File: gawk.info, Node: When, Next: Intro Summary, Prev: Other Features,
Up: Getting Started
+
+1.8 When to Use 'awk'
+=====================
+
+Now that you've seen some of what 'awk' can do, you might wonder how
+'awk' could be useful for you. By using utility programs, advanced
+patterns, field separators, arithmetic statements, and other selection
+criteria, you can produce much more complex output. The 'awk' language
+is very useful for producing reports from large amounts of raw data,
+such as summarizing information from the output of other utility
+programs like 'ls'. (*Note More Complex::.)
+
+ Programs written with 'awk' are usually much smaller than they would
+be in other languages. This makes 'awk' programs easy to compose and
+use. Often, 'awk' programs can be quickly composed at your keyboard,
+used once, and thrown away. Because 'awk' programs are interpreted, you
+can avoid the (usually lengthy) compilation part of the typical
+edit-compile-test-debug cycle of software development.
+
+ Complex programs have been written in 'awk', including a complete
+retargetable assembler for eight-bit microprocessors (*note Glossary::,
+for more information), and a microcode assembler for a special-purpose
+Prolog computer. The original 'awk''s capabilities were strained by
+tasks of such complexity, but modern versions are more capable.
+
+ If you find yourself writing 'awk' scripts of more than, say, a few
+hundred lines, you might consider using a different programming
+language. The shell is good at string and pattern matching; in
+addition, it allows powerful use of the system utilities. Python offers
+a nice balance between high-level ease of programming and access to
+system facilities.(1)
+
+ ---------- Footnotes ----------
+
+ (1) Other popular scripting languages include Ruby and Perl.
+
+
+File: gawk.info, Node: Intro Summary, Prev: When, Up: Getting Started
+
+1.9 Summary
+===========
+
+ * Programs in 'awk' consist of PATTERN-ACTION pairs.
+
+ * An ACTION without a PATTERN always runs. The default ACTION for a
+ pattern without one is '{ print $0 }'.
+
+ * Use either 'awk 'PROGRAM' FILES' or 'awk -f PROGRAM-FILE FILES' to
+ run 'awk'.
+
+ * You may use the special '#!' header line to create 'awk' programs
+ that are directly executable.
+
+ * Comments in 'awk' programs start with '#' and continue to the end
+ of the same line.
+
+ * Be aware of quoting issues when writing 'awk' programs as part of a
+ larger shell script (or MS-Windows batch file).
+
+ * You may use backslash continuation to continue a source line.
+ Lines are automatically continued after a comma, open brace,
+ question mark, colon, '||', '&&', 'do', and 'else'.
+
+
+File: gawk.info, Node: Invoking Gawk, Next: Regexp, Prev: Getting Started,
Up: Top
+
+2 Running 'awk' and 'gawk'
+**************************
+
+This major node covers how to run 'awk', both POSIX-standard and
+'gawk'-specific command-line options, and what 'awk' and 'gawk' do with
+nonoption arguments. It then proceeds to cover how 'gawk' searches for
+source files, reading standard input along with other files, 'gawk''s
+environment variables, 'gawk''s exit status, using include files, and
+obsolete and undocumented options and/or features.
+
+ Many of the options and features described here are discussed in more
+detail later in the Info file; feel free to skip over things in this
+major node that don't interest you right now.
+
+* Menu:
+
+* Command Line:: How to run 'awk'.
+* Options:: Command-line options and their meanings.
+* Other Arguments:: Input file names and variable assignments.
+* Naming Standard Input:: How to specify standard input with other
+ files.
+* Environment Variables:: The environment variables 'gawk' uses.
+* Exit Status:: 'gawk''s exit status.
+* Include Files:: Including other files into your program.
+* Loading Shared Libraries:: Loading shared libraries into your program.
+* Obsolete:: Obsolete Options and/or features.
+* Undocumented:: Undocumented Options and Features.
+* Invoking Summary:: Invocation summary.
+
+
+File: gawk.info, Node: Command Line, Next: Options, Up: Invoking Gawk
+
+2.1 Invoking 'awk'
+==================
+
+There are two ways to run 'awk'--with an explicit program or with one or
+more program files. Here are templates for both of them; items enclosed
+in [...] in these templates are optional:
+
+ 'awk' [OPTIONS] '-f' PROGFILE ['--'] FILE ...
+ 'awk' [OPTIONS] ['--'] ''PROGRAM'' FILE ...
+
+ In addition to traditional one-letter POSIX-style options, 'gawk'
+also supports GNU long options.
+
+ It is possible to invoke 'awk' with an empty program:
+
+ awk '' datafile1 datafile2
+
+Doing so makes little sense, though; 'awk' exits silently when given an
+empty program. (d.c.) If '--lint' has been specified on the command
+line, 'gawk' issues a warning that the program is empty.
+
+
+File: gawk.info, Node: Options, Next: Other Arguments, Prev: Command Line,
Up: Invoking Gawk
+
+2.2 Command-Line Options
+========================
+
+Options begin with a dash and consist of a single character. GNU-style
+long options consist of two dashes and a keyword. The keyword can be
+abbreviated, as long as the abbreviation allows the option to be
+uniquely identified. If the option takes an argument, either the
+keyword is immediately followed by an equals sign ('=') and the
+argument's value, or the keyword and the argument's value are separated
+by whitespace (spaces or TABs). If a particular option with a value is
+given more than once, it is (usually) the last value that counts.
+
+ Each long option for 'gawk' has a corresponding POSIX-style short
+option. The long and short options are interchangeable in all contexts.
+The following list describes options mandated by the POSIX standard:
+
+'-F FS'
+'--field-separator FS'
+ Set the 'FS' variable to FS (*note Field Separators::).
+
+'-f SOURCE-FILE'
+'--file SOURCE-FILE'
+ Read the 'awk' program source from SOURCE-FILE instead of in the
+ first nonoption argument. This option may be given multiple times;
+ the 'awk' program consists of the concatenation of the contents of
+ each specified SOURCE-FILE.
+
+ Files named with '-f' are treated as if they had '@namespace "awk"'
+ at their beginning. *Note Changing The Namespace::, for more
+ information on this advanced feature.
+
+'-v VAR=VAL'
+'--assign VAR=VAL'
+ Set the variable VAR to the value VAL _before_ execution of the
+ program begins. Such variable values are available inside the
+ 'BEGIN' rule (*note Other Arguments::).
+
+ The '-v' option can only set one variable, but it can be used more
+ than once, setting another variable each time, like this: 'awk
+ -v foo=1 -v bar=2 ...'.
+
+ CAUTION: Using '-v' to set the values of the built-in
+ variables may lead to surprising results. 'awk' will reset
+ the values of those variables as it needs to, possibly
+ ignoring any initial value you may have given.
+
+'-W GAWK-OPT'
+ Provide an implementation-specific option. This is the POSIX
+ convention for providing implementation-specific options. These
+ options also have corresponding GNU-style long options. Note that
+ the long options may be abbreviated, as long as the abbreviations
+ remain unique. The full list of 'gawk'-specific options is
+ provided next.
+
+'--'
+ Signal the end of the command-line options. The following
+ arguments are not treated as options even if they begin with '-'.
+ This interpretation of '--' follows the POSIX argument parsing
+ conventions.
+
+ This is useful if you have file names that start with '-', or in
+ shell scripts, if you have file names that will be specified by the
+ user that could start with '-'. It is also useful for passing
+ options on to the 'awk' program; see *note Getopt Function::.
+
+ The following list describes 'gawk'-specific options:
+
+'-b'
+'--characters-as-bytes'
+ Cause 'gawk' to treat all input data as single-byte characters. In
+ addition, all output written with 'print' or 'printf' is treated as
+ single-byte characters.
+
+ Normally, 'gawk' follows the POSIX standard and attempts to process
+ its input data according to the current locale (*note Locales::).
+ This can often involve converting multibyte characters into wide
+ characters (internally), and can lead to problems or confusion if
+ the input data does not contain valid multibyte characters. This
+ option is an easy way to tell 'gawk', "Hands off my data!"
+
+'-c'
+'--traditional'
+ Specify "compatibility mode", in which the GNU extensions to the
+ 'awk' language are disabled, so that 'gawk' behaves just like BWK
+ 'awk'. *Note POSIX/GNU::, which summarizes the extensions. Also
+ see *note Compatibility Mode::.
+
+'-C'
+'--copyright'
+ Print the short version of the General Public License and then
+ exit.
+
+'-d'[FILE]
+'--dump-variables'['='FILE]
+ Print a sorted list of global variables, their types, and final
+ values to FILE. If no FILE is provided, print this list to a file
+ named 'awkvars.out' in the current directory. No space is allowed
+ between the '-d' and FILE, if FILE is supplied.
+
+ Having a list of all global variables is a good way to look for
+ typographical errors in your programs. You would also use this
+ option if you have a large program with a lot of functions, and you
+ want to be sure that your functions don't inadvertently use global
+ variables that you meant to be local. (This is a particularly easy
+ mistake to make with simple variable names like 'i', 'j', etc.)
+
+'-D'[FILE]
+'--debug'['='FILE]
+ Enable debugging of 'awk' programs (*note Debugging::). By
+ default, the debugger reads commands interactively from the
+ keyboard (standard input). The optional FILE argument allows you
+ to specify a file with a list of commands for the debugger to
+ execute noninteractively. No space is allowed between the '-D' and
+ FILE, if FILE is supplied.
+
+'-e' PROGRAM-TEXT
+'--source' PROGRAM-TEXT
+ Provide program source code in the PROGRAM-TEXT. This option
+ allows you to mix source code in files with source code that you
+ enter on the command line. This is particularly useful when you
+ have library functions that you want to use from your command-line
+ programs (*note AWKPATH Variable::).
+
+ Note that 'gawk' treats each string as if it ended with a newline
+ character (even if it doesn't). This makes building the total
+ program easier.
+
+ CAUTION: Prior to version 5.0, there was no requirement that
+ each PROGRAM-TEXT be a full syntactic unit. I.e., the
+ following worked:
+
+ $ gawk -e 'BEGIN { a = 5 ;' -e 'print a }'
+ -| 5
+
+ However, this is no longer true. If you have any scripts that
+ rely upon this feature, you should revise them.
+
+ This is because each PROGRAM-TEXT is treated as if it had
+ '@namespace "awk"' at its beginning. *Note Changing The
+ Namespace::, for more information.
+
+'-E' FILE
+'--exec' FILE
+ Similar to '-f', read 'awk' program text from FILE. There are two
+ differences from '-f':
+
+ * This option terminates option processing; anything else on the
+ command line is passed on directly to the 'awk' program.
+
+ * Command-line variable assignments of the form 'VAR=VALUE' are
+ disallowed.
+
+ This option is particularly necessary for World Wide Web CGI
+ applications that pass arguments through the URL; using this option
+ prevents a malicious (or other) user from passing in options,
+ assignments, or 'awk' source code (via '-e') to the CGI
+ application.(1) This option should be used with '#!' scripts
+ (*note Executable Scripts::), like so:
+
+ #! /usr/local/bin/gawk -E
+
+ AWK PROGRAM HERE ...
+
+'-g'
+'--gen-pot'
+ Analyze the source program and generate a GNU 'gettext' portable
+ object template file on standard output for all string constants
+ that have been marked for translation. *Note
+ Internationalization::, for information about this option.
+
+'-h'
+'--help'
+ Print a "usage" message summarizing the short- and long-style
+ options that 'gawk' accepts and then exit.
+
+'-i' SOURCE-FILE
+'--include' SOURCE-FILE
+ Read an 'awk' source library from SOURCE-FILE. This option is
+ completely equivalent to using the '@include' directive inside your
+ program. It is very similar to the '-f' option, but there are two
+ important differences. First, when '-i' is used, the program
+ source is not loaded if it has been previously loaded, whereas with
+ '-f', 'gawk' always loads the file. Second, because this option is
+ intended to be used with code libraries, 'gawk' does not recognize
+ such files as constituting main program input. Thus, after
+ processing an '-i' argument, 'gawk' still expects to find the main
+ source code via the '-f' option or on the command line.
+
+ Files named with '-i' are treated as if they had '@namespace "awk"'
+ at their beginning. *Note Changing The Namespace::, for more
+ information.
+
+'-I'
+'--trace'
+ Print the internal byte code names as they are executed when
+ running the program. The trace is printed to standard error. Each
+ "op code" is preceded by a '+' sign in the output.
+
+'-l' EXT
+'--load' EXT
+ Load a dynamic extension named EXT. Extensions are stored as
+ system shared libraries. This option searches for the library
+ using the 'AWKLIBPATH' environment variable. The correct library
+ suffix for your platform will be supplied by default, so it need
+ not be specified in the extension name. The extension
+ initialization routine should be named 'dl_load()'. An alternative
+ is to use the '@load' keyword inside the program to load a shared
+ library. This advanced feature is described in detail in *note
+ Dynamic Extensions::.
+
+'-L'[VALUE]
+'--lint'['='VALUE]
+ Warn about constructs that are dubious or nonportable to other
+ 'awk' implementations. No space is allowed between the '-L' and
+ VALUE, if VALUE is supplied. Some warnings are issued when 'gawk'
+ first reads your program. Others are issued at runtime, as your
+ program executes. The optional argument may be one of the
+ following:
+
+ 'fatal'
+ Cause lint warnings become fatal errors. This may be drastic,
+ but its use will certainly encourage the development of
+ cleaner 'awk' programs.
+
+ 'invalid'
+ Only issue warnings about things that are actually invalid are
+ issued. (This is not fully implemented yet.)
+
+ 'no-ext'
+ Disable warnings about 'gawk' extensions.
+
+ Some warnings are only printed once, even if the dubious constructs
+ they warn about occur multiple times in your 'awk' program. Thus,
+ when eliminating problems pointed out by '--lint', you should take
+ care to search for all occurrences of each inappropriate construct.
+ As 'awk' programs are usually short, doing so is not burdensome.
+
+'-M'
+'--bignum'
+ Select arbitrary-precision arithmetic on numbers. This option has
+ no effect if 'gawk' is not compiled to use the GNU MPFR and MP
+ libraries (*note Arbitrary Precision Arithmetic::).
+
+'-n'
+'--non-decimal-data'
+ Enable automatic interpretation of octal and hexadecimal values in
+ input data (*note Nondecimal Data::).
+
+ CAUTION: This option can severely break old programs. Use
+ with care. Also note that this option may disappear in a
+ future version of 'gawk'.
+
+'-N'
+'--use-lc-numeric'
+ Force the use of the locale's decimal point character when parsing
+ numeric input data (*note Locales::).
+
+'-o'[FILE]
+'--pretty-print'['='FILE]
+ Enable pretty-printing of 'awk' programs. Implies '--no-optimize'.
+ By default, the output program is created in a file named
+ 'awkprof.out' (*note Profiling::). The optional FILE argument
+ allows you to specify a different file name for the output. No
+ space is allowed between the '-o' and FILE, if FILE is supplied.
+
+ NOTE: In the past, this option would also execute your
+ program. This is no longer the case.
+
+'-O'
+'--optimize'
+ Enable 'gawk''s default optimizations on the internal
+ representation of the program. At the moment, this includes just
+ simple constant folding.
+
+ Optimization is enabled by default. This option remains primarily
+ for backwards compatibility. However, it may be used to cancel the
+ effect of an earlier '-s' option (see later in this list).
+
+'-p'[FILE]
+'--profile'['='FILE]
+ Enable profiling of 'awk' programs (*note Profiling::). Implies
+ '--no-optimize'. By default, profiles are created in a file named
+ 'awkprof.out'. The optional FILE argument allows you to specify a
+ different file name for the profile file. No space is allowed
+ between the '-p' and FILE, if FILE is supplied.
+
+ The profile contains execution counts for each statement in the
+ program in the left margin, and function call counts for each
+ function.
+
+'-P'
+'--posix'
+ Operate in strict POSIX mode. This disables all 'gawk' extensions
+ (just like '--traditional') and disables all extensions not allowed
+ by POSIX. *Note Common Extensions:: for a summary of the extensions
+ in 'gawk' that are disabled by this option. Also, the following
+ additional restrictions apply:
+
+ * Newlines are not allowed after '?' or ':' (*note Conditional
+ Exp::).
+
+ * Specifying '-Ft' on the command line does not set the value of
+ 'FS' to be a single TAB character (*note Field Separators::).
+
+ * The locale's decimal point character is used for parsing input
+ data (*note Locales::).
+
+ If you supply both '--traditional' and '--posix' on the command
+ line, '--posix' takes precedence. 'gawk' issues a warning if both
+ options are supplied.
+
+'-r'
+'--re-interval'
+ Allow interval expressions (*note Regexp Operators::) in regexps.
+ This is now 'gawk''s default behavior. Nevertheless, this option
+ remains (both for backward compatibility and for use in combination
+ with '--traditional').
+
+'-s'
+'--no-optimize'
+ Disable 'gawk''s default optimizations on the internal
+ representation of the program.
+
+'-S'
+'--sandbox'
+ Disable the 'system()' function, input redirections with 'getline',
+ output redirections with 'print' and 'printf', and dynamic
+ extensions. Also, disallow adding file names to 'ARGV' that were
+ not there when 'gawk' started running. This is particularly useful
+ when you want to run 'awk' scripts from questionable sources and
+ need to make sure the scripts can't access your system (other than
+ the specified input data files).
+
+'-t'
+'--lint-old'
+ Warn about constructs that are not available in the original
+ version of 'awk' from Version 7 Unix (*note V7/SVR3.1::).
+
+'-V'
+'--version'
+ Print version information for this particular copy of 'gawk'. This
+ allows you to determine if your copy of 'gawk' is up to date with
+ respect to whatever the Free Software Foundation is currently
+ distributing. It is also useful for bug reports (*note Bugs::).
+
+'--'
+ Mark the end of all options. Any command-line arguments following
+ '--' are placed in 'ARGV', even if they start with a minus sign.
+
+ In compatibility mode, as long as program text has been supplied, any
+other options are flagged as invalid with a warning message but are
+otherwise ignored.
+
+ In compatibility mode, as a special case, if the value of FS supplied
+to the '-F' option is 't', then 'FS' is set to the TAB character
+('"\t"'). This is true only for '--traditional' and not for '--posix'
+(*note Field Separators::).
+
+ The '-f' option may be used more than once on the command line. If
+it is, 'awk' reads its program source from all of the named files, as if
+they had been concatenated together into one big file. This is useful
+for creating libraries of 'awk' functions. These functions can be
+written once and then retrieved from a standard place, instead of having
+to be included in each individual program. The '-i' option is similar
+in this regard. (As mentioned in *note Definition Syntax::, function
+names must be unique.)
+
+ With standard 'awk', library functions can still be used, even if the
+program is entered at the keyboard, by specifying '-f /dev/tty'. After
+typing your program, type 'Ctrl-d' (the end-of-file character) to
+terminate it. (You may also use '-f -' to read program source from the
+standard input, but then you will not be able to also use the standard
+input as a source of data.)
+
+ Because it is clumsy using the standard 'awk' mechanisms to mix
+source file and command-line 'awk' programs, 'gawk' provides the '-e'
+option. This does not require you to preempt the standard input for
+your source code, and it allows you to easily mix command-line and
+library source code (*note AWKPATH Variable::). As with '-f', the '-e'
+and '-i' options may also be used multiple times on the command line.
+
+ If no '-f' option (or '-e' option for 'gawk') is specified, then
+'awk' uses the first nonoption command-line argument as the text of the
+program source code. Arguments on the command line that follow the
+program text are entered into the 'ARGV' array; 'awk' does _not_
+continue to parse the command line looking for options.
+
+ If the environment variable 'POSIXLY_CORRECT' exists, then 'gawk'
+behaves in strict POSIX mode, exactly as if you had supplied '--posix'.
+Many GNU programs look for this environment variable to suppress
+extensions that conflict with POSIX, but 'gawk' behaves differently: it
+suppresses all extensions, even those that do not conflict with POSIX,
+and behaves in strict POSIX mode. If '--lint' is supplied on the
+command line and 'gawk' turns on POSIX mode because of
+'POSIXLY_CORRECT', then it issues a warning message indicating that
+POSIX mode is in effect. You would typically set this variable in your
+shell's startup file. For a Bourne-compatible shell (such as Bash), you
+would add these lines to the '.profile' file in your home directory:
+
+ POSIXLY_CORRECT=true
+ export POSIXLY_CORRECT
+
+ For a C shell-compatible shell,(2) you would add this line to the
+'.login' file in your home directory:
+
+ setenv POSIXLY_CORRECT true
+
+ Having 'POSIXLY_CORRECT' set is not recommended for daily use, but it
+is good for testing the portability of your programs to other
+environments.
+
+ ---------- Footnotes ----------
+
+ (1) For more detail, please see Section 4.4 of RFC 3875
+(http://www.ietf.org/rfc/rfc3875). Also see the explanatory note sent
+to the 'gawk' bug mailing list
+(https://lists.gnu.org/archive/html/bug-gawk/2014-11/msg00022.html).
+
+ (2) Not recommended.
+
+
+File: gawk.info, Node: Other Arguments, Next: Naming Standard Input, Prev:
Options, Up: Invoking Gawk
+
+2.3 Other Command-Line Arguments
+================================
+
+Any additional arguments on the command line are normally treated as
+input files to be processed in the order specified. However, an
+argument that has the form 'VAR=VALUE', assigns the value VALUE to the
+variable VAR--it does not specify a file at all. (See *note Assignment
+Options::.) In the following example, 'count=1' is a variable
+assignment, not a file name:
+
+ awk -f program.awk file1 count=1 file2
+
+As a side point, should you really need to have 'awk' process a file
+named 'count=1' (or any file whose name looks like a variable
+assignment), precede the file name with './', like so:
+
+ awk -f program.awk file1 ./count=1 file2
+
+ All the command-line arguments are made available to your 'awk'
+program in the 'ARGV' array (*note Built-in Variables::). Command-line
+options and the program text (if present) are omitted from 'ARGV'. All
+other arguments, including variable assignments, are included. As each
+element of 'ARGV' is processed, 'gawk' sets 'ARGIND' to the index in
+'ARGV' of the current element. ('gawk' makes the full command line,
+including program text and options, available in 'PROCINFO["argv"]';
+*note Auto-set::.)
+
+ Changing 'ARGC' and 'ARGV' in your 'awk' program lets you control how
+'awk' processes the input files; this is described in more detail in
+*note ARGC and ARGV::.
+
+ The distinction between file name arguments and variable-assignment
+arguments is made when 'awk' is about to open the next input file. At
+that point in execution, it checks the file name to see whether it is
+really a variable assignment; if so, 'awk' sets the variable instead of
+reading a file.
+
+ Therefore, the variables actually receive the given values after all
+previously specified files have been read. In particular, the values of
+variables assigned in this fashion are _not_ available inside a 'BEGIN'
+rule (*note BEGIN/END::), because such rules are run before 'awk' begins
+scanning the argument list.
+
+ The variable values given on the command line are processed for
+escape sequences (*note Escape Sequences::). (d.c.)
+
+ In some very early implementations of 'awk', when a variable
+assignment occurred before any file names, the assignment would happen
+_before_ the 'BEGIN' rule was executed. 'awk''s behavior was thus
+inconsistent; some command-line assignments were available inside the
+'BEGIN' rule, while others were not. Unfortunately, some applications
+came to depend upon this "feature." When 'awk' was changed to be more
+consistent, the '-v' option was added to accommodate applications that
+depended upon the old behavior.
+
+ The variable assignment feature is most useful for assigning to
+variables such as 'RS', 'OFS', and 'ORS', which control input and output
+formats, before scanning the data files. It is also useful for
+controlling state if multiple passes are needed over a data file. For
+example:
+
+ awk 'pass == 1 { PASS 1 STUFF }
+ pass == 2 { PASS 2 STUFF }' pass=1 mydata pass=2 mydata
+
+ Given the variable assignment feature, the '-F' option for setting
+the value of 'FS' is not strictly necessary. It remains for historical
+compatibility.
+
+ Quoting Shell Variables On The 'awk' Command Line
+
+ Small 'awk' programs are often embedded in larger shell scripts, so
+it's worthwhile to understand some shell basics. Consider the
+following:
+
+ f=""
+ awk '{ print("hi") }' $f
+
+ In this case, 'awk' reads from standard input instead of trying to
+open any command line files. To the unwary, this looks like 'awk' is
+hanging.
+
+ However 'awk' doesn't see an explicit empty string. When a variable
+expansion is the null string, _and_ it's not quoted, the shell simply
+removes it from the command line. To demonstrate:
+
+ $ f=""
+ $ awk 'BEGIN { print ARGC }' $f
+ -| 1
+ $ awk 'BEGIN { print ARGC }' "$f"
+ -| 2
+
+
+File: gawk.info, Node: Naming Standard Input, Next: Environment Variables,
Prev: Other Arguments, Up: Invoking Gawk
+
+2.4 Naming Standard Input
+=========================
+
+Often, you may wish to read standard input together with other files.
+For example, you may wish to read one file, read standard input coming
+from a pipe, and then read another file.
+
+ The way to name the standard input, with all versions of 'awk', is to
+use a single, standalone minus sign or dash, '-'. For example:
+
+ SOME_COMMAND | awk -f myprog.awk file1 - file2
+
+Here, 'awk' first reads 'file1', then it reads the output of
+SOME_COMMAND, and finally it reads 'file2'.
+
+ You may also use '"-"' to name standard input when reading files with
+'getline' (*note Getline/File::). And, you can even use '"-"' with the
+'-f' option to read program source code from standard input (*note
+Options::).
+
+ In addition, 'gawk' allows you to specify the special file name
+'/dev/stdin', both on the command line and with 'getline'. Some other
+versions of 'awk' also support this, but it is not standard. (Some
+operating systems provide a '/dev/stdin' file in the filesystem;
+however, 'gawk' always processes this file name itself.)
+
+
+File: gawk.info, Node: Environment Variables, Next: Exit Status, Prev:
Naming Standard Input, Up: Invoking Gawk
+
+2.5 The Environment Variables 'gawk' Uses
+=========================================
+
+A number of environment variables influence how 'gawk' behaves.
+
+* Menu:
+
+* AWKPATH Variable:: Searching directories for 'awk'
+ programs.
+* AWKLIBPATH Variable:: Searching directories for 'awk' shared
+ libraries.
+* Other Environment Variables:: The environment variables.
+
+
+File: gawk.info, Node: AWKPATH Variable, Next: AWKLIBPATH Variable, Up:
Environment Variables
+
+2.5.1 The 'AWKPATH' Environment Variable
+----------------------------------------
+
+The previous minor node described how 'awk' program files can be named
+on the command line with the '-f' option. In most 'awk'
+implementations, you must supply a precise pathname for each program
+file, unless the file is in the current directory. But with 'gawk', if
+the file name supplied to the '-f' or '-i' options does not contain a
+directory separator '/', then 'gawk' searches a list of directories
+(called the "search path") one by one, looking for a file with the
+specified name.
+
+ The search path is a string consisting of directory names separated
+by colons.(1) 'gawk' gets its search path from the 'AWKPATH'
+environment variable. If that variable does not exist, or if it has an
+empty value, 'gawk' uses a default path (described shortly).
+
+ The search path feature is particularly helpful for building
+libraries of useful 'awk' functions. The library files can be placed in
+a standard directory in the default path and then specified on the
+command line with a short file name. Otherwise, you would have to type
+the full file name for each file.
+
+ By using the '-i' or '-f' options, your command-line 'awk' programs
+can use facilities in 'awk' library files (*note Library Functions::).
+Path searching is not done if 'gawk' is in compatibility mode. This is
+true for both '--traditional' and '--posix'. *Note Options::.
+
+ If the source code file is not found after the initial search, the
+path is searched again after adding the suffix '.awk' to the file name.
+
+ 'gawk''s path search mechanism is similar to the shell's. (See 'The
+Bourne-Again SHell manual' (https://www.gnu.org/software/bash/manual/).)
+It treats a null entry in the path as indicating the current directory.
+(A null entry is indicated by starting or ending the path with a colon
+or by placing two colons next to each other ['::'].)
+
+ NOTE: To include the current directory in the path, either place
+ '.' as an entry in the path or write a null entry in the path.
+
+ Different past versions of 'gawk' would also look explicitly in the
+ current directory, either before or after the path search. As of
+ version 4.1.2, this no longer happens; if you wish to look in the
+ current directory, you must include '.' either as a separate entry
+ or as a null entry in the search path.
+
+ The default value for 'AWKPATH' is '.:/usr/local/share/awk'.(2)
+Since '.' is included at the beginning, 'gawk' searches first in the
+current directory and then in '/usr/local/share/awk'. In practice, this
+means that you will rarely need to change the value of 'AWKPATH'.
+
+ *Note Shell Startup Files::, for information on functions that help
+to manipulate the 'AWKPATH' variable.
+
+ 'gawk' places the value of the search path that it used into
+'ENVIRON["AWKPATH"]'. This provides access to the actual search path
+value from within an 'awk' program.
+
+ Although you can change 'ENVIRON["AWKPATH"]' within your 'awk'
+program, this has no effect on the running program's behavior. This
+makes sense: the 'AWKPATH' environment variable is used to find the
+program source files. Once your program is running, all the files have
+been found, and 'gawk' no longer needs to use 'AWKPATH'.
+
+ ---------- Footnotes ----------
+
+ (1) Semicolons on MS-Windows.
+
+ (2) Your version of 'gawk' may use a different directory; it will
+depend upon how 'gawk' was built and installed. The actual directory is
+the value of '$(pkgdatadir)' generated when 'gawk' was configured. (For
+more detail, see the 'INSTALL' file in the source distribution, and see
+*note Quick Installation::. You probably don't need to worry about
+this, though.)
+
+
+File: gawk.info, Node: AWKLIBPATH Variable, Next: Other Environment
Variables, Prev: AWKPATH Variable, Up: Environment Variables
+
+2.5.2 The 'AWKLIBPATH' Environment Variable
+-------------------------------------------
+
+The 'AWKLIBPATH' environment variable is similar to the 'AWKPATH'
+variable, but it is used to search for loadable extensions (stored as
+system shared libraries) specified with the '-l' option rather than for
+source files. If the extension is not found, the path is searched again
+after adding the appropriate shared library suffix for the platform.
+For example, on GNU/Linux systems, the suffix '.so' is used. The search
+path specified is also used for extensions loaded via the '@load'
+keyword (*note Loading Shared Libraries::).
+
+ If 'AWKLIBPATH' does not exist in the environment, or if it has an
+empty value, 'gawk' uses a default path; this is typically
+'/usr/local/lib/gawk', although it can vary depending upon how 'gawk'
+was built.(1)
+
+ *Note Shell Startup Files::, for information on functions that help
+to manipulate the 'AWKLIBPATH' variable.
+
+ 'gawk' places the value of the search path that it used into
+'ENVIRON["AWKLIBPATH"]'. This provides access to the actual search path
+value from within an 'awk' program.
+
+ Although you can change 'ENVIRON["AWKLIBPATH"]' within your 'awk'
+program, this has no effect on the running program's behavior. This
+makes sense: the 'AWKLIBPATH' environment variable is used to find any
+requested extensions, and they are loaded before the program starts to
+run. Once your program is running, all the extensions have been found,
+and 'gawk' no longer needs to use 'AWKLIBPATH'.
+
+ ---------- Footnotes ----------
+
+ (1) Your version of 'gawk' may use a different directory; it will
+depend upon how 'gawk' was built and installed. The actual directory is
+the value of '$(pkgextensiondir)' generated when 'gawk' was configured.
+(For more detail, see the 'INSTALL' file in the source distribution, and
+see *note Quick Installation::. You probably don't need to worry about
+this, though.)
+
+
+File: gawk.info, Node: Other Environment Variables, Prev: AWKLIBPATH
Variable, Up: Environment Variables
+
+2.5.3 Other Environment Variables
+---------------------------------
+
+A number of other environment variables affect 'gawk''s behavior, but
+they are more specialized. Those in the following list are meant to be
+used by regular users:
+
+'GAWK_MSEC_SLEEP'
+ Specifies the interval between connection retries, in milliseconds.
+ On systems that do not support the 'usleep()' system call, the
+ value is rounded up to an integral number of seconds.
+
+'GAWK_READ_TIMEOUT'
+ Specifies the time, in milliseconds, for 'gawk' to wait for input
+ before returning with an error. *Note Read Timeout::.
+
+'GAWK_SOCK_RETRIES'
+ Controls the number of times 'gawk' attempts to retry a two-way
+ TCP/IP (socket) connection before giving up. *Note TCP/IP
+ Networking::. Note that when nonfatal I/O is enabled (*note
+ Nonfatal::), 'gawk' only tries to open a TCP/IP socket once.
+
+'POSIXLY_CORRECT'
+ Causes 'gawk' to switch to POSIX-compatibility mode, disabling all
+ traditional and GNU extensions. *Note Options::.
+
+ The environment variables in the following list are meant for use by
+the 'gawk' developers for testing and tuning. They are subject to
+change. The variables are:
+
+'AWKBUFSIZE'
+ This variable only affects 'gawk' on POSIX-compliant systems. With
+ a value of 'exact', 'gawk' uses the size of each input file as the
+ size of the memory buffer to allocate for I/O. Otherwise, the value
+ should be a number, and 'gawk' uses that number as the size of the
+ buffer to allocate. (When this variable is not set, 'gawk' uses
+ the smaller of the file's size and the "default" blocksize, which
+ is usually the filesystem's I/O blocksize.)
+
+'AWK_HASH'
+ If this variable exists with a value of 'gst', 'gawk' switches to
+ using the hash function from GNU Smalltalk for managing arrays.
+ With a value of 'fnv1a', 'gawk' uses the FNV1-A hash function
+ (http://www.isthe.com/chongo/tech/comp/fnv/index.html). These
+ functions may be marginally faster than the standard function.
+
+'AWKREADFUNC'
+ If this variable exists, 'gawk' switches to reading source files
+ one line at a time, instead of reading in blocks. This exists for
+ debugging problems on filesystems on non-POSIX operating systems
+ where I/O is performed in records, not in blocks.
+
+'GAWK_MSG_SRC'
+ If this variable exists, 'gawk' includes the file name and line
+ number within the 'gawk' source code from which warning and/or
+ fatal messages are generated. Its purpose is to help isolate the
+ source of a message, as there are multiple places that produce the
+ same warning or error message.
+
+'GAWK_LOCALE_DIR'
+ Specifies the location of compiled message object files for 'gawk'
+ itself. This is passed to the 'bindtextdomain()' function when
+ 'gawk' starts up.
+
+'GAWK_NO_DFA'
+ If this variable exists, 'gawk' does not use the DFA regexp matcher
+ for "does it match" kinds of tests. This can cause 'gawk' to be
+ slower. Its purpose is to help isolate differences between the two
+ regexp matchers that 'gawk' uses internally. (There aren't
+ supposed to be differences, but occasionally theory and practice
+ don't coordinate with each other.)
+
+'GAWK_STACKSIZE'
+ This specifies the amount by which 'gawk' should grow its internal
+ evaluation stack, when needed.
+
+'INT_CHAIN_MAX'
+ This specifies intended maximum number of items 'gawk' will
+ maintain on a hash chain for managing arrays indexed by integers.
+
+'STR_CHAIN_MAX'
+ This specifies intended maximum number of items 'gawk' will
+ maintain on a hash chain for managing arrays indexed by strings.
+
+'TIDYMEM'
+ If this variable exists, 'gawk' uses the 'mtrace()' library calls
+ from the GNU C library to help track down possible memory leaks.
+
+
+File: gawk.info, Node: Exit Status, Next: Include Files, Prev: Environment
Variables, Up: Invoking Gawk
+
+2.6 'gawk''s Exit Status
+========================
+
+If the 'exit' statement is used with a value (*note Exit Statement::),
+then 'gawk' exits with the numeric value given to it.
+
+ Otherwise, if there were no problems during execution, 'gawk' exits
+with the value of the C constant 'EXIT_SUCCESS'. This is usually zero.
+
+ If an error occurs, 'gawk' exits with the value of the C constant
+'EXIT_FAILURE'. This is usually one.
+
+ If 'gawk' exits because of a fatal error, the exit status is two. On
+non-POSIX systems, this value may be mapped to 'EXIT_FAILURE'.
+
+
+File: gawk.info, Node: Include Files, Next: Loading Shared Libraries, Prev:
Exit Status, Up: Invoking Gawk
+
+2.7 Including Other Files into Your Program
+===========================================
+
+This minor node describes a feature that is specific to 'gawk'.
+
+ The '@include' keyword can be used to read external 'awk' source
+files. This gives you the ability to split large 'awk' source files
+into smaller, more manageable pieces, and also lets you reuse common
+'awk' code from various 'awk' scripts. In other words, you can group
+together 'awk' functions used to carry out specific tasks into external
+files. These files can be used just like function libraries, using the
+'@include' keyword in conjunction with the 'AWKPATH' environment
+variable. Note that source files may also be included using the '-i'
+option.
+
+ Let's see an example. We'll start with two (trivial) 'awk' scripts,
+namely 'test1' and 'test2'. Here is the 'test1' script:
+
+ BEGIN {
+ print "This is script test1."
+ }
+
+and here is 'test2':
+
+ @include "test1"
+ BEGIN {
+ print "This is script test2."
+ }
+
+ Running 'gawk' with 'test2' produces the following result:
+
+ $ gawk -f test2
+ -| This is script test1.
+ -| This is script test2.
+
+ 'gawk' runs the 'test2' script, which includes 'test1' using the
+'@include' keyword. So, to include external 'awk' source files, you
+just use '@include' followed by the name of the file to be included,
+enclosed in double quotes.
+
+ NOTE: Keep in mind that this is a language construct and the file
+ name cannot be a string variable, but rather just a literal string
+ constant in double quotes.
+
+ The files to be included may be nested; e.g., given a third script,
+namely 'test3':
+
+ @include "test2"
+ BEGIN {
+ print "This is script test3."
+ }
+
+Running 'gawk' with the 'test3' script produces the following results:
+
+ $ gawk -f test3
+ -| This is script test1.
+ -| This is script test2.
+ -| This is script test3.
+
+ The file name can, of course, be a pathname. For example:
+
+ @include "../io_funcs"
+
+and:
+
+ @include "/usr/awklib/network"
+
+are both valid. The 'AWKPATH' environment variable can be of great
+value when using '@include'. The same rules for the use of the
+'AWKPATH' variable in command-line file searches (*note AWKPATH
+Variable::) apply to '@include' also.
+
+ This is very helpful in constructing 'gawk' function libraries. If
+you have a large script with useful, general-purpose 'awk' functions,
+you can break it down into library files and put those files in a
+special directory. You can then include those "libraries," either by
+using the full pathnames of the files, or by setting the 'AWKPATH'
+environment variable accordingly and then using '@include' with just the
+file part of the full pathname. Of course, you can keep library files
+in more than one directory; the more complex the working environment is,
+the more directories you may need to organize the files to be included.
+
+ Given the ability to specify multiple '-f' options, the '@include'
+mechanism is not strictly necessary. However, the '@include' keyword
+can help you in constructing self-contained 'gawk' programs, thus
+reducing the need for writing complex and tedious command lines. In
+particular, '@include' is very useful for writing CGI scripts to be run
+from web pages.
+
+ The rules for finding a source file described in *note AWKPATH
+Variable:: also apply to files loaded with '@include'.
+
+ Finally, files included with '@include' are treated as if they had
+'@namespace "awk"' at their beginning. *Note Changing The Namespace::,
+for more information.
+
+
+File: gawk.info, Node: Loading Shared Libraries, Next: Obsolete, Prev:
Include Files, Up: Invoking Gawk
+
+2.8 Loading Dynamic Extensions into Your Program
+================================================
+
+This minor node describes a feature that is specific to 'gawk'.
+
+ The '@load' keyword can be used to read external 'awk' extensions
+(stored as system shared libraries). This allows you to link in
+compiled code that may offer superior performance and/or give you access
+to extended capabilities not supported by the 'awk' language. The
+'AWKLIBPATH' variable is used to search for the extension. Using
+'@load' is completely equivalent to using the '-l' command-line option.
+
+ If the extension is not initially found in 'AWKLIBPATH', another
+search is conducted after appending the platform's default shared
+library suffix to the file name. For example, on GNU/Linux systems, the
+suffix '.so' is used:
+
+ $ gawk '@load "ordchr"; BEGIN {print chr(65)}'
+ -| A
+
+This is equivalent to the following example:
+
+ $ gawk -lordchr 'BEGIN {print chr(65)}'
+ -| A
+
+For command-line usage, the '-l' option is more convenient, but '@load'
+is useful for embedding inside an 'awk' source file that requires access
+to an extension.
+
+ *note Dynamic Extensions::, describes how to write extensions (in C
+or C++) that can be loaded with either '@load' or the '-l' option. It
+also describes the 'ordchr' extension.
+
+
+File: gawk.info, Node: Obsolete, Next: Undocumented, Prev: Loading Shared
Libraries, Up: Invoking Gawk
+
+2.9 Obsolete Options and/or Features
+====================================
+
+This minor node describes features and/or command-line options from
+previous releases of 'gawk' that either are not available in the current
+version or are still supported but deprecated (meaning that they will
+_not_ be in the next release).
+
+ The process-related special files '/dev/pid', '/dev/ppid',
+'/dev/pgrpid', and '/dev/user' were deprecated in 'gawk' 3.1, but still
+worked. As of version 4.0, they are no longer interpreted specially by
+'gawk'. (Use 'PROCINFO' instead; see *note Auto-set::.)
+
+
+File: gawk.info, Node: Undocumented, Next: Invoking Summary, Prev:
Obsolete, Up: Invoking Gawk
+
+2.10 Undocumented Options and Features
+======================================
+
+ Use the Source, Luke!
+ -- _Obi-Wan_
+
+ This minor node intentionally left blank.
+
+
+File: gawk.info, Node: Invoking Summary, Prev: Undocumented, Up: Invoking
Gawk
+
+2.11 Summary
+============
+
+ * 'gawk' parses arguments on the command line, left to right, to
+ determine if they should be treated as options or as non-option
+ arguments.
+
+ * 'gawk' recognizes several options which control its operation, as
+ described in *note Options::. All options begin with '-'.
+
+ * Any argument that is not recognized as an option is treated as a
+ non-option argument, even if it begins with '-'.
+
+ - However, when an option itself requires an argument, and the
+ option is separated from that argument on the command line by
+ at least one space, the space is ignored, and the argument is
+ considered to be related to the option. Thus, in the
+ invocation, 'gawk -F x', the 'x' is treated as belonging to
+ the '-F' option, not as a separate non-option argument.
+
+ * Once 'gawk' finds a non-option argument, it stops looking for
+ options. Therefore, all following arguments are also non-option
+ arguments, even if they resemble recognized options.
+
+ * If no '-e' or '-f' options are present, 'gawk' expects the program
+ text to be in the first non-option argument.
+
+ * All non-option arguments, except program text provided in the first
+ non-option argument, are placed in 'ARGV' as explained in *note
+ ARGC and ARGV::, and are processed as described in *note Other
+ Arguments::. Adjusting 'ARGC' and 'ARGV' affects how 'awk'
+ processes input.
+
+ * The three standard options for all versions of 'awk' are '-f',
+ '-F', and '-v'. 'gawk' supplies these and many others, as well as
+ corresponding GNU-style long options.
+
+ * Nonoption command-line arguments are usually treated as file names,
+ unless they have the form 'VAR=VALUE', in which case they are taken
+ as variable assignments to be performed at that point in processing
+ the input.
+
+ * You can use a single minus sign ('-') to refer to standard input on
+ the command line. 'gawk' also lets you use the special file name
+ '/dev/stdin'.
+
+ * 'gawk' pays attention to a number of environment variables.
+ 'AWKPATH', 'AWKLIBPATH', and 'POSIXLY_CORRECT' are the most
+ important ones.
+
+ * 'gawk''s exit status conveys information to the program that
+ invoked it. Use the 'exit' statement from within an 'awk' program
+ to set the exit status.
+
+ * 'gawk' allows you to include other 'awk' source files into your
+ program using the '@include' statement and/or the '-i' and '-f'
+ command-line options.
+
+ * 'gawk' allows you to load additional functions written in C or C++
+ using the '@load' statement and/or the '-l' option. (This advanced
+ feature is described later, in *note Dynamic Extensions::.)
+
+
+File: gawk.info, Node: Regexp, Next: Reading Files, Prev: Invoking Gawk,
Up: Top
+
+3 Regular Expressions
+*********************
+
+A "regular expression", or "regexp", is a way of describing a set of
+strings. Because regular expressions are such a fundamental part of
+'awk' programming, their format and use deserve a separate major node.
+
+ A regular expression enclosed in slashes ('/') is an 'awk' pattern
+that matches every input record whose text belongs to that set. The
+simplest regular expression is a sequence of letters, numbers, or both.
+Such a regexp matches any string that contains that sequence. Thus, the
+regexp 'foo' matches any string containing 'foo'. Thus, the pattern
+'/foo/' matches any input record containing the three adjacent
+characters 'foo' _anywhere_ in the record. Other kinds of regexps let
+you specify more complicated classes of strings.
+
+* Menu:
+
+* Regexp Usage:: How to Use Regular Expressions.
+* Escape Sequences:: How to write nonprinting characters.
+* Regexp Operators:: Regular Expression Operators.
+* Bracket Expressions:: What can go between '[...]'.
+* Leftmost Longest:: How much text matches.
+* Computed Regexps:: Using Dynamic Regexps.
+* GNU Regexp Operators:: Operators specific to GNU software.
+* Case-sensitivity:: How to do case-insensitive matching.
+* Regexp Summary:: Regular expressions summary.
+
+
+File: gawk.info, Node: Regexp Usage, Next: Escape Sequences, Up: Regexp
+
+3.1 How to Use Regular Expressions
+==================================
+
+A regular expression can be used as a pattern by enclosing it in
+slashes. Then the regular expression is tested against the entire text
+of each record. (Normally, it only needs to match some part of the text
+in order to succeed.) For example, the following prints the second
+field of each record where the string 'li' appears anywhere in the
+record:
+
+ $ awk '/li/ { print $2 }' mail-list
+ -| 555-5553
+ -| 555-0542
+ -| 555-6699
+ -| 555-3430
+
+ Regular expressions can also be used in matching expressions. These
+expressions allow you to specify the string to match against; it need
+not be the entire current input record. The two operators '~' and '!~'
+perform regular expression comparisons. Expressions using these
+operators can be used as patterns, or in 'if', 'while', 'for', and 'do'
+statements. (*Note Statements::.) For example, the following is true
+if the expression EXP (taken as a string) matches REGEXP:
+
+ EXP ~ /REGEXP/
+
+This example matches, or selects, all input records with the uppercase
+letter 'J' somewhere in the first field:
+
+ $ awk '$1 ~ /J/' inventory-shipped
+ -| Jan 13 25 15 115
+ -| Jun 31 42 75 492
+ -| Jul 24 34 67 436
+ -| Jan 21 36 64 620
+
+ So does this:
+
+ awk '{ if ($1 ~ /J/) print }' inventory-shipped
+
+ This next example is true if the expression EXP (taken as a character
+string) does _not_ match REGEXP:
+
+ EXP !~ /REGEXP/
+
+ The following example matches, or selects, all input records whose
+first field _does not_ contain the uppercase letter 'J':
+
+ $ awk '$1 !~ /J/' inventory-shipped
+ -| Feb 15 32 24 226
+ -| Mar 15 24 34 228
+ -| Apr 31 52 63 420
+ -| May 16 34 29 208
+ ...
+
+ When a regexp is enclosed in slashes, such as '/foo/', we call it a
+"regexp constant", much like '5.27' is a numeric constant and '"foo"' is
+a string constant.
+
+
+File: gawk.info, Node: Escape Sequences, Next: Regexp Operators, Prev:
Regexp Usage, Up: Regexp
+
+3.2 Escape Sequences
+====================
+
+Some characters cannot be included literally in string constants
+('"foo"') or regexp constants ('/foo/'). Instead, they should be
+represented with "escape sequences", which are character sequences
+beginning with a backslash ('\'). One use of an escape sequence is to
+include a double-quote character in a string constant. Because a plain
+double quote ends the string, you must use '\"' to represent an actual
+double-quote character as a part of the string. For example:
+
+ $ awk 'BEGIN { print "He said \"hi!\" to her." }'
+ -| He said "hi!" to her.
+
+ The backslash character itself is another character that cannot be
+included normally; you must write '\\' to put one backslash in the
+string or regexp. Thus, the string whose contents are the two
+characters '"' and '\' must be written '"\"\\"'.
+
+ Other escape sequences represent unprintable characters such as TAB
+or newline. There is nothing to stop you from entering most unprintable
+characters directly in a string constant or regexp constant, but they
+may look ugly.
+
+ The following list presents all the escape sequences used in 'awk'
+and what they represent. Unless noted otherwise, all these escape
+sequences apply to both string constants and regexp constants:
+
+'\\'
+ A literal backslash, '\'.
+
+'\a'
+ The "alert" character, 'Ctrl-g', ASCII code 7 (BEL). (This often
+ makes some sort of audible noise.)
+
+'\b'
+ Backspace, 'Ctrl-h', ASCII code 8 (BS).
+
+'\f'
+ Formfeed, 'Ctrl-l', ASCII code 12 (FF).
+
+'\n'
+ Newline, 'Ctrl-j', ASCII code 10 (LF).
+
+'\r'
+ Carriage return, 'Ctrl-m', ASCII code 13 (CR).
+
+'\t'
+ Horizontal TAB, 'Ctrl-i', ASCII code 9 (HT).
+
+'\v'
+ Vertical TAB, 'Ctrl-k', ASCII code 11 (VT).
+
+'\NNN'
+ The octal value NNN, where NNN stands for 1 to 3 digits between '0'
+ and '7'. For example, the code for the ASCII ESC (escape)
+ character is '\033'.
+
+'\xHH...'
+ The hexadecimal value HH, where HH stands for a sequence of
+ hexadecimal digits ('0'-'9', and either 'A'-'F' or 'a'-'f'). A
+ maximum of two digts are allowed after the '\x'. Any further
+ hexadecimal digits are treated as simple letters or numbers.
+ (c.e.) (The '\x' escape sequence is not allowed in POSIX awk.)
+
+ CAUTION: In ISO C, the escape sequence continues until the
+ first nonhexadecimal digit is seen. For many years, 'gawk'
+ would continue incorporating hexadecimal digits into the value
+ until a non-hexadecimal digit or the end of the string was
+ encountered. However, using more than two hexadecimal digits
+ produced undefined results. As of version 4.2, only two
+ digits are processed.
+
+'\/'
+ A literal slash (should be used for regexp constants only). This
+ sequence is used when you want to write a regexp constant that
+ contains a slash (such as '/.*:\/home\/[[:alnum:]]+:.*/'; the
+ '[[:alnum:]]' notation is discussed in *note Bracket
+ Expressions::). Because the regexp is delimited by slashes, you
+ need to escape any slash that is part of the pattern, in order to
+ tell 'awk' to keep processing the rest of the regexp.
+
+'\"'
+ A literal double quote (should be used for string constants only).
+ This sequence is used when you want to write a string constant that
+ contains a double quote (such as '"He said \"hi!\" to her."').
+ Because the string is delimited by double quotes, you need to
+ escape any quote that is part of the string, in order to tell 'awk'
+ to keep processing the rest of the string.
+
+ In 'gawk', a number of additional two-character sequences that begin
+with a backslash have special meaning in regexps. *Note GNU Regexp
+Operators::.
+
+ In a regexp, a backslash before any character that is not in the
+previous list and not listed in *note GNU Regexp Operators:: means that
+the next character should be taken literally, even if it would normally
+be a regexp operator. For example, '/a\+b/' matches the three
+characters 'a+b'.
+
+ For complete portability, do not use a backslash before any character
+not shown in the previous list or that is not an operator.
+
+ Backslash Before Regular Characters
+
+ If you place a backslash in a string constant before something that
+is not one of the characters previously listed, POSIX 'awk' purposely
+leaves what happens as undefined. There are two choices:
+
+Strip the backslash out
+ This is what BWK 'awk' and 'gawk' both do. For example, '"a\qc"'
+ is the same as '"aqc"'. (Because this is such an easy bug both to
+ introduce and to miss, 'gawk' warns you about it.) Consider 'FS =
+ "[ \t]+\|[ \t]+"' to use vertical bars surrounded by whitespace as
+ the field separator. There should be two backslashes in the
+ string: 'FS = "[ \t]+\\|[ \t]+"'.)
+
+Leave the backslash alone
+ Some other 'awk' implementations do this. In such implementations,
+ typing '"a\qc"' is the same as typing '"a\\qc"'.
+
+ To summarize:
+
+ * The escape sequences in the preceding list are always processed
+ first, for both string constants and regexp constants. This
+ happens very early, as soon as 'awk' reads your program.
+
+ * 'gawk' processes both regexp constants and dynamic regexps (*note
+ Computed Regexps::), for the special operators listed in *note GNU
+ Regexp Operators::.
+
+ * A backslash before any other character means to treat that
+ character literally.
+
+ Escape Sequences for Metacharacters
+
+ Suppose you use an octal or hexadecimal escape to represent a regexp
+metacharacter. (See *note Regexp Operators::.) Does 'awk' treat the
+character as a literal character or as a regexp operator?
+
+ Historically, such characters were taken literally. (d.c.) However,
+the POSIX standard indicates that they should be treated as real
+metacharacters, which is what 'gawk' does. In compatibility mode (*note
+Options::), 'gawk' treats the characters represented by octal and
+hexadecimal escape sequences literally when used in regexp constants.
+Thus, '/a\52b/' is equivalent to '/a\*b/'.
+
+
+File: gawk.info, Node: Regexp Operators, Next: Bracket Expressions, Prev:
Escape Sequences, Up: Regexp
+
+3.3 Regular Expression Operators
+================================
+
+You can combine regular expressions with special characters, called
+"regular expression operators" or "metacharacters", to increase the
+power and versatility of regular expressions.
+
+* Menu:
+
+* Regexp Operator Details:: The actual details.
+* Interval Expressions:: Notes on interval expressions.
+
+
+File: gawk.info, Node: Regexp Operator Details, Next: Interval Expressions,
Up: Regexp Operators
+
+3.3.1 Regexp Operators in 'awk'
+-------------------------------
+
+The escape sequences described in *note Escape Sequences:: are valid
+inside a regexp. They are introduced by a '\' and are recognized and
+converted into corresponding real characters as the very first step in
+processing regexps.
+
+ Here is a list of metacharacters. All characters that are not escape
+sequences and that are not listed here stand for themselves:
+
+'\'
+ This suppresses the special meaning of a character when matching.
+ For example, '\$' matches the character '$'.
+
+'^'
+ This matches the beginning of a string. '^@chapter' matches
+ '@chapter' at the beginning of a string, for example, and can be
+ used to identify chapter beginnings in Texinfo source files. The
+ '^' is known as an "anchor", because it anchors the pattern to
+ match only at the beginning of the string.
+
+ It is important to realize that '^' does not match the beginning of
+ a line (the point right after a '\n' newline character) embedded in
+ a string. The condition is not true in the following example:
+
+ if ("line1\nLINE 2" ~ /^L/) ...
+
+'$'
+ This is similar to '^', but it matches only at the end of a string.
+ For example, 'p$' matches a record that ends with a 'p'. The '$'
+ is an anchor and does not match the end of a line (the point right
+ before a '\n' newline character) embedded in a string. The
+ condition in the following example is not true:
+
+ if ("line1\nLINE 2" ~ /1$/) ...
+
+'.' (period)
+ This matches any single character, _including_ the newline
+ character. For example, '.P' matches any single character followed
+ by a 'P' in a string. Using concatenation, we can make a regular
+ expression such as 'U.A', which matches any three-character
+ sequence that begins with 'U' and ends with 'A'.
+
+ In strict POSIX mode (*note Options::), '.' does not match the NUL
+ character, which is a character with all bits equal to zero.
+ Otherwise, NUL is just another character. Other versions of 'awk'
+ may not be able to match the NUL character.
+
+'['...']'
+ This is called a "bracket expression".(1) It matches any _one_ of
+ the characters that are enclosed in the square brackets. For
+ example, '[MVX]' matches any one of the characters 'M', 'V', or 'X'
+ in a string. A full discussion of what can be inside the square
+ brackets of a bracket expression is given in *note Bracket
+ Expressions::.
+
+'[^'...']'
+ This is a "complemented bracket expression". The first character
+ after the '[' _must_ be a '^'. It matches any characters _except_
+ those in the square brackets. For example, '[^awk]' matches any
+ character that is not an 'a', 'w', or 'k'.
+
+'|'
+ This is the "alternation operator" and it is used to specify
+ alternatives. The '|' has the lowest precedence of all the regular
+ expression operators. For example, '^P|[aeiouy]' matches any
+ string that matches either '^P' or '[aeiouy]'. This means it
+ matches any string that starts with 'P' or contains (anywhere
+ within it) a lowercase English vowel.
+
+ The alternation applies to the largest possible regexps on either
+ side.
+
+'('...')'
+ Parentheses are used for grouping in regular expressions, as in
+ arithmetic. They can be used to concatenate regular expressions
+ containing the alternation operator, '|'. For example,
+ '@(samp|code)\{[^}]+\}' matches both '@code{foo}' and '@samp{bar}'.
+ (These are Texinfo formatting control sequences. The '+' is
+ explained further on in this list.)
+
+ The left or opening parenthesis is always a metacharacter; to match
+ one literally, precede it with a backslash. However, the right or
+ closing parenthesis is only special when paired with a left
+ parenthesis; an unpaired right parenthesis is (silently) treated as
+ a regular character.
+
+'*'
+ This symbol means that the preceding regular expression should be
+ repeated as many times as necessary to find a match. For example,
+ 'ph*' applies the '*' symbol to the preceding 'h' and looks for
+ matches of one 'p' followed by any number of 'h's. This also
+ matches just 'p' if no 'h's are present.
+
+ There are two subtle points to understand about how '*' works.
+ First, the '*' applies only to the single preceding regular
+ expression component (e.g., in 'ph*', it applies just to the 'h').
+ To cause '*' to apply to a larger subexpression, use parentheses:
+ '(ph)*' matches 'ph', 'phph', 'phphph', and so on.
+
+ Second, '*' finds as many repetitions as possible. If the text to
+ be matched is 'phhhhhhhhhhhhhhooey', 'ph*' matches all of the 'h's.
+
+'+'
+ This symbol is similar to '*', except that the preceding expression
+ must be matched at least once. This means that 'wh+y' would match
+ 'why' and 'whhy', but not 'wy', whereas 'wh*y' would match all
+ three.
+
+'?'
+ This symbol is similar to '*', except that the preceding expression
+ can be matched either once or not at all. For example, 'fe?d'
+ matches 'fed' and 'fd', but nothing else.
+
+'{'N'}'
+'{'N',}'
+'{'N','M'}'
+ One or two numbers inside braces denote an "interval expression".
+ If there is one number in the braces, the preceding regexp is
+ repeated N times. If there are two numbers separated by a comma,
+ the preceding regexp is repeated N to M times. If there is one
+ number followed by a comma, then the preceding regexp is repeated
+ at least N times:
+
+ 'wh{3}y'
+ Matches 'whhhy', but not 'why' or 'whhhhy'.
+
+ 'wh{3,5}y'
+ Matches 'whhhy', 'whhhhy', or 'whhhhhy' only.
+
+ 'wh{2,}y'
+ Matches 'whhy', 'whhhy', and so on.
+
+ In regular expressions, the '*', '+', and '?' operators, as well as
+the braces '{' and '}', have the highest precedence, followed by
+concatenation, and finally by '|'. As in arithmetic, parentheses can
+change how operators are grouped.
+
+ In POSIX 'awk' and 'gawk', the '*', '+', and '?' operators stand for
+themselves when there is nothing in the regexp that precedes them. For
+example, '/+/' matches a literal plus sign. However, many other
+versions of 'awk' treat such a usage as a syntax error.
+
+ What About The Empty Regexp?
+
+ We describe here an advanced regexp usage. Feel free to skip it upon
+first reading.
+
+ You can supply an empty regexp constant ('//') in all places where a
+regexp is expected. Is this useful? What does it match?
+
+ It is useful. It matches the (invisible) empty string at the start
+and end of a string of characters, as well as the empty string between
+characters. This is best illustrated with the 'gsub()' function, which
+makes global substitutions in a string (*note String Functions::).
+Normal usage of 'gsub()' is like so:
+
+ $ awk '
+ > BEGIN {
+ > x = "ABC_CBA"
+ > gsub(/B/, "bb", x)
+ > print x
+ > }'
+ -| AbbC_CbbA
+
+ We can use 'gsub()' to see where the empty strings are that match the
+empty regexp:
+
+ $ awk '
+ > BEGIN {
+ > x = "ABC"
+ > gsub(//, "x", x)
+ > print x
+ > }'
+ -| xAxBxCx
+
+ ---------- Footnotes ----------
+
+ (1) In other literature, you may see a bracket expression referred to
+as either a "character set", a "character class", or a "character list".
+
+
+File: gawk.info, Node: Interval Expressions, Prev: Regexp Operator Details,
Up: Regexp Operators
+
+3.3.2 Some Notes On Interval Expressions
+----------------------------------------
+
+Interval expressions were not traditionally available in 'awk'. They
+were added as part of the POSIX standard to make 'awk' and 'egrep'
+consistent with each other.
+
+ Initially, because old programs may use '{' and '}' in regexp
+constants, 'gawk' did _not_ match interval expressions in regexps.
+
+ However, beginning with version 4.0, 'gawk' does match interval
+expressions by default. This is because compatibility with POSIX has
+become more important to most 'gawk' users than compatibility with old
+programs.
+
+ For programs that use '{' and '}' in regexp constants, it is good
+practice to always escape them with a backslash. Then the regexp
+constants are valid and work the way you want them to, using any version
+of 'awk'.(1)
+
+ When '{' and '}' appear in regexp constants in a way that cannot be
+interpreted as an interval expression (such as '/q{a}/'), then they
+stand for themselves.
+
+ As mentioned, interval expressions were not traditionally available
+in 'awk'. In March of 2019, BWK 'awk' (finally) acquired them.
+Nonetheless, because they were not available for so many decades, 'gawk'
+continues to not supply them when in compatibility mode (*note
+Options::).
+
+ POSIX says that interval expressions containing repetition counts
+greater than 255 produce unspecified results.
+
+ In the manual for GNU 'grep', Paul Eggert notes the following:
+
+ Interval expressions may be implemented internally via repetition.
+ For example, '^(a|bc){2,4}$' might be implemented as
+ '^(a|bc)(a|bc)((a|bc)(a|bc)?)?$'. A large repetition count may
+ exhaust memory or greatly slow matching. Even small counts can
+ cause problems if cascaded; for example, 'grep -E
+ ".*{10,}{10,}{10,}{10,}{10,}"' is likely to overflow a stack.
+ Fortunately, regular expressions like these are typically
+ artificial, and cascaded repetitions do not conform to POSIX so
+ cannot be used in portable programs anyway.
+
+This same caveat applies to 'gawk'.
+
+ ---------- Footnotes ----------
+
+ (1) Use two backslashes if you're using a string constant with a
+regexp operator or function.
+
+
+File: gawk.info, Node: Bracket Expressions, Next: Leftmost Longest, Prev:
Regexp Operators, Up: Regexp
+
+3.4 Using Bracket Expressions
+=============================
+
+As mentioned earlier, a bracket expression matches any character among
+those listed between the opening and closing square brackets.
+
+ Within a bracket expression, a "range expression" consists of two
+characters separated by a hyphen. It matches any single character that
+sorts between the two characters, based upon the system's native
+character set. For example, '[0-9]' is equivalent to '[0123456789]'.
+(See *note Ranges and Locales:: for an explanation of how the POSIX
+standard and 'gawk' have changed over time. This is mainly of
+historical interest.)
+
+ With the increasing popularity of the Unicode character standard
+(http://www.unicode.org), there is an additional wrinkle to consider.
+Octal and hexadecimal escape sequences inside bracket expressions are
+taken to represent only single-byte characters (characters whose values
+fit within the range 0-256). To match a range of characters where the
+endpoints of the range are larger than 256, enter the multibyte
+encodings of the characters directly.
+
+ To include one of the characters '\', ']', '-', or '^' in a bracket
+expression, put a '\' in front of it. For example:
+
+ [d\]]
+
+matches either 'd' or ']'. Additionally, if you place ']' right after
+the opening '[', the closing bracket is treated as one of the characters
+to be matched.
+
+ The treatment of '\' in bracket expressions is compatible with other
+'awk' implementations and is also mandated by POSIX. The regular
+expressions in 'awk' are a superset of the POSIX specification for
+Extended Regular Expressions (EREs). POSIX EREs are based on the
+regular expressions accepted by the traditional 'egrep' utility.
+
+ "Character classes" are a feature introduced in the POSIX standard.
+A character class is a special notation for describing lists of
+characters that have a specific attribute, but the actual characters can
+vary from country to country and/or from character set to character set.
+For example, the notion of what is an alphabetic character differs
+between the United States and France.
+
+ A character class is only valid in a regexp _inside_ the brackets of
+a bracket expression. Character classes consist of '[:', a keyword
+denoting the class, and ':]'. *note Table 3.1: table-char-classes.
+lists the character classes defined by the POSIX standard.
+
+
+Class Meaning
+--------------------------------------------------------------------------
+'[:alnum:]' Alphanumeric characters
+'[:alpha:]' Alphabetic characters
+'[:blank:]' Space and TAB characters
+'[:cntrl:]' Control characters
+'[:digit:]' Numeric characters
+'[:graph:]' Characters that are both printable and visible (a space is
+ printable but not visible, whereas an 'a' is both)
+'[:lower:]' Lowercase alphabetic characters
+'[:print:]' Printable characters (characters that are not control
+ characters)
+'[:punct:]' Punctuation characters (characters that are not letters,
+ digits, control characters, or space characters)
+'[:space:]' Space characters (these are: space, TAB, newline, carriage
+ return, formfeed and vertical tab)
+'[:upper:]' Uppercase alphabetic characters
+'[:xdigit:]'Characters that are hexadecimal digits
+
+Table 3.1: POSIX character classes
+
+ For example, before the POSIX standard, you had to write
+'/[A-Za-z0-9]/' to match alphanumeric characters. If your character set
+had other alphabetic characters in it, this would not match them. With
+the POSIX character classes, you can write '/[[:alnum:]]/' to match the
+alphabetic and numeric characters in your character set.
+
+ Some utilities that match regular expressions provide a nonstandard
+'[:ascii:]' character class; 'awk' does not. However, you can simulate
+such a construct using '[\x00-\x7F]'. This matches all values
+numerically between zero and 127, which is the defined range of the
+ASCII character set. Use a complemented character list ('[^\x00-\x7F]')
+to match any single-byte characters that are not in the ASCII range.
+
+ NOTE: Some older versions of Unix 'awk' treat '[:blank:]' like
+ '[:space:]', incorrectly matching more characters than they should.
+ Caveat Emptor.
+
+ Two additional special sequences can appear in bracket expressions.
+These apply to non-ASCII character sets, which can have single symbols
+(called "collating elements") that are represented with more than one
+character. They can also have several characters that are equivalent
+for "collating", or sorting, purposes. (For example, in French, a plain
+"e" and a grave-accented "è" are equivalent.) These sequences are:
+
+Collating symbols
+ Multicharacter collating elements enclosed between '[.' and '.]'.
+ For example, if 'ch' is a collating element, then '[[.ch.]]' is a
+ regexp that matches this collating element, whereas '[ch]' is a
+ regexp that matches either 'c' or 'h'.
+
+Equivalence classes
+ Locale-specific names for a list of characters that are equal. The
+ name is enclosed between '[=' and '=]'. For example, the name 'e'
+ might be used to represent all of "e," "ê," "è," and "é." In this
+ case, '[[=e=]]' is a regexp that matches any of 'e', 'ê', 'é', or
+ 'è'.
+
+ These features are very valuable in non-English-speaking locales.
+
+ CAUTION: The library functions that 'gawk' uses for regular
+ expression matching currently recognize only POSIX character
+ classes; they do not recognize collating symbols or equivalence
+ classes.
+
+ Inside a bracket expression, an opening bracket ('[') that does not
+start a character class, collating element or equivalence class is taken
+literally. This is also true of '.' and '*'.
+
+
+File: gawk.info, Node: Leftmost Longest, Next: Computed Regexps, Prev:
Bracket Expressions, Up: Regexp
+
+3.5 How Much Text Matches?
+==========================
+
+Consider the following:
+
+ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
+
+ This example uses the 'sub()' function to make a change to the input
+record. ('sub()' replaces the first instance of any text matched by the
+first argument with the string provided as the second argument; *note
+String Functions::.) Here, the regexp '/a+/' indicates "one or more 'a'
+characters," and the replacement text is '<A>'.
+
+ The input contains four 'a' characters. 'awk' (and POSIX) regular
+expressions always match the leftmost, _longest_ sequence of input
+characters that can match. Thus, all four 'a' characters are replaced
+with '<A>' in this example:
+
+ $ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
+ -| <A>bcd
+
+ For simple match/no-match tests, this is not so important. But when
+doing text matching and substitutions with the 'match()', 'sub()',
+'gsub()', and 'gensub()' functions, it is very important. *Note String
+Functions::, for more information on these functions. Understanding
+this principle is also important for regexp-based record and field
+splitting (*note Records::, and also *note Field Separators::).
+
+
+File: gawk.info, Node: Computed Regexps, Next: GNU Regexp Operators, Prev:
Leftmost Longest, Up: Regexp
+
+3.6 Using Dynamic Regexps
+=========================
+
+The righthand side of a '~' or '!~' operator need not be a regexp
+constant (i.e., a string of characters between slashes). It may be any
+expression. The expression is evaluated and converted to a string if
+necessary; the contents of the string are then used as the regexp. A
+regexp computed in this way is called a "dynamic regexp" or a "computed
+regexp":
+
+ BEGIN { digits_regexp = "[[:digit:]]+" }
+ $0 ~ digits_regexp { print }
+
+This sets 'digits_regexp' to a regexp that describes one or more digits,
+and tests whether the input record matches this regexp.
+
+ NOTE: When using the '~' and '!~' operators, be aware that there is
+ a difference between a regexp constant enclosed in slashes and a
+ string constant enclosed in double quotes. If you are going to use
+ a string constant, you have to understand that the string is, in
+ essence, scanned _twice_: the first time when 'awk' reads your
+ program, and the second time when it goes to match the string on
+ the lefthand side of the operator with the pattern on the right.
+ This is true of any string-valued expression (such as
+ 'digits_regexp', shown in the previous example), not just string
+ constants.
+
+ What difference does it make if the string is scanned twice? The
+answer has to do with escape sequences, and particularly with
+backslashes. To get a backslash into a regular expression inside a
+string, you have to type two backslashes.
+
+ For example, '/\*/' is a regexp constant for a literal '*'. Only one
+backslash is needed. To do the same thing with a string, you have to
+type '"\\*"'. The first backslash escapes the second one so that the
+string actually contains the two characters '\' and '*'.
+
+ Given that you can use both regexp and string constants to describe
+regular expressions, which should you use? The answer is "regexp
+constants," for several reasons:
+
+ * String constants are more complicated to write and more difficult
+ to read. Using regexp constants makes your programs less
+ error-prone. Not understanding the difference between the two
+ kinds of constants is a common source of errors.
+
+ * It is more efficient to use regexp constants. 'awk' can note that
+ you have supplied a regexp and store it internally in a form that
+ makes pattern matching more efficient. When using a string
+ constant, 'awk' must first convert the string into this internal
+ form and then perform the pattern matching.
+
+ * Using regexp constants is better form; it shows clearly that you
+ intend a regexp match.
+
+ Using '\n' in Bracket Expressions of Dynamic Regexps
+
+ Some older versions of 'awk' do not allow the newline character to be
+used inside a bracket expression for a dynamic regexp:
+
+ $ awk '$0 ~ "[ \t\n]"'
+ error-> awk: newline in character class [
+ error-> ]...
+ error-> source line number 1
+ error-> context is
+ error-> $0 ~ "[ >>> \t\n]" <<<
+
+ But a newline in a regexp constant works with no problem:
+
+ $ awk '$0 ~ /[ \t\n]/'
+ here is a sample line
+ -| here is a sample line
+ Ctrl-d
+
+ 'gawk' does not have this problem, and it isn't likely to occur often
+in practice, but it's worth noting for future reference.
+
+
+File: gawk.info, Node: GNU Regexp Operators, Next: Case-sensitivity, Prev:
Computed Regexps, Up: Regexp
+
+3.7 'gawk'-Specific Regexp Operators
+====================================
+
+GNU software that deals with regular expressions provides a number of
+additional regexp operators. These operators are described in this
+minor node and are specific to 'gawk'; they are not available in other
+'awk' implementations. Most of the additional operators deal with word
+matching. For our purposes, a "word" is a sequence of one or more
+letters, digits, or underscores ('_'):
+
+'\s'
+ Matches any space character as defined by the current locale.
+ Think of it as shorthand for '[[:space:]]'.
+
+'\S'
+ Matches any character that is not a space, as defined by the
+ current locale. Think of it as shorthand for '[^[:space:]]'.
+
+'\w'
+ Matches any word-constituent character--that is, it matches any
+ letter, digit, or underscore. Think of it as shorthand for
+ '[[:alnum:]_]'.
+
+'\W'
+ Matches any character that is not word-constituent. Think of it as
+ shorthand for '[^[:alnum:]_]'.
+
+'\<'
+ Matches the empty string at the beginning of a word. For example,
+ '/\<away/' matches 'away' but not 'stowaway'.
+
+'\>'
+ Matches the empty string at the end of a word. For example,
+ '/stow\>/' matches 'stow' but not 'stowaway'.
+
+'\y'
+ Matches the empty string at either the beginning or the end of a
+ word (i.e., the word boundar*y*). For example, '\yballs?\y'
+ matches either 'ball' or 'balls', as a separate word.
+
+'\B'
+ Matches the empty string that occurs between two word-constituent
+ characters. For example, '/\Brat\B/' matches 'crate', but it does
+ not match 'dirty rat'. '\B' is essentially the opposite of '\y'.
+
+ There are two other operators that work on buffers. In Emacs, a
+"buffer" is, naturally, an Emacs buffer. Other GNU programs, including
+'gawk', consider the entire string to match as the buffer. The
+operators are:
+
+'\`'
+ Matches the empty string at the beginning of a buffer (string)
+
+'\''
+ Matches the empty string at the end of a buffer (string)
+
+ Because '^' and '$' always work in terms of the beginning and end of
+strings, these operators don't add any new capabilities for 'awk'. They
+are provided for compatibility with other GNU software.
+
+ In other GNU software, the word-boundary operator is '\b'. However,
+that conflicts with the 'awk' language's definition of '\b' as
+backspace, so 'gawk' uses a different letter. An alternative method
+would have been to require two backslashes in the GNU operators, but
+this was deemed too confusing. The current method of using '\y' for the
+GNU '\b' appears to be the lesser of two evils.
+
+ The various command-line options (*note Options::) control how 'gawk'
+interprets characters in regexps:
+
+No options
+ In the default case, 'gawk' provides all the facilities of POSIX
+ regexps and the GNU regexp operators described in *note Regexp
+ Operators::.
+
+'--posix'
+ Match only POSIX regexps; the GNU operators are not special (e.g.,
+ '\w' matches a literal 'w'). Interval expressions are allowed.
+
+'--traditional'
+ Match traditional Unix 'awk' regexps. The GNU operators are not
+ special, and interval expressions are not available. Because BWK
+ 'awk' supports them, the POSIX character classes ('[[:alnum:]]',
+ etc.) are available. Characters described by octal and
+ hexadecimal escape sequences are treated literally, even if they
+ represent regexp metacharacters.
+
+'--re-interval'
+ Allow interval expressions in regexps, if '--traditional' has been
+ provided. Otherwise, interval expressions are available by
+ default.
+
+
+File: gawk.info, Node: Case-sensitivity, Next: Regexp Summary, Prev: GNU
Regexp Operators, Up: Regexp
+
+3.8 Case Sensitivity in Matching
+================================
+
+Case is normally significant in regular expressions, both when matching
+ordinary characters (i.e., not metacharacters) and inside bracket
+expressions. Thus, a 'w' in a regular expression matches only a
+lowercase 'w' and not an uppercase 'W'.
+
+ The simplest way to do a case-independent match is to use a bracket
+expression--for example, '[Ww]'. However, this can be cumbersome if you
+need to use it often, and it can make the regular expressions harder to
+read. There are two alternatives that you might prefer.
+
+ One way to perform a case-insensitive match at a particular point in
+the program is to convert the data to a single case, using the
+'tolower()' or 'toupper()' built-in string functions (which we haven't
+discussed yet; *note String Functions::). For example:
+
+ tolower($1) ~ /foo/ { ... }
+
+converts the first field to lowercase before matching against it. This
+works in any POSIX-compliant 'awk'.
+
+ Another method, specific to 'gawk', is to set the variable
+'IGNORECASE' to a nonzero value (*note Built-in Variables::). When
+'IGNORECASE' is not zero, _all_ regexp and string operations ignore
+case.
+
+ Changing the value of 'IGNORECASE' dynamically controls the case
+sensitivity of the program as it runs. Case is significant by default
+because 'IGNORECASE' (like most variables) is initialized to zero:
+
+ x = "aB"
+ if (x ~ /ab/) ... # this test will fail
+
+ IGNORECASE = 1
+ if (x ~ /ab/) ... # now it will succeed
+
+ In general, you cannot use 'IGNORECASE' to make certain rules case
+insensitive and other rules case sensitive, as there is no
+straightforward way to set 'IGNORECASE' just for the pattern of a
+particular rule.(1) To do this, use either bracket expressions or
+'tolower()'. However, one thing you can do with 'IGNORECASE' only is
+dynamically turn case sensitivity on or off for all the rules at once.
+
+ 'IGNORECASE' can be set on the command line or in a 'BEGIN' rule
+(*note Other Arguments::; also *note Using BEGIN/END::). Setting
+'IGNORECASE' from the command line is a way to make a program case
+insensitive without having to edit it.
+
+ In multibyte locales, the equivalences between upper- and lowercase
+characters are tested based on the wide-character values of the locale's
+character set. Prior to version 5.0, single-byte characters were tested
+based on the ISO-8859-1 (ISO Latin-1) character set. However, as of
+version 5.0, single-byte characters are also tested based on the values
+of the locale's character set.(2)
+
+ The value of 'IGNORECASE' has no effect if 'gawk' is in compatibility
+mode (*note Options::). Case is always significant in compatibility
+mode.
+
+ ---------- Footnotes ----------
+
+ (1) Experienced C and C++ programmers will note that it is possible,
+using something like 'IGNORECASE = 1 && /foObAr/ { ... }' and
+'IGNORECASE = 0 || /foobar/ { ... }'. However, this is somewhat obscure
+and we don't recommend it.
+
+ (2) If you don't understand this, don't worry about it; it just means
+that 'gawk' does the right thing.
+
+
+File: gawk.info, Node: Regexp Summary, Prev: Case-sensitivity, Up: Regexp
+
+3.9 Summary
+===========
+
+ * Regular expressions describe sets of strings to be matched. In
+ 'awk', regular expression constants are written enclosed between
+ slashes: '/'...'/'.
+
+ * Regexp constants may be used standalone in patterns and in
+ conditional expressions, or as part of matching expressions using
+ the '~' and '!~' operators.
+
+ * Escape sequences let you represent nonprintable characters and also
+ let you represent regexp metacharacters as literal characters to be
+ matched.
+
+ * Regexp operators provide grouping, alternation, and repetition.
+
+ * Bracket expressions give you a shorthand for specifying sets of
+ characters that can match at a particular point in a regexp.
+ Within bracket expressions, POSIX character classes let you specify
+ certain groups of characters in a locale-independent fashion.
+
+ * Regular expressions match the leftmost longest text in the string
+ being matched. This matters for cases where you need to know the
+ extent of the match, such as for text substitution and when the
+ record separator is a regexp.
+
+ * Matching expressions may use dynamic regexps (i.e., string values
+ treated as regular expressions).
+
+ * 'gawk''s 'IGNORECASE' variable lets you control the case
+ sensitivity of regexp matching. In other 'awk' versions, use
+ 'tolower()' or 'toupper()'.
+
+
+File: gawk.info, Node: Reading Files, Next: Printing, Prev: Regexp, Up: Top
+
+4 Reading Input Files
+*********************
+
+In the typical 'awk' program, 'awk' reads all input either from the
+standard input (by default, this is the keyboard, but often it is a pipe
+from another command) or from files whose names you specify on the 'awk'
+command line. If you specify input files, 'awk' reads them in order,
+processing all the data from one before going on to the next. The name
+of the current input file can be found in the predefined variable
+'FILENAME' (*note Built-in Variables::).
+
+ The input is read in units called "records", and is processed by the
+rules of your program one record at a time. By default, each record is
+one line. Each record is automatically split into chunks called
+"fields". This makes it more convenient for programs to work on the
+parts of a record.
+
+ On rare occasions, you may need to use the 'getline' command. The
+'getline' command is valuable both because it can do explicit input from
+any number of files, and because the files used with it do not have to
+be named on the 'awk' command line (*note Getline::).
+
+* Menu:
+
+* Records:: Controlling how data is split into records.
+* Fields:: An introduction to fields.
+* Nonconstant Fields:: Nonconstant Field Numbers.
+* Changing Fields:: Changing the Contents of a Field.
+* Field Separators:: The field separator and how to change it.
+* Constant Size:: Reading constant width data.
+* Splitting By Content:: Defining Fields By Content
+* Testing field creation:: Checking how 'gawk' is splitting
+ records.
+* Multiple Line:: Reading multiline records.
+* Getline:: Reading files under explicit program control
+ using the 'getline' function.
+* Read Timeout:: Reading input with a timeout.
+* Retrying Input:: Retrying input after certain errors.
+* Command-line directories:: What happens if you put a directory on the
+ command line.
+* Input Summary:: Input summary.
+* Input Exercises:: Exercises.
+
+
+File: gawk.info, Node: Records, Next: Fields, Up: Reading Files
+
+4.1 How Input Is Split into Records
+===================================
+
+'awk' divides the input for your program into records and fields. It
+keeps track of the number of records that have been read so far from the
+current input file. This value is stored in a predefined variable
+called 'FNR', which is reset to zero every time a new file is started.
+Another predefined variable, 'NR', records the total number of input
+records read so far from all data files. It starts at zero, but is
+never automatically reset to zero.
+
+ Normally, records are separated by newline characters. You can
+control how records are separated by assigning values to the built-in
+variable 'RS'. If 'RS' is any single character, that character
+separates records. Otherwise (in 'gawk'), 'RS' is treated as a regular
+expression. This mechanism is explained in greater detail shortly.
+
+* Menu:
+
+* awk split records:: How standard 'awk' splits records.
+* gawk split records:: How 'gawk' splits records.
+
+
+File: gawk.info, Node: awk split records, Next: gawk split records, Up:
Records
+
+4.1.1 Record Splitting with Standard 'awk'
+------------------------------------------
+
+Records are separated by a character called the "record separator". By
+default, the record separator is the newline character. This is why
+records are, by default, single lines. To use a different character for
+the record separator, simply assign that character to the predefined
+variable 'RS'.
+
+ Like any other variable, the value of 'RS' can be changed in the
+'awk' program with the assignment operator, '=' (*note Assignment
+Ops::). The new record-separator character should be enclosed in
+quotation marks, which indicate a string constant. Often, the right
+time to do this is at the beginning of execution, before any input is
+processed, so that the very first record is read with the proper
+separator. To do this, use the special 'BEGIN' pattern (*note
+BEGIN/END::). For example:
+
+ awk 'BEGIN { RS = "u" }
+ { print $0 }' mail-list
+
+changes the value of 'RS' to 'u', before reading any input. The new
+value is a string whose first character is the letter "u"; as a result,
+records are separated by the letter "u". Then the input file is read,
+and the second rule in the 'awk' program (the action with no pattern)
+prints each record. Because each 'print' statement adds a newline at
+the end of its output, this 'awk' program copies the input with each 'u'
+changed to a newline. Here are the results of running the program on
+'mail-list':
+
+ $ awk 'BEGIN { RS = "u" }
+ > { print $0 }' mail-list
+ -| Amelia 555-5553 amelia.zodiac
+ -| sq
+ -| e@gmail.com F
+ -| Anthony 555-3412 anthony.assert
+ -| ro@hotmail.com A
+ -| Becky 555-7685 becky.algebrar
+ -| m@gmail.com A
+ -| Bill 555-1675 bill.drowning@hotmail.com A
+ -| Broderick 555-0542 broderick.aliq
+ -| otiens@yahoo.com R
+ -| Camilla 555-2912 camilla.inf
+ -| sar
+ -| m@skynet.be R
+ -| Fabi
+ -| s 555-1234 fabi
+ -| s.
+ -| ndevicesim
+ -| s@
+ -| cb.ed
+ -| F
+ -| J
+ -| lie 555-6699 j
+ -| lie.perscr
+ -| tabor@skeeve.com F
+ -| Martin 555-6480 martin.codicib
+ -| s@hotmail.com A
+ -| Sam
+ -| el 555-3430 sam
+ -| el.lanceolis@sh
+ -| .ed
+ -| A
+ -| Jean-Pa
+ -| l 555-2127 jeanpa
+ -| l.campanor
+ -| m@ny
+ -| .ed
+ -| R
+ -|
+
+Note that the entry for the name 'Bill' is not split. In the original
+data file (*note Sample Data Files::), the line looks like this:
+
+ Bill 555-1675 bill.drowning@hotmail.com A
+
+It contains no 'u', so there is no reason to split the record, unlike
+the others, which each have one or more occurrences of the 'u'. In
+fact, this record is treated as part of the previous record; the newline
+separating them in the output is the original newline in the data file,
+not the one added by 'awk' when it printed the record!
+
+ Another way to change the record separator is on the command line,
+using the variable-assignment feature (*note Other Arguments::):
+
+ awk '{ print $0 }' RS="u" mail-list
+
+This sets 'RS' to 'u' before processing 'mail-list'.
+
+ Using an alphabetic character such as 'u' for the record separator is
+highly likely to produce strange results. Using an unusual character
+such as '/' is more likely to produce correct behavior in the majority
+of cases, but there are no guarantees. The moral is: Know Your Data.
+
+ 'gawk' allows 'RS' to be a full regular expression (discussed
+shortly; *note gawk split records::). Even so, using a regular
+expression metacharacter, such as '.' as the single character in the
+value of 'RS' has no special effect: it is treated literally. This is
+required for backwards compatibility with both Unix 'awk' and with
+POSIX.
+
+ Reaching the end of an input file terminates the current input
+record, even if the last character in the file is not the character in
+'RS'. (d.c.)
+
+ The empty string '""' (a string without any characters) has a special
+meaning as the value of 'RS'. It means that records are separated by
+one or more blank lines and nothing else. *Note Multiple Line:: for
+more details.
+
+ If you change the value of 'RS' in the middle of an 'awk' run, the
+new value is used to delimit subsequent records, but the record
+currently being processed, as well as records already processed, are not
+affected.
+
+ After the end of the record has been determined, 'gawk' sets the
+variable 'RT' to the text in the input that matched 'RS'.
+
+
+File: gawk.info, Node: gawk split records, Prev: awk split records, Up:
Records
+
+4.1.2 Record Splitting with 'gawk'
+----------------------------------
+
+When using 'gawk', the value of 'RS' is not limited to a one-character
+string. If it contains more than one character, it is treated as a
+regular expression (*note Regexp::). (c.e.) In general, each record
+ends at the next string that matches the regular expression; the next
+record starts at the end of the matching string. This general rule is
+actually at work in the usual case, where 'RS' contains just a newline:
+a record ends at the beginning of the next matching string (the next
+newline in the input), and the following record starts just after the
+end of this string (at the first character of the following line). The
+newline, because it matches 'RS', is not part of either record.
+
+ When 'RS' is a single character, 'RT' contains the same single
+character. However, when 'RS' is a regular expression, 'RT' contains
+the actual input text that matched the regular expression.
+
+ If the input file ends without any text matching 'RS', 'gawk' sets
+'RT' to the null string.
+
+ The following example illustrates both of these features. It sets
+'RS' equal to a regular expression that matches either a newline or a
+series of one or more uppercase letters with optional leading and/or
+trailing whitespace:
+
+ $ echo record 1 AAAA record 2 BBBB record 3 |
+ > gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
+ > { print "Record =", $0,"and RT = [" RT "]" }'
+ -| Record = record 1 and RT = [ AAAA ]
+ -| Record = record 2 and RT = [ BBBB ]
+ -| Record = record 3 and RT = [
+ -| ]
+
+The square brackets delineate the contents of 'RT', letting you see the
+leading and trailing whitespace. The final value of 'RT' is a newline.
+*Note Simple Sed:: for a more useful example of 'RS' as a regexp and
+'RT'.
+
+ If you set 'RS' to a regular expression that allows optional trailing
+text, such as 'RS = "abc(XYZ)?"', it is possible, due to implementation
+constraints, that 'gawk' may match the leading part of the regular
+expression, but not the trailing part, particularly if the input text
+that could match the trailing part is fairly long. 'gawk' attempts to
+avoid this problem, but currently, there's no guarantee that this will
+never happen.
+
+ Caveats When Using Regular Expressions for 'RS'
+
+ Remember that in 'awk', the '^' and '$' anchor metacharacters match
+the beginning and end of a _string_, and not the beginning and end of a
+_line_. As a result, something like 'RS = "^[[:upper:]]"' can only
+match at the beginning of a file. This is because 'gawk' views the
+input file as one long string that happens to contain newline
+characters. It is thus best to avoid anchor metacharacters in the value
+of 'RS'.
+
+ Record splitting with regular expressions works differently than
+regexp matching with the 'sub()', 'gsub()', and 'gensub()' (*note String
+Functions::). Those functions allow a regexp to match the empty string;
+record splitting does not. Thus, for example 'RS = "()"' does _not_
+split records between characters.
+
+ The use of 'RS' as a regular expression and the 'RT' variable are
+'gawk' extensions; they are not available in compatibility mode (*note
+Options::). In compatibility mode, only the first character of the
+value of 'RS' determines the end of the record.
+
+ 'mawk' has allowed 'RS' to be a regexp for decades. As of October,
+2019, BWK 'awk' also supports it. Neither version supplies 'RT',
+however.
+
+ 'RS = "\0"' Is Not Portable
+
+ There are times when you might want to treat an entire data file as a
+single record. The only way to make this happen is to give 'RS' a value
+that you know doesn't occur in the input file. This is hard to do in a
+general way, such that a program always works for arbitrary input files.
+
+ You might think that for text files, the NUL character, which
+consists of a character with all bits equal to zero, is a good value to
+use for 'RS' in this case:
+
+ BEGIN { RS = "\0" } # whole file becomes one record?
+
+ 'gawk' in fact accepts this, and uses the NUL character for the
+record separator. This works for certain special files, such as
+'/proc/environ' on GNU/Linux systems, where the NUL character is in fact
+the record separator. However, this usage is _not_ portable to most
+other 'awk' implementations.
+
+ Almost all other 'awk' implementations(1) store strings internally as
+C-style strings. C strings use the NUL character as the string
+terminator. In effect, this means that 'RS = "\0"' is the same as 'RS =
+""'. (d.c.)
+
+ It happens that recent versions of 'mawk' can use the NUL character
+as a record separator. However, this is a special case: 'mawk' does not
+allow embedded NUL characters in strings. (This may change in a future
+version of 'mawk'.)
+
+ *Note Readfile Function:: for an interesting way to read whole files.
+If you are using 'gawk', see *note Extension Sample Readfile:: for
+another option.
+
+ ---------- Footnotes ----------
+
+ (1) At least that we know about.
+
+
+File: gawk.info, Node: Fields, Next: Nonconstant Fields, Prev: Records,
Up: Reading Files
+
+4.2 Examining Fields
+====================
+
+When 'awk' reads an input record, the record is automatically "parsed"
+or separated by the 'awk' utility into chunks called "fields". By
+default, fields are separated by "whitespace", like words in a line.
+Whitespace in 'awk' means any string of one or more spaces, TABs, or
+newlines; other characters that are considered whitespace by other
+languages (such as formfeed, vertical tab, etc.) are _not_ considered
+whitespace by 'awk'.
+
+ The purpose of fields is to make it more convenient for you to refer
+to these pieces of the record. You don't have to use them--you can
+operate on the whole record if you want--but fields are what make simple
+'awk' programs so powerful.
+
+ You use a dollar sign ('$') to refer to a field in an 'awk' program,
+followed by the number of the field you want. Thus, '$1' refers to the
+first field, '$2' to the second, and so on. (Unlike in the Unix shells,
+the field numbers are not limited to single digits. '$127' is the 127th
+field in the record.) For example, suppose the following is a line of
+input:
+
+ This seems like a pretty nice example.
+
+Here the first field, or '$1', is 'This', the second field, or '$2', is
+'seems', and so on. Note that the last field, '$7', is 'example.'.
+Because there is no space between the 'e' and the '.', the period is
+considered part of the seventh field.
+
+ 'NF' is a predefined variable whose value is the number of fields in
+the current record. 'awk' automatically updates the value of 'NF' each
+time it reads a record. No matter how many fields there are, the last
+field in a record can be represented by '$NF'. So, '$NF' is the same as
+'$7', which is 'example.'. If you try to reference a field beyond the
+last one (such as '$8' when the record has only seven fields), you get
+the empty string. (If used in a numeric operation, you get zero.)
+
+ The use of '$0', which looks like a reference to the "zeroth" field,
+is a special case: it represents the whole input record. Use it when
+you are not interested in specific fields. Here are some more examples:
+
+ $ awk '$1 ~ /li/ { print $0 }' mail-list
+ -| Amelia 555-5553 amelia.zodiacusque@gmail.com F
+ -| Julie 555-6699 julie.perscrutabor@skeeve.com F
+
+This example prints each record in the file 'mail-list' whose first
+field contains the string 'li'.
+
+ By contrast, the following example looks for 'li' in _the entire
+record_ and prints the first and last fields for each matching input
+record:
+
+ $ awk '/li/ { print $1, $NF }' mail-list
+ -| Amelia F
+ -| Broderick R
+ -| Julie F
+ -| Samuel A
+
+
+File: gawk.info, Node: Nonconstant Fields, Next: Changing Fields, Prev:
Fields, Up: Reading Files
+
+4.3 Nonconstant Field Numbers
+=============================
+
+A field number need not be a constant. Any expression in the 'awk'
+language can be used after a '$' to refer to a field. The value of the
+expression specifies the field number. If the value is a string, rather
+than a number, it is converted to a number. Consider this example:
+
+ awk '{ print $NR }'
+
+Recall that 'NR' is the number of records read so far: one in the first
+record, two in the second, and so on. So this example prints the first
+field of the first record, the second field of the second record, and so
+on. For the twentieth record, field number 20 is printed; most likely,
+the record has fewer than 20 fields, so this prints a blank line. Here
+is another example of using expressions as field numbers:
+
+ awk '{ print $(2*2) }' mail-list
+
+ 'awk' evaluates the expression '(2*2)' and uses its value as the
+number of the field to print. The '*' represents multiplication, so the
+expression '2*2' evaluates to four. The parentheses are used so that
+the multiplication is done before the '$' operation; they are necessary
+whenever there is a binary operator(1) in the field-number expression.
+This example, then, prints the type of relationship (the fourth field)
+for every line of the file 'mail-list'. (All of the 'awk' operators are
+listed, in order of decreasing precedence, in *note Precedence::.)
+
+ If the field number you compute is zero, you get the entire record.
+Thus, '$(2-2)' has the same value as '$0'. Negative field numbers are
+not allowed; trying to reference one usually terminates the program.
+(The POSIX standard does not define what happens when you reference a
+negative field number. 'gawk' notices this and terminates your program.
+Other 'awk' implementations may behave differently.)
+
+ As mentioned in *note Fields::, 'awk' stores the current record's
+number of fields in the built-in variable 'NF' (also *note Built-in
+Variables::). Thus, the expression '$NF' is not a special feature--it
+is the direct consequence of evaluating 'NF' and using its value as a
+field number.
+
+ ---------- Footnotes ----------
+
+ (1) A "binary operator", such as '*' for multiplication, is one that
+takes two operands. The distinction is required because 'awk' also has
+unary (one-operand) and ternary (three-operand) operators.
+
+
+File: gawk.info, Node: Changing Fields, Next: Field Separators, Prev:
Nonconstant Fields, Up: Reading Files
+
+4.4 Changing the Contents of a Field
+====================================
+
+The contents of a field, as seen by 'awk', can be changed within an
+'awk' program; this changes what 'awk' perceives as the current input
+record. (The actual input is untouched; 'awk' _never_ modifies the
+input file.) Consider the following example and its output:
+
+ $ awk '{ nboxes = $3 ; $3 = $3 - 10
+ > print nboxes, $3 }' inventory-shipped
+ -| 25 15
+ -| 32 22
+ -| 24 14
+ ...
+
+The program first saves the original value of field three in the
+variable 'nboxes'. The '-' sign represents subtraction, so this program
+reassigns field three, '$3', as the original value of field three minus
+ten: '$3 - 10'. (*Note Arithmetic Ops::.) Then it prints the original
+and new values for field three. (Someone in the warehouse made a
+consistent mistake while inventorying the red boxes.)
+
+ For this to work, the text in '$3' must make sense as a number; the
+string of characters must be converted to a number for the computer to
+do arithmetic on it. The number resulting from the subtraction is
+converted back to a string of characters that then becomes field three.
+*Note Conversion::.
+
+ When the value of a field is changed (as perceived by 'awk'), the
+text of the input record is recalculated to contain the new field where
+the old one was. In other words, '$0' changes to reflect the altered
+field. Thus, this program prints a copy of the input file, with 10
+subtracted from the second field of each line:
+
+ $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
+ -| Jan 3 25 15 115
+ -| Feb 5 32 24 226
+ -| Mar 5 24 34 228
+ ...
+
+ It is also possible to assign contents to fields that are out of
+range. For example:
+
+ $ awk '{ $6 = ($5 + $4 + $3 + $2)
+ > print $6 }' inventory-shipped
+ -| 168
+ -| 297
+ -| 301
+ ...
+
+We've just created '$6', whose value is the sum of fields '$2', '$3',
+'$4', and '$5'. The '+' sign represents addition. For the file
+'inventory-shipped', '$6' represents the total number of parcels shipped
+for a particular month.
+
+ Creating a new field changes 'awk''s internal copy of the current
+input record, which is the value of '$0'. Thus, if you do 'print $0'
+after adding a field, the record printed includes the new field, with
+the appropriate number of field separators between it and the previously
+existing fields.
+
+ This recomputation affects and is affected by 'NF' (the number of
+fields; *note Fields::). For example, the value of 'NF' is set to the
+number of the highest field you create. The exact format of '$0' is
+also affected by a feature that has not been discussed yet: the "output
+field separator", 'OFS', used to separate the fields (*note Output
+Separators::).
+
+ Note, however, that merely _referencing_ an out-of-range field does
+_not_ change the value of either '$0' or 'NF'. Referencing an
+out-of-range field only produces an empty string. For example:
+
+ if ($(NF+1) != "")
+ print "can't happen"
+ else
+ print "everything is normal"
+
+should print 'everything is normal', because 'NF+1' is certain to be out
+of range. (*Note If Statement:: for more information about 'awk''s
+'if-else' statements. *Note Typing and Comparison:: for more
+information about the '!=' operator.)
+
+ It is important to note that making an assignment to an existing
+field changes the value of '$0' but does not change the value of 'NF',
+even when you assign the empty string to a field. For example:
+
+ $ echo a b c d | awk '{ OFS = ":"; $2 = ""
+ > print $0; print NF }'
+ -| a::c:d
+ -| 4
+
+The field is still there; it just has an empty value, delimited by the
+two colons between 'a' and 'c'. This example shows what happens if you
+create a new field:
+
+ $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new"
+ > print $0; print NF }'
+ -| a::c:d::new
+ -| 6
+
+The intervening field, '$5', is created with an empty value (indicated
+by the second pair of adjacent colons), and 'NF' is updated with the
+value six.
+
+ Decrementing 'NF' throws away the values of the fields after the new
+value of 'NF' and recomputes '$0'. (d.c.) Here is an example:
+
+ $ echo a b c d e f | awk '{ print "NF =", NF;
+ > NF = 3; print $0 }'
+ -| NF = 6
+ -| a b c
+
+ CAUTION: Some versions of 'awk' don't rebuild '$0' when 'NF' is
+ decremented. Until August, 2018, this included BWK 'awk';
+ fortunately his version now handles this correctly.
+
+ Finally, there are times when it is convenient to force 'awk' to
+rebuild the entire record, using the current values of the fields and
+'OFS'. To do this, use the seemingly innocuous assignment:
+
+ $1 = $1 # force record to be reconstituted
+ print $0 # or whatever else with $0
+
+This forces 'awk' to rebuild the record. It does help to add a comment,
+as we've shown here.
+
+ There is a flip side to the relationship between '$0' and the fields.
+Any assignment to '$0' causes the record to be reparsed into fields
+using the _current_ value of 'FS'. This also applies to any built-in
+function that updates '$0', such as 'sub()' and 'gsub()' (*note String
+Functions::).
+
+ Understanding '$0'
+
+ It is important to remember that '$0' is the _full_ record, exactly
+as it was read from the input. This includes any leading or trailing
+whitespace, and the exact whitespace (or other characters) that
+separates the fields.
+
+ It is a common error to try to change the field separators in a
+record simply by setting 'FS' and 'OFS', and then expecting a plain
+'print' or 'print $0' to print the modified record.
+
+ But this does not work, because nothing was done to change the record
+itself. Instead, you must force the record to be rebuilt, typically
+with a statement such as '$1 = $1', as described earlier.
+
+
+File: gawk.info, Node: Field Separators, Next: Constant Size, Prev:
Changing Fields, Up: Reading Files
+
+4.5 Specifying How Fields Are Separated
+=======================================
+
+* Menu:
+
+* Default Field Splitting:: How fields are normally separated.
+* Regexp Field Splitting:: Using regexps as the field separator.
+* Single Character Fields:: Making each character a separate field.
+* Command Line Field Separator:: Setting 'FS' from the command line.
+* Full Line Fields:: Making the full line be a single field.
+* Field Splitting Summary:: Some final points and a summary table.
+
+The "field separator", which is either a single character or a regular
+expression, controls the way 'awk' splits an input record into fields.
+'awk' scans the input record for character sequences that match the
+separator; the fields themselves are the text between the matches.
+
+ In the examples that follow, we use the bullet symbol (*) to
+represent spaces in the output. If the field separator is 'oo', then
+the following line:
+
+ moo goo gai pan
+
+is split into three fields: 'm', '*g', and '*gai*pan'. Note the leading
+spaces in the values of the second and third fields.
+
+ The field separator is represented by the predefined variable 'FS'.
+Shell programmers take note: 'awk' does _not_ use the name 'IFS' that is
+used by the POSIX-compliant shells (such as the Unix Bourne shell, 'sh',
+or Bash).
+
+ The value of 'FS' can be changed in the 'awk' program with the
+assignment operator, '=' (*note Assignment Ops::). Often, the right
+time to do this is at the beginning of execution before any input has
+been processed, so that the very first record is read with the proper
+separator. To do this, use the special 'BEGIN' pattern (*note
+BEGIN/END::). For example, here we set the value of 'FS' to the string
+'","':
+
+ awk 'BEGIN { FS = "," } ; { print $2 }'
+
+Given the input line:
+
+ John Q. Smith, 29 Oak St., Walamazoo, MI 42139
+
+this 'awk' program extracts and prints the string '*29*Oak*St.'.
+
+ Sometimes the input data contains separator characters that don't
+separate fields the way you thought they would. For instance, the
+person's name in the example we just used might have a title or suffix
+attached, such as:
+
+ John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
+
+The same program would extract '*LXIX' instead of '*29*Oak*St.'. If you
+were expecting the program to print the address, you would be surprised.
+The moral is to choose your data layout and separator characters
+carefully to prevent such problems. (If the data is not in a form that
+is easy to process, perhaps you can massage it first with a separate
+'awk' program.)
+
+
+File: gawk.info, Node: Default Field Splitting, Next: Regexp Field
Splitting, Up: Field Separators
+
+4.5.1 Whitespace Normally Separates Fields
+------------------------------------------
+
+Fields are normally separated by whitespace sequences (spaces, TABs, and
+newlines), not by single spaces. Two spaces in a row do not delimit an
+empty field. The default value of the field separator 'FS' is a string
+containing a single space, '" "'. If 'awk' interpreted this value in
+the usual way, each space character would separate fields, so two spaces
+in a row would make an empty field between them. The reason this does
+not happen is that a single space as the value of 'FS' is a special
+case--it is taken to specify the default manner of delimiting fields.
+
+ If 'FS' is any other single character, such as '","', then each
+occurrence of that character separates two fields. Two consecutive
+occurrences delimit an empty field. If the character occurs at the
+beginning or the end of the line, that too delimits an empty field. The
+space character is the only single character that does not follow these
+rules.
+
+
+File: gawk.info, Node: Regexp Field Splitting, Next: Single Character
Fields, Prev: Default Field Splitting, Up: Field Separators
+
+4.5.2 Using Regular Expressions to Separate Fields
+--------------------------------------------------
+
+The previous node discussed the use of single characters or simple
+strings as the value of 'FS'. More generally, the value of 'FS' may be
+a string containing any regular expression. In this case, each match in
+the record for the regular expression separates fields. For example,
+the assignment:
+
+ FS = ", \t"
+
+makes every area of an input line that consists of a comma followed by a
+space and a TAB into a field separator. ('\t' is an "escape sequence"
+that stands for a TAB; *note Escape Sequences::, for the complete list
+of similar escape sequences.)
+
+ For a less trivial example of a regular expression, try using single
+spaces to separate fields the way single commas are used. 'FS' can be
+set to '"[ ]"' (left bracket, space, right bracket). This regular
+expression matches a single space and nothing else (*note Regexp::).
+
+ There is an important difference between the two cases of 'FS = " "'
+(a single space) and 'FS = "[ \t\n]+"' (a regular expression matching
+one or more spaces, TABs, or newlines). For both values of 'FS', fields
+are separated by "runs" (multiple adjacent occurrences) of spaces, TABs,
+and/or newlines. However, when the value of 'FS' is '" "', 'awk' first
+strips leading and trailing whitespace from the record and then decides
+where the fields are. For example, the following pipeline prints 'b':
+
+ $ echo ' a b c d ' | awk '{ print $2 }'
+ -| b
+
+However, this pipeline prints 'a' (note the extra spaces around each
+letter):
+
+ $ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t\n]+" }
+ > { print $2 }'
+ -| a
+
+In this case, the first field is null, or empty.
+
+ The stripping of leading and trailing whitespace also comes into play
+whenever '$0' is recomputed. For instance, study this pipeline:
+
+ $ echo ' a b c d' | awk '{ print; $2 = $2; print }'
+ -| a b c d
+ -| a b c d
+
+The first 'print' statement prints the record as it was read, with
+leading whitespace intact. The assignment to '$2' rebuilds '$0' by
+concatenating '$1' through '$NF' together, separated by the value of
+'OFS' (which is a space by default). Because the leading whitespace was
+ignored when finding '$1', it is not part of the new '$0'. Finally, the
+last 'print' statement prints the new '$0'.
+
+ There is an additional subtlety to be aware of when using regular
+expressions for field splitting. It is not well specified in the POSIX
+standard, or anywhere else, what '^' means when splitting fields. Does
+the '^' match only at the beginning of the entire record? Or is each
+field separator a new string? It turns out that different 'awk'
+versions answer this question differently, and you should not rely on
+any specific behavior in your programs. (d.c.)
+
+ As a point of information, BWK 'awk' allows '^' to match only at the
+beginning of the record. 'gawk' also works this way. For example:
+
+ $ echo 'xxAA xxBxx C' |
+ > gawk -F '(^x+)|( +)' '{ for (i = 1; i <= NF; i++)
+ > printf "-->%s<--\n", $i }'
+ -| --><--
+ -| -->AA<--
+ -| -->xxBxx<--
+ -| -->C<--
+
+ Finally, field splitting with regular expressions works differently
+than regexp matching with the 'sub()', 'gsub()', and 'gensub()' (*note
+String Functions::). Those functions allow a regexp to match the empty
+string; field splitting does not. Thus, for example 'FS = "()"' does
+_not_ split fields between characters.
+
+
+File: gawk.info, Node: Single Character Fields, Next: Command Line Field
Separator, Prev: Regexp Field Splitting, Up: Field Separators
+
+4.5.3 Making Each Character a Separate Field
+--------------------------------------------
+
+There are times when you may want to examine each character of a record
+separately. This can be done in 'gawk' by simply assigning the null
+string ('""') to 'FS'. (c.e.) In this case, each individual character
+in the record becomes a separate field. For example:
+
+ $ echo a b | gawk 'BEGIN { FS = "" }
+ > {
+ > for (i = 1; i <= NF; i = i + 1)
+ > print "Field", i, "is", $i
+ > }'
+ -| Field 1 is a
+ -| Field 2 is
+ -| Field 3 is b
+
+ Traditionally, the behavior of 'FS' equal to '""' was not defined.
+In this case, most versions of Unix 'awk' simply treat the entire record
+as only having one field. (d.c.) In compatibility mode (*note
+Options::), if 'FS' is the null string, then 'gawk' also behaves this
+way.
+
+
+File: gawk.info, Node: Command Line Field Separator, Next: Full Line Fields,
Prev: Single Character Fields, Up: Field Separators
+
+4.5.4 Setting 'FS' from the Command Line
+----------------------------------------
+
+'FS' can be set on the command line. Use the '-F' option to do so. For
+example:
+
+ awk -F, 'PROGRAM' INPUT-FILES
+
+sets 'FS' to the ',' character. Notice that the option uses an
+uppercase 'F' instead of a lowercase 'f'. The latter option ('-f')
+specifies a file containing an 'awk' program.
+
+ The value used for the argument to '-F' is processed in exactly the
+same way as assignments to the predefined variable 'FS'. Any special
+characters in the field separator must be escaped appropriately. For
+example, to use a '\' as the field separator on the command line, you
+would have to type:
+
+ # same as FS = "\\"
+ awk -F\\\\ '...' files ...
+
+Because '\' is used for quoting in the shell, 'awk' sees '-F\\'. Then
+'awk' processes the '\\' for escape characters (*note Escape
+Sequences::), finally yielding a single '\' to use for the field
+separator.
+
+ As a special case, in compatibility mode (*note Options::), if the
+argument to '-F' is 't', then 'FS' is set to the TAB character. If you
+type '-F\t' at the shell, without any quotes, the '\' gets deleted, so
+'awk' figures that you really want your fields to be separated with TABs
+and not 't's. Use '-v FS="t"' or '-F"[t]"' on the command line if you
+really do want to separate your fields with 't's. Use '-F '\t'' when
+not in compatibility mode to specify that TABs separate fields.
+
+ As an example, let's use an 'awk' program file called 'edu.awk' that
+contains the pattern '/edu/' and the action 'print $1':
+
+ /edu/ { print $1 }
+
+ Let's also set 'FS' to be the '-' character and run the program on
+the file 'mail-list'. The following command prints a list of the names
+of the people that work at or attend a university, and the first three
+digits of their phone numbers:
+
+ $ awk -F- -f edu.awk mail-list
+ -| Fabius 555
+ -| Samuel 555
+ -| Jean
+
+Note the third line of output. The third line in the original file
+looked like this:
+
+ Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+
+ The '-' as part of the person's name was used as the field separator,
+instead of the '-' in the phone number that was originally intended.
+This demonstrates why you have to be careful in choosing your field and
+record separators.
+
+ Perhaps the most common use of a single character as the field
+separator occurs when processing the Unix system password file. On many
+Unix systems, each user has a separate entry in the system password
+file, with one line per user. The information in these lines is
+separated by colons. The first field is the user's login name and the
+second is the user's encrypted or shadow password. (A shadow password
+is indicated by the presence of a single 'x' in the second field.) A
+password file entry might look like this:
+
+ arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash
+
+ The following program searches the system password file and prints
+the entries for users whose full name is not indicated:
+
+ awk -F: '$5 == ""' /etc/passwd
+
+
+File: gawk.info, Node: Full Line Fields, Next: Field Splitting Summary,
Prev: Command Line Field Separator, Up: Field Separators
+
+4.5.5 Making the Full Line Be a Single Field
+--------------------------------------------
+
+Occasionally, it's useful to treat the whole input line as a single
+field. This can be done easily and portably simply by setting 'FS' to
+'"\n"' (a newline):(1)
+
+ awk -F'\n' 'PROGRAM' FILES ...
+
+When you do this, '$1' is the same as '$0'.
+
+ Changing 'FS' Does Not Affect the Fields
+
+ According to the POSIX standard, 'awk' is supposed to behave as if
+each record is split into fields at the time it is read. In particular,
+this means that if you change the value of 'FS' after a record is read,
+the values of the fields (i.e., how they were split) should reflect the
+old value of 'FS', not the new one.
+
+ However, many older implementations of 'awk' do not work this way.
+Instead, they defer splitting the fields until a field is actually
+referenced. The fields are split using the _current_ value of 'FS'!
+(d.c.) This behavior can be difficult to diagnose. The following
+example illustrates the difference between the two methods:
+
+ sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'
+
+which usually prints:
+
+ root
+
+on an incorrect implementation of 'awk', while 'gawk' prints the full
+first line of the file, something like:
+
+ root:x:0:0:Root:/:
+
+ (The 'sed'(2) command prints just the first line of '/etc/passwd'.)
+
+ ---------- Footnotes ----------
+
+ (1) Thanks to Andrew Schorr for this tip.
+
+ (2) The 'sed' utility is a "stream editor." Its behavior is also
+defined by the POSIX standard.
+
+
+File: gawk.info, Node: Field Splitting Summary, Prev: Full Line Fields, Up:
Field Separators
+
+4.5.6 Field-Splitting Summary
+-----------------------------
+
+It is important to remember that when you assign a string constant as
+the value of 'FS', it undergoes normal 'awk' string processing. For
+example, with Unix 'awk' and 'gawk', the assignment 'FS = "\.."' assigns
+the character string '".."' to 'FS' (the backslash is stripped). This
+creates a regexp meaning "fields are separated by occurrences of any two
+characters." If instead you want fields to be separated by a literal
+period followed by any single character, use 'FS = "\\.."'.
+
+ The following list summarizes how fields are split, based on the
+value of 'FS' ('==' means "is equal to"):
+
+'FS == " "'
+ Fields are separated by runs of whitespace. Leading and trailing
+ whitespace are ignored. This is the default.
+
+'FS == ANY OTHER SINGLE CHARACTER'
+ Fields are separated by each occurrence of the character. Multiple
+ successive occurrences delimit empty fields, as do leading and
+ trailing occurrences. The character can even be a regexp
+ metacharacter; it does not need to be escaped.
+
+'FS == REGEXP'
+ Fields are separated by occurrences of characters that match
+ REGEXP. Leading and trailing matches of REGEXP delimit empty
+ fields.
+
+'FS == ""'
+ Each individual character in the record becomes a separate field.
+ (This is a common extension; it is not specified by the POSIX
+ standard.)
+
+ 'FS' and 'IGNORECASE'
+
+ The 'IGNORECASE' variable (*note User-modified::) affects field
+splitting _only_ when the value of 'FS' is a regexp. It has no effect
+when 'FS' is a single character, even if that character is a letter.
+Thus, in the following code:
+
+ FS = "c"
+ IGNORECASE = 1
+ $0 = "aCa"
+ print $1
+
+The output is 'aCa'. If you really want to split fields on an
+alphabetic character while ignoring case, use a regexp that will do it
+for you (e.g., 'FS = "[c]"'). In this case, 'IGNORECASE' will take
+effect.
+
+
+File: gawk.info, Node: Constant Size, Next: Splitting By Content, Prev:
Field Separators, Up: Reading Files
+
+4.6 Reading Fixed-Width Data
+============================
+
+This minor node discusses an advanced feature of 'gawk'. If you are a
+novice 'awk' user, you might want to skip it on the first reading.
+
+ 'gawk' provides a facility for dealing with fixed-width fields with
+no distinctive field separator. We discuss this feature in the
+following nodes.
+
+* Menu:
+
+* Fixed width data:: Processing fixed-width data.
+* Skipping intervening:: Skipping intervening fields.
+* Allowing trailing data:: Capturing optional trailing data.
+* Fields with fixed data:: Field values with fixed-width data.
+
+
+File: gawk.info, Node: Fixed width data, Next: Skipping intervening, Up:
Constant Size
+
+4.6.1 Processing Fixed-Width Data
+---------------------------------
+
+An example of fixed-width data would be the input for old Fortran
+programs where numbers are run together, or the output of programs that
+did not anticipate the use of their output as input for other programs.
+
+ An example of the latter is a table where all the columns are lined
+up by the use of a variable number of spaces and _empty fields are just
+spaces_. Clearly, 'awk''s normal field splitting based on 'FS' does not
+work well in this case. Although a portable 'awk' program can use a
+series of 'substr()' calls on '$0' (*note String Functions::), this is
+awkward and inefficient for a large number of fields.
+
+ The splitting of an input record into fixed-width fields is specified
+by assigning a string containing space-separated numbers to the built-in
+variable 'FIELDWIDTHS'. Each number specifies the width of the field,
+_including_ columns between fields. If you want to ignore the columns
+between fields, you can specify the width as a separate field that is
+subsequently ignored. It is a fatal error to supply a field width that
+has a negative value.
+
+ The following data is the output of the Unix 'w' utility. It is
+useful to illustrate the use of 'FIELDWIDTHS':
+
+ 10:06pm up 21 days, 14:04, 23 users
+ User tty login idle JCPU PCPU what
+ hzuo ttyV0 8:58pm 9 5 vi p24.tex
+ hzang ttyV3 6:37pm 50 -csh
+ eklye ttyV5 9:53pm 7 1 em thes.tex
+ dportein ttyV6 8:17pm 1:47 -csh
+ gierd ttyD3 10:00pm 1 elm
+ dave ttyD4 9:47pm 4 4 w
+ brent ttyp0 26Jun91 4:46 26:46 4:41 bash
+ dave ttyq4 26Jun9115days 46 46 wnewmail
+
+ The following program takes this input, converts the idle time to
+number of seconds, and prints out the first two fields and the
+calculated idle time:
+
+ BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" }
+ NR > 2 {
+ idle = $4
+ sub(/^ +/, "", idle) # strip leading spaces
+ if (idle == "")
+ idle = 0
+ if (idle ~ /:/) { # hh:mm
+ split(idle, t, ":")
+ idle = t[1] * 60 + t[2]
+ }
+ if (idle ~ /days/)
+ idle *= 24 * 60 * 60
+
+ print $1, $2, idle
+ }
+
+ NOTE: The preceding program uses a number of 'awk' features that
+ haven't been introduced yet.
+
+ Running the program on the data produces the following results:
+
+ hzuo ttyV0 0
+ hzang ttyV3 50
+ eklye ttyV5 0
+ dportein ttyV6 107
+ gierd ttyD3 1
+ dave ttyD4 0
+ brent ttyp0 286
+ dave ttyq4 1296000
+
+ Another (possibly more practical) example of fixed-width input data
+is the input from a deck of balloting cards. In some parts of the
+United States, voters mark their choices by punching holes in computer
+cards. These cards are then processed to count the votes for any
+particular candidate or on any particular issue. Because a voter may
+choose not to vote on some issue, any column on the card may be empty.
+An 'awk' program for processing such data could use the 'FIELDWIDTHS'
+feature to simplify reading the data. (Of course, getting 'gawk' to run
+on a system with card readers is another story!)
+
+
+File: gawk.info, Node: Skipping intervening, Next: Allowing trailing data,
Prev: Fixed width data, Up: Constant Size
+
+4.6.2 Skipping Intervening Fields
+---------------------------------
+
+Starting in version 4.2, each field width may optionally be preceded by
+a colon-separated value specifying the number of characters to skip
+before the field starts. Thus, the preceding program could be rewritten
+to specify 'FIELDWIDTHS' like so:
+
+ BEGIN { FIELDWIDTHS = "8 1:5 4:7 6 1:6 1:6 2:33" }
+
+ This strips away some of the white space separating the fields. With
+such a change, the program produces the following results:
+
+ hzang ttyV3 50
+ eklye ttyV5 0
+ dportein ttyV6 107
+ gierd ttyD3 1
+ dave ttyD4 0
+ brent ttyp0 286
+ dave ttyq4 1296000
+
+
+File: gawk.info, Node: Allowing trailing data, Next: Fields with fixed data,
Prev: Skipping intervening, Up: Constant Size
+
+4.6.3 Capturing Optional Trailing Data
+--------------------------------------
+
+There are times when fixed-width data may be followed by additional data
+that has no fixed length. Such data may or may not be present, but if
+it is, it should be possible to get at it from an 'awk' program.
+
+ Starting with version 4.2, in order to provide a way to say "anything
+else in the record after the defined fields," 'gawk' allows you to add a
+final '*' character to the value of 'FIELDWIDTHS'. There can only be
+one such character, and it must be the final non-whitespace character in
+'FIELDWIDTHS'. For example:
+
+ $ cat fw.awk Show the program
+ -| BEGIN { FIELDWIDTHS = "2 2 *" }
+ -| { print NF, $1, $2, $3 }
+ $ cat fw.in Show sample input
+ -| 1234abcdefghi
+ $ gawk -f fw.awk fw.in Run the program
+ -| 3 12 34 abcdefghi
+
+
+File: gawk.info, Node: Fields with fixed data, Prev: Allowing trailing data,
Up: Constant Size
+
+4.6.4 Field Values With Fixed-Width Data
+----------------------------------------
+
+So far, so good. But what happens if there isn't as much data as there
+should be based on the contents of 'FIELDWIDTHS'? Or, what happens if
+there is more data than expected?
+
+ For many years, what happens in these cases was not well defined.
+Starting with version 4.2, the rules are as follows:
+
+Enough data for some fields
+ For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
+ record is 'aabbb'. In this case, 'NF' is set to two.
+
+Not enough data for a field
+ For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
+ record is 'aab'. In this case, 'NF' is set to two and '$2' has the
+ value '"b"'. The idea is that even though there aren't as many
+ characters as were expected, there are some, so the data should be
+ made available to the program.
+
+Too much data
+ For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
+ record is 'aabbbccccddd'. In this case, 'NF' is set to three and
+ the extra characters ('ddd') are ignored. If you want 'gawk' to
+ capture the extra characters, supply a final '*' in the value of
+ 'FIELDWIDTHS'.
+
+Too much data, but with '*' supplied
+ For example, if 'FIELDWIDTHS' is set to '"2 3 4 *"' and the input
+ record is 'aabbbccccddd'. In this case, 'NF' is set to four, and
+ '$4' has the value '"ddd"'.
+
+
+File: gawk.info, Node: Splitting By Content, Next: Testing field creation,
Prev: Constant Size, Up: Reading Files
+
+4.7 Defining Fields by Content
+==============================
+
+* Menu:
+
+* More CSV:: More on CSV files.
+* FS versus FPAT:: A subtle difference.
+
+This minor node discusses an advanced feature of 'gawk'. If you are a
+novice 'awk' user, you might want to skip it on the first reading.
+
+ Normally, when using 'FS', 'gawk' defines the fields as the parts of
+the record that occur in between each field separator. In other words,
+'FS' defines what a field _is not_, instead of what a field _is_.
+However, there are times when you really want to define the fields by
+what they are, and not by what they are not.
+
+ The most notorious such case is so-called "comma-separated values"
+(CSV) data. Many spreadsheet programs, for example, can export their
+data into text files, where each record is terminated with a newline,
+and fields are separated by commas. If commas only separated the data,
+there wouldn't be an issue. The problem comes when one of the fields
+contains an _embedded_ comma. In such cases, most programs embed the
+field in double quotes.(1) So, we might have data like this:
+
+ Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA
+
+ The 'FPAT' variable offers a solution for cases like this. The value
+of 'FPAT' should be a string that provides a regular expression. This
+regular expression describes the contents of each field.
+
+ In the case of CSV data as presented here, each field is either
+"anything that is not a comma," or "a double quote, anything that is not
+a double quote, and a closing double quote." (There are more
+complicated definitions of CSV data, treated shortly.) If written as a
+regular expression constant (*note Regexp::), we would have
+'/([^,]+)|("[^"]+")/'. Writing this as a string requires us to escape
+the double quotes, leading to:
+
+ FPAT = "([^,]+)|(\"[^\"]+\")"
+
+ Putting this to use, here is a simple program to parse the data:
+
+ BEGIN {
+ FPAT = "([^,]+)|(\"[^\"]+\")"
+ }
+
+ {
+ print "NF = ", NF
+ for (i = 1; i <= NF; i++) {
+ printf("$%d = <%s>\n", i, $i)
+ }
+ }
+
+ When run, we get the following:
+
+ $ gawk -f simple-csv.awk addresses.csv
+ NF = 7
+ $1 = <Robbins>
+ $2 = <Arnold>
+ $3 = <"1234 A Pretty Street, NE">
+ $4 = <MyTown>
+ $5 = <MyState>
+ $6 = <12345-6789>
+ $7 = <USA>
+
+ Note the embedded comma in the value of '$3'.
+
+ A straightforward improvement when processing CSV data of this sort
+would be to remove the quotes when they occur, with something like this:
+
+ if (substr($i, 1, 1) == "\"") {
+ len = length($i)
+ $i = substr($i, 2, len - 2) # Get text within the two quotes
+ }
+
+ NOTE: Some programs export CSV data that contains embedded newlines
+ between the double quotes. 'gawk' provides no way to deal with
+ this. Even though a formal specification for CSV data exists,
+ there isn't much more to be done; the 'FPAT' mechanism provides an
+ elegant solution for the majority of cases, and the 'gawk'
+ developers are satisfied with that.
+
+ As written, the regexp used for 'FPAT' requires that each field
+contain at least one character. A straightforward modification
+(changing the first '+' to '*') allows fields to be empty:
+
+ FPAT = "([^,]*)|(\"[^\"]+\")"
+
+ As with 'FS', the 'IGNORECASE' variable (*note User-modified::)
+affects field splitting with 'FPAT'.
+
+ Assigning a value to 'FPAT' overrides field splitting with 'FS' and
+with 'FIELDWIDTHS'.
+
+ Finally, the 'patsplit()' function makes the same functionality
+available for splitting regular strings (*note String Functions::).
+
+ ---------- Footnotes ----------
+
+ (1) The CSV format lacked a formal standard definition for many
+years. RFC 4180 (http://www.ietf.org/rfc/rfc4180.txt) standardizes the
+most common practices.
+
+
+File: gawk.info, Node: More CSV, Next: FS versus FPAT, Up: Splitting By
Content
+
+4.7.1 More on CSV Files
+-----------------------
+
+Manuel Collado notes that in addition to commas, a CSV field can also
+contains quotes, that have to be escaped by doubling them. The
+previously described regexps fail to accept quoted fields with both
+commas and quotes inside. He suggests that the simplest 'FPAT'
+expression that recognizes this kind of fields is
+'/([^,]*)|("([^"]|"")+")/'. He provides the following input data to
+test these variants:
+
+ p,"q,r",s
+ p,"q""r",s
+ p,"q,""r",s
+ p,"",s
+ p,,s
+
+And here is his test program:
+
+ BEGIN {
+ fp[0] = "([^,]+)|(\"[^\"]+\")"
+ fp[1] = "([^,]*)|(\"[^\"]+\")"
+ fp[2] = "([^,]*)|(\"([^\"]|\"\")+\")"
+ FPAT = fp[fpat+0]
+ }
+
+ {
+ print "<" $0 ">"
+ printf("NF = %s ", NF)
+ for (i = 1; i <= NF; i++) {
+ printf("<%s>", $i)
+ }
+ print ""
+ }
+
+ When run on the third variant, it produces:
+
+ $ gawk -v fpat=2 -f test-csv.awk sample.csv
+ -| <p,"q,r",s>
+ -| NF = 3 <p><"q,r"><s>
+ -| <p,"q""r",s>
+ -| NF = 3 <p><"q""r"><s>
+ -| <p,"q,""r",s>
+ -| NF = 3 <p><"q,""r"><s>
+ -| <p,"",s>
+ -| NF = 3 <p><""><s>
+ -| <p,,s>
+ -| NF = 3 <p><><s>
+
+ In general, using 'FPAT' to do your own CSV parsing is like having a
+bed with a blanket that's not quite big enough. There's always a corner
+that isn't covered. We recommend, instead, that you use Manuel
+Collado's 'CSVMODE' library for 'gawk' (http://mcollado.z15.es/xgawk/).
+
+
+File: gawk.info, Node: FS versus FPAT, Prev: More CSV, Up: Splitting By
Content
+
+4.7.2 'FS' Versus 'FPAT': A Subtle Difference
+---------------------------------------------
+
+As we discussed earlier, 'FS' describes the data between fields ("what
+fields are not") and 'FPAT' describes the fields themselves ("what
+fields are"). This leads to a subtle difference in how fields are found
+when using regexps as the value for 'FS' or 'FPAT'.
+
+ In order to distinguish one field from another, there must be a
+non-empty separator between each field. This makes intuitive
+sense--otherwise one could not distinguish fields from separators.
+
+ Thus, regular expression matching as done when splitting fields with
+'FS' is not allowed to match the null string; it must always match at
+least one character, in order to be able to proceed through the entire
+record.
+
+ On the other hand, regular expression matching with 'FPAT' can match
+the null string, and the non-matching intervening characters function as
+the separators.
+
+ This same difference is reflected in how matching is done with the
+'split()' and 'patsplit()' functions (*note String Functions::).
+
+
+File: gawk.info, Node: Testing field creation, Next: Multiple Line, Prev:
Splitting By Content, Up: Reading Files
+
+4.8 Checking How 'gawk' Is Splitting Records
+============================================
+
+As we've seen, 'gawk' provides three independent methods to split input
+records into fields. The mechanism used is based on which of the three
+variables--'FS', 'FIELDWIDTHS', or 'FPAT'--was last assigned to. In
+addition, an API input parser may choose to override the record parsing
+mechanism; please refer to *note Input Parsers:: for further information
+about this feature.
+
+ To restore normal field splitting after using 'FIELDWIDTHS' and/or
+'FPAT', simply assign a value to 'FS'. You can use 'FS = FS' to do
+this, without having to know the current value of 'FS'.
+
+ In order to tell which kind of field splitting is in effect, use
+'PROCINFO["FS"]' (*note Auto-set::). The value is '"FS"' if regular
+field splitting is being used, '"FIELDWIDTHS"' if fixed-width field
+splitting is being used, or '"FPAT"' if content-based field splitting is
+being used:
+
+ if (PROCINFO["FS"] == "FS")
+ REGULAR FIELD SPLITTING ...
+ else if (PROCINFO["FS"] == "FIELDWIDTHS")
+ FIXED-WIDTH FIELD SPLITTING ...
+ else if (PROCINFO["FS"] == "FPAT")
+ CONTENT-BASED FIELD SPLITTING ...
+ else
+ API INPUT PARSER FIELD SPLITTING ... (advanced feature)
+
+ This information is useful when writing a function that needs to
+temporarily change 'FS' or 'FIELDWIDTHS', read some records, and then
+restore the original settings (*note Passwd Functions:: for an example
+of such a function).
+
+
+File: gawk.info, Node: Multiple Line, Next: Getline, Prev: Testing field
creation, Up: Reading Files
+
+4.9 Multiple-Line Records
+=========================
+
+In some databases, a single line cannot conveniently hold all the
+information in one entry. In such cases, you can use multiline records.
+The first step in doing this is to choose your data format.
+
+ One technique is to use an unusual character or string to separate
+records. For example, you could use the formfeed character (written
+'\f' in 'awk', as in C) to separate them, making each record a page of
+the file. To do this, just set the variable 'RS' to '"\f"' (a string
+containing the formfeed character). Any other character could equally
+well be used, as long as it won't be part of the data in a record.
+
+ Another technique is to have blank lines separate records. By a
+special dispensation, an empty string as the value of 'RS' indicates
+that records are separated by one or more blank lines. When 'RS' is set
+to the empty string, each record always ends at the first blank line
+encountered. The next record doesn't start until the first nonblank
+line that follows. No matter how many blank lines appear in a row, they
+all act as one record separator. (Blank lines must be completely empty;
+lines that contain only whitespace do not count.)
+
+ You can achieve the same effect as 'RS = ""' by assigning the string
+'"\n\n+"' to 'RS'. This regexp matches the newline at the end of the
+record and one or more blank lines after the record. In addition, a
+regular expression always matches the longest possible sequence when
+there is a choice (*note Leftmost Longest::). So, the next record
+doesn't start until the first nonblank line that follows--no matter how
+many blank lines appear in a row, they are considered one record
+separator.
+
+ However, there is an important difference between 'RS = ""' and 'RS =
+"\n\n+"'. In the first case, leading newlines in the input data file
+are ignored, and if a file ends without extra blank lines after the last
+record, the final newline is removed from the record. In the second
+case, this special processing is not done. (d.c.)
+
+ Now that the input is separated into records, the second step is to
+separate the fields in the records. One way to do this is to divide
+each of the lines into fields in the normal manner. This happens by
+default as the result of a special feature. When 'RS' is set to the
+empty string _and_ 'FS' is set to a single character, the newline
+character _always_ acts as a field separator. This is in addition to
+whatever field separations result from 'FS'.
+
+ NOTE: When 'FS' is the null string ('""') or a regexp, this special
+ feature of 'RS' does not apply. It does apply to the default field
+ separator of a single space: 'FS = " "'.
+
+ Note that language in the POSIX specification implies that this
+ special feature should apply when 'FS' is a regexp. However, Unix
+ 'awk' has never behaved that way, nor has 'gawk'. This is
+ essentially a bug in POSIX.
+
+ The original motivation for this special exception was probably to
+provide useful behavior in the default case (i.e., 'FS' is equal to
+'" "'). This feature can be a problem if you really don't want the
+newline character to separate fields, because there is no way to prevent
+it. However, you can work around this by using the 'split()' function
+to break up the record manually (*note String Functions::). If you have
+a single-character field separator, you can work around the special
+feature in a different way, by making 'FS' into a regexp for that single
+character. For example, if the field separator is a percent character,
+instead of 'FS = "%"', use 'FS = "[%]"'.
+
+ Another way to separate fields is to put each field on a separate
+line: to do this, just set the variable 'FS' to the string '"\n"'.
+(This single-character separator matches a single newline.) A practical
+example of a data file organized this way might be a mailing list, where
+blank lines separate the entries. Consider a mailing list in a file
+named 'addresses', which looks like this:
+
+ Jane Doe
+ 123 Main Street
+ Anywhere, SE 12345-6789
+
+ John Smith
+ 456 Tree-lined Avenue
+ Smallville, MW 98765-4321
+ ...
+
+A simple program to process this file is as follows:
+
+ # addrs.awk --- simple mailing list program
+
+ # Records are separated by blank lines.
+ # Each line is one field.
+ BEGIN { RS = "" ; FS = "\n" }
+
+ {
+ print "Name is:", $1
+ print "Address is:", $2
+ print "City and State are:", $3
+ print ""
+ }
+
+ Running the program produces the following output:
+
+ $ awk -f addrs.awk addresses
+ -| Name is: Jane Doe
+ -| Address is: 123 Main Street
+ -| City and State are: Anywhere, SE 12345-6789
+ -|
+ -| Name is: John Smith
+ -| Address is: 456 Tree-lined Avenue
+ -| City and State are: Smallville, MW 98765-4321
+ -|
+ ...
+
+ *Note Labels Program:: for a more realistic program dealing with
+address lists. The following list summarizes how records are split,
+based on the value of 'RS'. ('==' means "is equal to.")
+
+'RS == "\n"'
+ Records are separated by the newline character ('\n'). In effect,
+ every line in the data file is a separate record, including blank
+ lines. This is the default.
+
+'RS == ANY SINGLE CHARACTER'
+ Records are separated by each occurrence of the character.
+ Multiple successive occurrences delimit empty records.
+
+'RS == ""'
+ Records are separated by runs of blank lines. When 'FS' is a
+ single character, then the newline character always serves as a
+ field separator, in addition to whatever value 'FS' may have.
+ Leading and trailing newlines in a file are ignored.
+
+'RS == REGEXP'
+ Records are separated by occurrences of characters that match
+ REGEXP. Leading and trailing matches of REGEXP delimit empty
+ records. (This is a 'gawk' extension; it is not specified by the
+ POSIX standard.)
+
+ If not in compatibility mode (*note Options::), 'gawk' sets 'RT' to
+the input text that matched the value specified by 'RS'. But if the
+input file ended without any text that matches 'RS', then 'gawk' sets
+'RT' to the null string.
+
+
+File: gawk.info, Node: Getline, Next: Read Timeout, Prev: Multiple Line,
Up: Reading Files
+
+4.10 Explicit Input with 'getline'
+==================================
+
+So far we have been getting our input data from 'awk''s main input
+stream--either the standard input (usually your keyboard, sometimes the
+output from another program) or the files specified on the command line.
+The 'awk' language has a special built-in command called 'getline' that
+can be used to read input under your explicit control.
+
+ The 'getline' command is used in several different ways and should
+_not_ be used by beginners. The examples that follow the explanation of
+the 'getline' command include material that has not been covered yet.
+Therefore, come back and study the 'getline' command _after_ you have
+reviewed the rest of this Info file and have a good knowledge of how
+'awk' works.
+
+ The 'getline' command returns 1 if it finds a record and 0 if it
+encounters the end of the file. If there is some error in getting a
+record, such as a file that cannot be opened, then 'getline' returns -1.
+In this case, 'gawk' sets the variable 'ERRNO' to a string describing
+the error that occurred.
+
+ If 'ERRNO' indicates that the I/O operation may be retried, and
+'PROCINFO["INPUT", "RETRY"]' is set, then 'getline' returns -2 instead
+of -1, and further calls to 'getline' may be attempted. *Note Retrying
+Input:: for further information about this feature.
+
+ In the following examples, COMMAND stands for a string value that
+represents a shell command.
+
+ NOTE: When '--sandbox' is specified (*note Options::), reading
+ lines from files, pipes, and coprocesses is disabled.
+
+* Menu:
+
+* Plain Getline:: Using 'getline' with no arguments.
+* Getline/Variable:: Using 'getline' into a variable.
+* Getline/File:: Using 'getline' from a file.
+* Getline/Variable/File:: Using 'getline' into a variable from a
+ file.
+* Getline/Pipe:: Using 'getline' from a pipe.
+* Getline/Variable/Pipe:: Using 'getline' into a variable from a
+ pipe.
+* Getline/Coprocess:: Using 'getline' from a coprocess.
+* Getline/Variable/Coprocess:: Using 'getline' into a variable from a
+ coprocess.
+* Getline Notes:: Important things to know about 'getline'.
+* Getline Summary:: Summary of 'getline' Variants.
+
+
+File: gawk.info, Node: Plain Getline, Next: Getline/Variable, Up: Getline
+
+4.10.1 Using 'getline' with No Arguments
+----------------------------------------
+
+The 'getline' command can be used without arguments to read input from
+the current input file. All it does in this case is read the next input
+record and split it up into fields. This is useful if you've finished
+processing the current record, but want to do some special processing on
+the next record _right now_. For example:
+
+ # Remove text between /* and */, inclusive
+ {
+ while ((start = index($0, "/*")) != 0) {
+ out = substr($0, 1, start - 1) # leading part of the string
+ rest = substr($0, start + 2) # ... */ ...
+ while ((end = index(rest, "*/")) == 0) { # is */ in trailing
part?
+ # get more text
+ if (getline <= 0) {
+ print("unexpected EOF or error:", ERRNO) > "/dev/stderr"
+ exit
+ }
+ # build up the line using string concatenation
+ rest = rest $0
+ }
+ rest = substr(rest, end + 2) # remove comment
+ # build up the output line using string concatenation
+ $0 = out rest
+ }
+ print $0
+ }
+
+ This 'awk' program deletes C-style comments ('/* ... */') from the
+input. It uses a number of features we haven't covered yet, including
+string concatenation (*note Concatenation::) and the 'index()' and
+'substr()' built-in functions (*note String Functions::). By replacing
+the 'print $0' with other statements, you could perform more complicated
+processing on the decommented input, such as searching for matches of a
+regular expression.
+
+ Here is some sample input:
+
+ mon/*comment*/key
+ rab/*commen
+ t*/bit
+ horse /*comment*/more text
+ part 1 /*comment*/part 2 /*comment*/part 3
+ no comment
+
+ When run, the output is:
+
+ $ awk -f strip_comments.awk example_text
+ -| monkey
+ -| rabbit
+ -| horse more text
+ -| part 1 part 2 part 3
+ -| no comment
+
+ This form of the 'getline' command sets 'NF', 'NR', 'FNR', 'RT', and
+the value of '$0'.
+
+ NOTE: The new value of '$0' is used to test the patterns of any
+ subsequent rules. The original value of '$0' that triggered the
+ rule that executed 'getline' is lost. By contrast, the 'next'
+ statement reads a new record but immediately begins processing it
+ normally, starting with the first rule in the program. *Note Next
+ Statement::.
+
+
+File: gawk.info, Node: Getline/Variable, Next: Getline/File, Prev: Plain
Getline, Up: Getline
+
+4.10.2 Using 'getline' into a Variable
+--------------------------------------
+
+You can use 'getline VAR' to read the next record from 'awk''s input
+into the variable VAR. No other processing is done. For example,
+suppose the next line is a comment or a special string, and you want to
+read it without triggering any rules. This form of 'getline' allows you
+to read that line and store it in a variable so that the main
+read-a-line-and-check-each-rule loop of 'awk' never sees it. The
+following example swaps every two lines of input:
+
+ {
+ if ((getline tmp) > 0) {
+ print tmp
+ print $0
+ } else
+ print $0
+ }
+
+It takes the following list:
+
+ wan
+ tew
+ free
+ phore
+
+and produces these results:
+
+ tew
+ wan
+ phore
+ free
+
+ The 'getline' command used in this way sets only the variables 'NR',
+'FNR', and 'RT' (and, of course, VAR). The record is not split into
+fields, so the values of the fields (including '$0') and the value of
+'NF' do not change.
+
+
+File: gawk.info, Node: Getline/File, Next: Getline/Variable/File, Prev:
Getline/Variable, Up: Getline
+
+4.10.3 Using 'getline' from a File
+----------------------------------
+
+Use 'getline < FILE' to read the next record from FILE. Here, FILE is a
+string-valued expression that specifies the file name. '< FILE' is
+called a "redirection" because it directs input to come from a different
+place. For example, the following program reads its input record from
+the file 'secondary.input' when it encounters a first field with a value
+equal to 10 in the current input file:
+
+ {
+ if ($1 == 10) {
+ getline < "secondary.input"
+ print
+ } else
+ print
+ }
+
+ Because the main input stream is not used, the values of 'NR' and
+'FNR' are not changed. However, the record it reads is split into
+fields in the normal manner, so the values of '$0' and the other fields
+are changed, resulting in a new value of 'NF'. 'RT' is also set.
+
+ According to POSIX, 'getline < EXPRESSION' is ambiguous if EXPRESSION
+contains unparenthesized operators other than '$'; for example, 'getline
+< dir "/" file' is ambiguous because the concatenation operator (not
+discussed yet; *note Concatenation::) is not parenthesized. You should
+write it as 'getline < (dir "/" file)' if you want your program to be
+portable to all 'awk' implementations.
+
+
+File: gawk.info, Node: Getline/Variable/File, Next: Getline/Pipe, Prev:
Getline/File, Up: Getline
+
+4.10.4 Using 'getline' into a Variable from a File
+--------------------------------------------------
+
+Use 'getline VAR < FILE' to read input from the file FILE, and put it in
+the variable VAR. As earlier, FILE is a string-valued expression that
+specifies the file from which to read.
+
+ In this version of 'getline', none of the predefined variables are
+changed and the record is not split into fields. The only variable
+changed is VAR.(1) For example, the following program copies all the
+input files to the output, except for records that say
+'@include FILENAME'. Such a record is replaced by the contents of the
+file FILENAME:
+
+ {
+ if (NF == 2 && $1 == "@include") {
+ while ((getline line < $2) > 0)
+ print line
+ close($2)
+ } else
+ print
+ }
+
+ Note here how the name of the extra input file is not built into the
+program; it is taken directly from the data, specifically from the
+second field on the '@include' line.
+
+ The 'close()' function is called to ensure that if two identical
+'@include' lines appear in the input, the entire specified file is
+included twice. *Note Close Files And Pipes::.
+
+ One deficiency of this program is that it does not process nested
+'@include' statements (i.e., '@include' statements in included files)
+the way a true macro preprocessor would. *Note Igawk Program:: for a
+program that does handle nested '@include' statements.
+
+ ---------- Footnotes ----------
+
+ (1) This is not quite true. 'RT' could be changed if 'RS' is a
+regular expression.
+
+
+File: gawk.info, Node: Getline/Pipe, Next: Getline/Variable/Pipe, Prev:
Getline/Variable/File, Up: Getline
+
+4.10.5 Using 'getline' from a Pipe
+----------------------------------
+
+ Omniscience has much to recommend it. Failing that, attention to
+ details would be useful.
+ -- _Brian Kernighan_
+
+ The output of a command can also be piped into 'getline', using
+'COMMAND | getline'. In this case, the string COMMAND is run as a shell
+command and its output is piped into 'awk' to be used as input. This
+form of 'getline' reads one record at a time from the pipe. For
+example, the following program copies its input to its output, except
+for lines that begin with '@execute', which are replaced by the output
+produced by running the rest of the line as a shell command:
+
+ {
+ if ($1 == "@execute") {
+ tmp = substr($0, 10) # Remove "@execute"
+ while ((tmp | getline) > 0)
+ print
+ close(tmp)
+ } else
+ print
+ }
+
+The 'close()' function is called to ensure that if two identical
+'@execute' lines appear in the input, the command is run for each one.
+*Note Close Files And Pipes::. Given the input:
+
+ foo
+ bar
+ baz
+ @execute who
+ bletch
+
+the program might produce:
+
+ foo
+ bar
+ baz
+ arnold ttyv0 Jul 13 14:22
+ miriam ttyp0 Jul 13 14:23 (murphy:0)
+ bill ttyp1 Jul 13 14:23 (murphy:0)
+ bletch
+
+Notice that this program ran the command 'who' and printed the result.
+(If you try this program yourself, you will of course get different
+results, depending upon who is logged in on your system.)
+
+ This variation of 'getline' splits the record into fields, sets the
+value of 'NF', and recomputes the value of '$0'. The values of 'NR' and
+'FNR' are not changed. 'RT' is set.
+
+ According to POSIX, 'EXPRESSION | getline' is ambiguous if EXPRESSION
+contains unparenthesized operators other than '$'--for example, '"echo "
+"date" | getline' is ambiguous because the concatenation operator is not
+parenthesized. You should write it as '("echo " "date") | getline' if
+you want your program to be portable to all 'awk' implementations.
+
+ NOTE: Unfortunately, 'gawk' has not been consistent in its
+ treatment of a construct like '"echo " "date" | getline'. Most
+ versions, including the current version, treat it as '("echo "
+ "date") | getline'. (This is also how BWK 'awk' behaves.) Some
+ versions instead treat it as '"echo " ("date" | getline)'. (This
+ is how 'mawk' behaves.) In short, _always_ use explicit
+ parentheses, and then you won't have to worry.
+
+
+File: gawk.info, Node: Getline/Variable/Pipe, Next: Getline/Coprocess,
Prev: Getline/Pipe, Up: Getline
+
+4.10.6 Using 'getline' into a Variable from a Pipe
+--------------------------------------------------
+
+When you use 'COMMAND | getline VAR', the output of COMMAND is sent
+through a pipe to 'getline' and into the variable VAR. For example, the
+following program reads the current date and time into the variable
+'current_time', using the 'date' utility, and then prints it:
+
+ BEGIN {
+ "date" | getline current_time
+ close("date")
+ print "Report printed on " current_time
+ }
+
+ In this version of 'getline', none of the predefined variables are
+changed and the record is not split into fields. However, 'RT' is set.
+
+ According to POSIX, 'EXPRESSION | getline VAR' is ambiguous if
+EXPRESSION contains unparenthesized operators other than '$'; for
+example, '"echo " "date" | getline VAR' is ambiguous because the
+concatenation operator is not parenthesized. You should write it as
+'("echo " "date") | getline VAR' if you want your program to be portable
+to other 'awk' implementations.
+
+
+File: gawk.info, Node: Getline/Coprocess, Next: Getline/Variable/Coprocess,
Prev: Getline/Variable/Pipe, Up: Getline
+
+4.10.7 Using 'getline' from a Coprocess
+---------------------------------------
+
+Reading input into 'getline' from a pipe is a one-way operation. The
+command that is started with 'COMMAND | getline' only sends data _to_
+your 'awk' program.
+
+ On occasion, you might want to send data to another program for
+processing and then read the results back. 'gawk' allows you to start a
+"coprocess", with which two-way communications are possible. This is
+done with the '|&' operator. Typically, you write data to the coprocess
+first and then read the results back, as shown in the following:
+
+ print "SOME QUERY" |& "db_server"
+ "db_server" |& getline
+
+which sends a query to 'db_server' and then reads the results.
+
+ The values of 'NR' and 'FNR' are not changed, because the main input
+stream is not used. However, the record is split into fields in the
+normal manner, thus changing the values of '$0', of the other fields,
+and of 'NF' and 'RT'.
+
+ Coprocesses are an advanced feature. They are discussed here only
+because this is the minor node on 'getline'. *Note Two-way I/O::, where
+coprocesses are discussed in more detail.
+
+
+File: gawk.info, Node: Getline/Variable/Coprocess, Next: Getline Notes,
Prev: Getline/Coprocess, Up: Getline
+
+4.10.8 Using 'getline' into a Variable from a Coprocess
+-------------------------------------------------------
+
+When you use 'COMMAND |& getline VAR', the output from the coprocess
+COMMAND is sent through a two-way pipe to 'getline' and into the
+variable VAR.
+
+ In this version of 'getline', none of the predefined variables are
+changed and the record is not split into fields. The only variable
+changed is VAR. However, 'RT' is set.
+
+ Coprocesses are an advanced feature. They are discussed here only
+because this is the minor node on 'getline'. *Note Two-way I/O::, where
+coprocesses are discussed in more detail.
+
+
+File: gawk.info, Node: Getline Notes, Next: Getline Summary, Prev:
Getline/Variable/Coprocess, Up: Getline
+
+4.10.9 Points to Remember About 'getline'
+-----------------------------------------
+
+Here are some miscellaneous points about 'getline' that you should bear
+in mind:
+
+ * When 'getline' changes the value of '$0' and 'NF', 'awk' does _not_
+ automatically jump to the start of the program and start testing
+ the new record against every pattern. However, the new record is
+ tested against any subsequent rules.
+
+ * Some very old 'awk' implementations limit the number of pipelines
+ that an 'awk' program may have open to just one. In 'gawk', there
+ is no such limit. You can open as many pipelines (and coprocesses)
+ as the underlying operating system permits.
+
+ * An interesting side effect occurs if you use 'getline' without a
+ redirection inside a 'BEGIN' rule. Because an unredirected
+ 'getline' reads from the command-line data files, the first
+ 'getline' command causes 'awk' to set the value of 'FILENAME'.
+ Normally, 'FILENAME' does not have a value inside 'BEGIN' rules,
+ because you have not yet started to process the command-line data
+ files. (d.c.) (See *note BEGIN/END::; also *note Auto-set::.)
+
+ * Using 'FILENAME' with 'getline' ('getline < FILENAME') is likely to
+ be a source of confusion. 'awk' opens a separate input stream from
+ the current input file. However, by not using a variable, '$0' and
+ 'NF' are still updated. If you're doing this, it's probably by
+ accident, and you should reconsider what it is you're trying to
+ accomplish.
+
+ * *note Getline Summary::, presents a table summarizing the 'getline'
+ variants and which variables they can affect. It is worth noting
+ that those variants that do not use redirection can cause
+ 'FILENAME' to be updated if they cause 'awk' to start reading a new
+ input file.
+
+ * If the variable being assigned is an expression with side effects,
+ different versions of 'awk' behave differently upon encountering
+ end-of-file. Some versions don't evaluate the expression; many
+ versions (including 'gawk') do. Here is an example, courtesy of
+ Duncan Moore:
+
+ BEGIN {
+ system("echo 1 > f")
+ while ((getline a[++c] < "f") > 0) { }
+ print c
+ }
+
+ Here, the side effect is the '++c'. Is 'c' incremented if
+ end-of-file is encountered before the element in 'a' is assigned?
+
+ 'gawk' treats 'getline' like a function call, and evaluates the
+ expression 'a[++c]' before attempting to read from 'f'. However,
+ some versions of 'awk' only evaluate the expression once they know
+ that there is a string value to be assigned.
+
+
+File: gawk.info, Node: Getline Summary, Prev: Getline Notes, Up: Getline
+
+4.10.10 Summary of 'getline' Variants
+-------------------------------------
+
+*note Table 4.1: table-getline-variants. summarizes the eight variants
+of 'getline', listing which predefined variables are set by each one,
+and whether the variant is standard or a 'gawk' extension. Note: for
+each variant, 'gawk' sets the 'RT' predefined variable.
+
+
+Variant Effect 'awk' / 'gawk'
+-------------------------------------------------------------------------
+'getline' Sets '$0', 'NF', 'FNR', 'awk'
+ 'NR', and 'RT'
+'getline' VAR Sets VAR, 'FNR', 'NR', 'awk'
+ and 'RT'
+'getline <' FILE Sets '$0', 'NF', and 'RT' 'awk'
+'getline VAR < FILE' Sets VAR and 'RT' 'awk'
+COMMAND '| getline' Sets '$0', 'NF', and 'RT' 'awk'
+COMMAND '| getline' Sets VAR and 'RT' 'awk'
+VAR
+COMMAND '|& getline' Sets '$0', 'NF', and 'RT' 'gawk'
+COMMAND '|& getline' Sets VAR and 'RT' 'gawk'
+VAR
+
+Table 4.1: 'getline' variants and what they set
+
+
+File: gawk.info, Node: Read Timeout, Next: Retrying Input, Prev: Getline,
Up: Reading Files
+
+4.11 Reading Input with a Timeout
+=================================
+
+This minor node describes a feature that is specific to 'gawk'.
+
+ You may specify a timeout in milliseconds for reading input from the
+keyboard, a pipe, or two-way communication, including TCP/IP sockets.
+This can be done on a per-input, per-command, or per-connection basis,
+by setting a special element in the 'PROCINFO' array (*note Auto-set::):
+
+ PROCINFO["input_name", "READ_TIMEOUT"] = TIMEOUT IN MILLISECONDS
+
+ When set, this causes 'gawk' to time out and return failure if no
+data is available to read within the specified timeout period. For
+example, a TCP client can decide to give up on receiving any response
+from the server after a certain amount of time:
+
+ Service = "/inet/tcp/0/localhost/daytime"
+ PROCINFO[Service, "READ_TIMEOUT"] = 100
+ if ((Service |& getline) > 0)
+ print $0
+ else if (ERRNO != "")
+ print ERRNO
+
+ Here is how to read interactively from the user(1) without waiting
+for more than five seconds:
+
+ PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000
+ while ((getline < "/dev/stdin") > 0)
+ print $0
+
+ 'gawk' terminates the read operation if input does not arrive after
+waiting for the timeout period, returns failure, and sets 'ERRNO' to an
+appropriate string value. A negative or zero value for the timeout is
+the same as specifying no timeout at all.
+
+ A timeout can also be set for reading from the keyboard in the
+implicit loop that reads input records and matches them against
+patterns, like so:
+
+ $ gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }
+ > { print "You entered: " $0 }'
+ gawk
+ -| You entered: gawk
+
+ In this case, failure to respond within five seconds results in the
+following error message:
+
+ error-> gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input
file `-': Connection timed out
+
+ The timeout can be set or changed at any time, and will take effect
+on the next attempt to read from the input device. In the following
+example, we start with a timeout value of one second, and progressively
+reduce it by one-tenth of a second until we wait indefinitely for the
+input to arrive:
+
+ PROCINFO[Service, "READ_TIMEOUT"] = 1000
+ while ((Service |& getline) > 0) {
+ print $0
+ PROCINFO[Service, "READ_TIMEOUT"] -= 100
+ }
+
+ NOTE: You should not assume that the read operation will block
+ exactly after the tenth record has been printed. It is possible
+ that 'gawk' will read and buffer more than one record's worth of
+ data the first time. Because of this, changing the value of
+ timeout like in the preceding example is not very useful.
+
+ If the 'PROCINFO' element is not present and the 'GAWK_READ_TIMEOUT'
+environment variable exists, 'gawk' uses its value to initialize the
+timeout value. The exclusive use of the environment variable to specify
+timeout has the disadvantage of not being able to control it on a
+per-command or per-connection basis.
+
+ 'gawk' considers a timeout event to be an error even though the
+attempt to read from the underlying device may succeed in a later
+attempt. This is a limitation, and it also means that you cannot use
+this to multiplex input from two or more sources. *Note Retrying
+Input:: for a way to enable later I/O attempts to succeed.
+
+ Assigning a timeout value prevents read operations from blocking
+indefinitely. But bear in mind that there are other ways 'gawk' can
+stall waiting for an input device to be ready. A network client can
+sometimes take a long time to establish a connection before it can start
+reading any data, or the attempt to open a FIFO special file for reading
+can block indefinitely until some other process opens it for writing.
+
+ ---------- Footnotes ----------
+
+ (1) This assumes that standard input is the keyboard.
+
+
+File: gawk.info, Node: Retrying Input, Next: Command-line directories,
Prev: Read Timeout, Up: Reading Files
+
+4.12 Retrying Reads After Certain Input Errors
+==============================================
+
+This minor node describes a feature that is specific to 'gawk'.
+
+ When 'gawk' encounters an error while reading input, by default
+'getline' returns -1, and subsequent attempts to read from that file
+result in an end-of-file indication. However, you may optionally
+instruct 'gawk' to allow I/O to be retried when certain errors are
+encountered by setting a special element in the 'PROCINFO' array (*note
+Auto-set::):
+
+ PROCINFO["INPUT_NAME", "RETRY"] = 1
+
+ When this element exists, 'gawk' checks the value of the system (C
+language) 'errno' variable when an I/O error occurs. If 'errno'
+indicates a subsequent I/O attempt may succeed, 'getline' instead
+returns -2 and further calls to 'getline' may succeed. This applies to
+the 'errno' values 'EAGAIN', 'EWOULDBLOCK', 'EINTR', or 'ETIMEDOUT'.
+
+ This feature is useful in conjunction with 'PROCINFO["INPUT_NAME",
+"READ_TIMEOUT"]' or situations where a file descriptor has been
+configured to behave in a non-blocking fashion.
+
+
+File: gawk.info, Node: Command-line directories, Next: Input Summary, Prev:
Retrying Input, Up: Reading Files
+
+4.13 Directories on the Command Line
+====================================
+
+According to the POSIX standard, files named on the 'awk' command line
+must be text files; it is a fatal error if they are not. Most versions
+of 'awk' treat a directory on the command line as a fatal error.
+
+ By default, 'gawk' produces a warning for a directory on the command
+line, but otherwise ignores it. This makes it easier to use shell
+wildcards with your 'awk' program:
+
+ $ gawk -f whizprog.awk * Directories could kill this program
+
+ If either of the '--posix' or '--traditional' options is given, then
+'gawk' reverts to treating a directory on the command line as a fatal
+error.
+
+ *Note Extension Sample Readdir:: for a way to treat directories as
+usable data from an 'awk' program.
+
+
+File: gawk.info, Node: Input Summary, Next: Input Exercises, Prev:
Command-line directories, Up: Reading Files
+
+4.14 Summary
+============
+
+ * Input is split into records based on the value of 'RS'. The
+ possibilities are as follows:
+
+ Value of 'RS' Records are split on 'awk' / 'gawk'
+ ...
+
---------------------------------------------------------------------------
+ Any single That character 'awk'
+ character
+ The empty string Runs of two or more 'awk'
+ ('""') newlines
+ A regexp Text that matches the 'gawk'
+ regexp
+
+ * 'FNR' indicates how many records have been read from the current
+ input file; 'NR' indicates how many records have been read in
+ total.
+
+ * 'gawk' sets 'RT' to the text matched by 'RS'.
+
+ * After splitting the input into records, 'awk' further splits the
+ records into individual fields, named '$1', '$2', and so on. '$0'
+ is the whole record, and 'NF' indicates how many fields there are.
+ The default way to split fields is between whitespace characters.
+
+ * Fields may be referenced using a variable, as in '$NF'. Fields may
+ also be assigned values, which causes the value of '$0' to be
+ recomputed when it is later referenced. Assigning to a field with
+ a number greater than 'NF' creates the field and rebuilds the
+ record, using 'OFS' to separate the fields. Incrementing 'NF' does
+ the same thing. Decrementing 'NF' throws away fields and rebuilds
+ the record.
+
+ * Field splitting is more complicated than record splitting:
+
+ Field separator value Fields are split ... 'awk' /
+ 'gawk'
+
---------------------------------------------------------------------------
+ 'FS == " "' On runs of whitespace 'awk'
+ 'FS == ANY SINGLE On that character 'awk'
+ CHARACTER'
+ 'FS == REGEXP' On text matching the regexp 'awk'
+ 'FS == ""' Such that each individual 'gawk'
+ character is a separate
+ field
+ 'FIELDWIDTHS == LIST OF Based on character position 'gawk'
+ COLUMNS'
+ 'FPAT == REGEXP' On the text surrounding 'gawk'
+ text matching the regexp
+
+ * Using 'FS = "\n"' causes the entire record to be a single field
+ (assuming that newlines separate records).
+
+ * 'FS' may be set from the command line using the '-F' option. This
+ can also be done using command-line variable assignment.
+
+ * Use 'PROCINFO["FS"]' to see how fields are being split.
+
+ * Use 'getline' in its various forms to read additional records from
+ the default input stream, from a file, or from a pipe or coprocess.
+
+ * Use 'PROCINFO[FILE, "READ_TIMEOUT"]' to cause reads to time out for
+ FILE.
+
+ * Directories on the command line are fatal for standard 'awk';
+ 'gawk' ignores them if not in POSIX mode.
+
+
+File: gawk.info, Node: Input Exercises, Prev: Input Summary, Up: Reading
Files
+
+4.15 Exercises
+==============
+
+ 1. Using the 'FIELDWIDTHS' variable (*note Constant Size::), write a
+ program to read election data, where each record represents one
+ voter's votes. Come up with a way to define which columns are
+ associated with each ballot item, and print the total votes,
+ including abstentions, for each item.
+
+
+File: gawk.info, Node: Printing, Next: Expressions, Prev: Reading Files,
Up: Top
+
+5 Printing Output
+*****************
+
+One of the most common programming actions is to "print", or output,
+some or all of the input. Use the 'print' statement for simple output,
+and the 'printf' statement for fancier formatting. The 'print'
+statement is not limited when computing _which_ values to print.
+However, with two exceptions, you cannot specify _how_ to print
+them--how many columns, whether to use exponential notation or not, and
+so on. (For the exceptions, *note Output Separators:: and *note
+OFMT::.) For printing with specifications, you need the 'printf'
+statement (*note Printf::).
+
+ Besides basic and formatted printing, this major node also covers I/O
+redirections to files and pipes, introduces the special file names that
+'gawk' processes internally, and discusses the 'close()' built-in
+function.
+
+* Menu:
+
+* Print:: The 'print' statement.
+* Print Examples:: Simple examples of 'print' statements.
+* Output Separators:: The output separators and how to change them.
+* OFMT:: Controlling Numeric Output With 'print'.
+* Printf:: The 'printf' statement.
+* Redirection:: How to redirect output to multiple files and
+ pipes.
+* Special FD:: Special files for I/O.
+* Special Files:: File name interpretation in 'gawk'.
+ 'gawk' allows access to inherited file
+ descriptors.
+* Close Files And Pipes:: Closing Input and Output Files and Pipes.
+* Nonfatal:: Enabling Nonfatal Output.
+* Output Summary:: Output summary.
+* Output Exercises:: Exercises.
+
+
+File: gawk.info, Node: Print, Next: Print Examples, Up: Printing
+
+5.1 The 'print' Statement
+=========================
+
+Use the 'print' statement to produce output with simple, standardized
+formatting. You specify only the strings or numbers to print, in a list
+separated by commas. They are output, separated by single spaces,
+followed by a newline. The statement looks like this:
+
+ print ITEM1, ITEM2, ...
+
+The entire list of items may be optionally enclosed in parentheses. The
+parentheses are necessary if any of the item expressions uses the '>'
+relational operator; otherwise it could be confused with an output
+redirection (*note Redirection::).
+
+ The items to print can be constant strings or numbers, fields of the
+current record (such as '$1'), variables, or any 'awk' expression.
+Numeric values are converted to strings and then printed.
+
+ The simple statement 'print' with no items is equivalent to 'print
+$0': it prints the entire current record. To print a blank line, use
+'print ""'. To print a fixed piece of text, use a string constant, such
+as '"Don't Panic"', as one item. If you forget to use the double-quote
+characters, your text is taken as an 'awk' expression, and you will
+probably get an error. Keep in mind that a space is printed between any
+two items.
+
+ Note that the 'print' statement is a statement and not an
+expression--you can't use it in the pattern part of a pattern-action
+statement, for example.
+
+
+File: gawk.info, Node: Print Examples, Next: Output Separators, Prev:
Print, Up: Printing
+
+5.2 'print' Statement Examples
+==============================
+
+Each 'print' statement makes at least one line of output. However, it
+isn't limited to only one line. If an item value is a string containing
+a newline, the newline is output along with the rest of the string. A
+single 'print' statement can make any number of lines this way.
+
+ The following is an example of printing a string that contains
+embedded newlines (the '\n' is an escape sequence, used to represent the
+newline character; *note Escape Sequences::):
+
+ $ awk 'BEGIN { print "line one\nline two\nline three" }'
+ -| line one
+ -| line two
+ -| line three
+
+ The next example, which is run on the 'inventory-shipped' file,
+prints the first two fields of each input record, with a space between
+them:
+
+ $ awk '{ print $1, $2 }' inventory-shipped
+ -| Jan 13
+ -| Feb 15
+ -| Mar 15
+ ...
+
+ A common mistake in using the 'print' statement is to omit the comma
+between two items. This often has the effect of making the items run
+together in the output, with no space. The reason for this is that
+juxtaposing two string expressions in 'awk' means to concatenate them.
+Here is the same program, without the comma:
+
+ $ awk '{ print $1 $2 }' inventory-shipped
+ -| Jan13
+ -| Feb15
+ -| Mar15
+ ...
+
+ To someone unfamiliar with the 'inventory-shipped' file, neither
+example's output makes much sense. A heading line at the beginning
+would make it clearer. Let's add some headings to our table of months
+('$1') and green crates shipped ('$2'). We do this using a 'BEGIN' rule
+(*note BEGIN/END::) so that the headings are only printed once:
+
+ awk 'BEGIN { print "Month Crates"
+ print "----- ------" }
+ { print $1, $2 }' inventory-shipped
+
+When run, the program prints the following:
+
+ Month Crates
+ ----- ------
+ Jan 13
+ Feb 15
+ Mar 15
+ ...
+
+The only problem, however, is that the headings and the table data don't
+line up! We can fix this by printing some spaces between the two
+fields:
+
+ awk 'BEGIN { print "Month Crates"
+ print "----- ------" }
+ { print $1, " ", $2 }' inventory-shipped
+
+ Lining up columns this way can get pretty complicated when there are
+many columns to fix. Counting spaces for two or three columns is
+simple, but any more than this can take up a lot of time. This is why
+the 'printf' statement was created (*note Printf::); one of its
+specialties is lining up columns of data.
+
+ NOTE: You can continue either a 'print' or 'printf' statement
+ simply by putting a newline after any comma (*note
+ Statements/Lines::).
+
+
+File: gawk.info, Node: Output Separators, Next: OFMT, Prev: Print Examples,
Up: Printing
+
+5.3 Output Separators
+=====================
+
+As mentioned previously, a 'print' statement contains a list of items
+separated by commas. In the output, the items are normally separated by
+single spaces. However, this doesn't need to be the case; a single
+space is simply the default. Any string of characters may be used as
+the "output field separator" by setting the predefined variable 'OFS'.
+The initial value of this variable is the string '" "' (i.e., a single
+space).
+
+ The output from an entire 'print' statement is called an "output
+record". Each 'print' statement outputs one output record, and then
+outputs a string called the "output record separator" (or 'ORS'). The
+initial value of 'ORS' is the string '"\n"' (i.e., a newline character).
+Thus, each 'print' statement normally makes a separate line.
+
+ In order to change how output fields and records are separated,
+assign new values to the variables 'OFS' and 'ORS'. The usual place to
+do this is in the 'BEGIN' rule (*note BEGIN/END::), so that it happens
+before any input is processed. It can also be done with assignments on
+the command line, before the names of the input files, or using the '-v'
+command-line option (*note Options::). The following example prints the
+first and second fields of each input record, separated by a semicolon,
+with a blank line added after each newline:
+
+ $ awk 'BEGIN { OFS = ";"; ORS = "\n\n" }
+ > { print $1, $2 }' mail-list
+ -| Amelia;555-5553
+ -|
+ -| Anthony;555-3412
+ -|
+ -| Becky;555-7685
+ -|
+ -| Bill;555-1675
+ -|
+ -| Broderick;555-0542
+ -|
+ -| Camilla;555-2912
+ -|
+ -| Fabius;555-1234
+ -|
+ -| Julie;555-6699
+ -|
+ -| Martin;555-6480
+ -|
+ -| Samuel;555-3430
+ -|
+ -| Jean-Paul;555-2127
+ -|
+
+ If the value of 'ORS' does not contain a newline, the program's
+output runs together on a single line.
+
+
+File: gawk.info, Node: OFMT, Next: Printf, Prev: Output Separators, Up:
Printing
+
+5.4 Controlling Numeric Output with 'print'
+===========================================
+
+When printing numeric values with the 'print' statement, 'awk'
+internally converts each number to a string of characters and prints
+that string. 'awk' uses the 'sprintf()' function to do this conversion
+(*note String Functions::). For now, it suffices to say that the
+'sprintf()' function accepts a "format specification" that tells it how
+to format numbers (or strings), and that there are a number of different
+ways in which numbers can be formatted. The different format
+specifications are discussed more fully in *note Control Letters::.
+
+ The predefined variable 'OFMT' contains the format specification that
+'print' uses with 'sprintf()' when it wants to convert a number to a
+string for printing. The default value of 'OFMT' is '"%.6g"'. The way
+'print' prints numbers can be changed by supplying a different format
+specification for the value of 'OFMT', as shown in the following
+example:
+
+ $ awk 'BEGIN {
+ > OFMT = "%.0f" # print numbers as integers (rounds)
+ > print 17.23, 17.54 }'
+ -| 17 18
+
+According to the POSIX standard, 'awk''s behavior is undefined if 'OFMT'
+contains anything but a floating-point conversion specification. (d.c.)
+
+
+File: gawk.info, Node: Printf, Next: Redirection, Prev: OFMT, Up: Printing
+
+5.5 Using 'printf' Statements for Fancier Printing
+==================================================
+
+For more precise control over the output format than what is provided by
+'print', use 'printf'. With 'printf' you can specify the width to use
+for each item, as well as various formatting choices for numbers (such
+as what output base to use, whether to print an exponent, whether to
+print a sign, and how many digits to print after the decimal point).
+
+* Menu:
+
+* Basic Printf:: Syntax of the 'printf' statement.
+* Control Letters:: Format-control letters.
+* Format Modifiers:: Format-specification modifiers.
+* Printf Examples:: Several examples.
+
+
+File: gawk.info, Node: Basic Printf, Next: Control Letters, Up: Printf
+
+5.5.1 Introduction to the 'printf' Statement
+--------------------------------------------
+
+A simple 'printf' statement looks like this:
+
+ printf FORMAT, ITEM1, ITEM2, ...
+
+As for 'print', the entire list of arguments may optionally be enclosed
+in parentheses. Here too, the parentheses are necessary if any of the
+item expressions uses the '>' relational operator; otherwise, it can be
+confused with an output redirection (*note Redirection::).
+
+ The difference between 'printf' and 'print' is the FORMAT argument.
+This is an expression whose value is taken as a string; it specifies how
+to output each of the other arguments. It is called the "format
+string".
+
+ The format string is very similar to that in the ISO C library
+function 'printf()'. Most of FORMAT is text to output verbatim.
+Scattered among this text are "format specifiers"--one per item. Each
+format specifier says to output the next item in the argument list at
+that place in the format.
+
+ The 'printf' statement does not automatically append a newline to its
+output. It outputs only what the format string specifies. So if a
+newline is needed, you must include one in the format string. The
+output separator variables 'OFS' and 'ORS' have no effect on 'printf'
+statements. For example:
+
+ $ awk 'BEGIN {
+ > ORS = "\nOUCH!\n"; OFS = "+"
+ > msg = "Don\47t Panic!"
+ > printf "%s\n", msg
+ > }'
+ -| Don't Panic!
+
+Here, neither the '+' nor the 'OUCH!' appears in the output message.
+
+
+File: gawk.info, Node: Control Letters, Next: Format Modifiers, Prev: Basic
Printf, Up: Printf
+
+5.5.2 Format-Control Letters
+----------------------------
+
+A format specifier starts with the character '%' and ends with a
+"format-control letter"--it tells the 'printf' statement how to output
+one item. The format-control letter specifies what _kind_ of value to
+print. The rest of the format specifier is made up of optional
+"modifiers" that control _how_ to print the value, such as the field
+width. Here is a list of the format-control letters:
+
+'%a', '%A'
+ A floating point number of the form ['-']'0xH.HHHHp+-DD' (C99
+ hexadecimal floating point format). For '%A', uppercase letters
+ are used instead of lowercase ones.
+
+ NOTE: The current POSIX standard requires support for '%a' and
+ '%A' in 'awk'. As far as we know, besides 'gawk', the only
+ other version of 'awk' that actually implements it is BWK
+ 'awk'. It's use is thus highly nonportable!
+
+ Furthermore, these formats are not available on any system
+ where the underlying C library 'printf()' function does not
+ support them. As of this writing, among current systems, only
+ OpenVMS is known to not support them.
+
+'%c'
+ Print a number as a character; thus, 'printf "%c", 65' outputs the
+ letter 'A'. The output for a string value is the first character
+ of the string.
+
+ NOTE: The POSIX standard says the first character of a string
+ is printed. In locales with multibyte characters, 'gawk'
+ attempts to convert the leading bytes of the string into a
+ valid wide character and then to print the multibyte encoding
+ of that character. Similarly, when printing a numeric value,
+ 'gawk' allows the value to be within the numeric range of
+ values that can be held in a wide character. If the
+ conversion to multibyte encoding fails, 'gawk' uses the low
+ eight bits of the value as the character to print.
+
+ Other 'awk' versions generally restrict themselves to printing
+ the first byte of a string or to numeric values within the
+ range of a single byte (0-255). (d.c.)
+
+'%d', '%i'
+ Print a decimal integer. The two control letters are equivalent.
+ (The '%i' specification is for compatibility with ISO C.)
+
+'%e', '%E'
+ Print a number in scientific (exponential) notation. For example:
+
+ printf "%4.3e\n", 1950
+
+ prints '1.950e+03', with a total of four significant figures, three
+ of which follow the decimal point. (The '4.3' represents two
+ modifiers, discussed in the next node.) '%E' uses 'E' instead of
+ 'e' in the output.
+
+'%f'
+ Print a number in floating-point notation. For example:
+
+ printf "%4.3f", 1950
+
+ prints '1950.000', with a minimum of four significant figures,
+ three of which follow the decimal point. (The '4.3' represents two
+ modifiers, discussed in the next node.)
+
+ On systems supporting IEEE 754 floating-point format, values
+ representing negative infinity are formatted as '-inf' or
+ '-infinity', and positive infinity as 'inf' or 'infinity'. The
+ special "not a number" value formats as '-nan' or 'nan' (*note
+ Strange values::).
+
+'%F'
+ Like '%f', but the infinity and "not a number" values are spelled
+ using uppercase letters.
+
+ The '%F' format is a POSIX extension to ISO C; not all systems
+ support it. On those that don't, 'gawk' uses '%f' instead.
+
+'%g', '%G'
+ Print a number in either scientific notation or in floating-point
+ notation, whichever uses fewer characters; if the result is printed
+ in scientific notation, '%G' uses 'E' instead of 'e'.
+
+'%o'
+ Print an unsigned octal integer (*note Nondecimal-numbers::).
+
+'%s'
+ Print a string.
+
+'%u'
+ Print an unsigned decimal integer. (This format is of marginal
+ use, because all numbers in 'awk' are floating point; it is
+ provided primarily for compatibility with C.)
+
+'%x', '%X'
+ Print an unsigned hexadecimal integer; '%X' uses the letters 'A'
+ through 'F' instead of 'a' through 'f' (*note
+ Nondecimal-numbers::).
+
+'%%'
+ Print a single '%'. This does not consume an argument and it
+ ignores any modifiers.
+
+ NOTE: When using the integer format-control letters for values that
+ are outside the range of the widest C integer type, 'gawk' switches
+ to the '%g' format specifier. If '--lint' is provided on the
+ command line (*note Options::), 'gawk' warns about this. Other
+ versions of 'awk' may print invalid values or do something else
+ entirely. (d.c.)
+
+ NOTE: The IEEE 754 standard for floating-point arithmetic allows
+ for special values that represent "infinity" (positive and
+ negative) and values that are "not a number" (NaN).
+
+ Input and output of these values occurs as text strings. This is
+ somewhat problematic for the 'awk' language, which predates the
+ IEEE standard. Further details are provided in *note POSIX
+ Floating Point Problems::; please see there.
+
+
+File: gawk.info, Node: Format Modifiers, Next: Printf Examples, Prev:
Control Letters, Up: Printf
+
+5.5.3 Modifiers for 'printf' Formats
+------------------------------------
+
+A format specification can also include "modifiers" that can control how
+much of the item's value is printed, as well as how much space it gets.
+The modifiers come between the '%' and the format-control letter. We
+use the bullet symbol "*" in the following examples to represent spaces
+in the output. Here are the possible modifiers, in the order in which
+they may appear:
+
+'N$'
+ An integer constant followed by a '$' is a "positional specifier".
+ Normally, format specifications are applied to arguments in the
+ order given in the format string. With a positional specifier, the
+ format specification is applied to a specific argument, instead of
+ what would be the next argument in the list. Positional specifiers
+ begin counting with one. Thus:
+
+ printf "%s %s\n", "don't", "panic"
+ printf "%2$s %1$s\n", "panic", "don't"
+
+ prints the famous friendly message twice.
+
+ At first glance, this feature doesn't seem to be of much use. It
+ is in fact a 'gawk' extension, intended for use in translating
+ messages at runtime. *Note Printf Ordering::, which describes how
+ and why to use positional specifiers. For now, we ignore them.
+
+'-' (Minus)
+ The minus sign, used before the width modifier (see later on in
+ this list), says to left-justify the argument within its specified
+ width. Normally, the argument is printed right-justified in the
+ specified width. Thus:
+
+ printf "%-4s", "foo"
+
+ prints 'foo*'.
+
+SPACE
+ For numeric conversions, prefix positive values with a space and
+ negative values with a minus sign.
+
+'+'
+ The plus sign, used before the width modifier (see later on in this
+ list), says to always supply a sign for numeric conversions, even
+ if the data to format is positive. The '+' overrides the space
+ modifier.
+
+'#'
+ Use an "alternative form" for certain control letters. For '%o',
+ supply a leading zero. For '%x' and '%X', supply a leading '0x' or
+ '0X' for a nonzero result. For '%e', '%E', '%f', and '%F', the
+ result always contains a decimal point. For '%g' and '%G',
+ trailing zeros are not removed from the result.
+
+'0'
+ A leading '0' (zero) acts as a flag indicating that output should
+ be padded with zeros instead of spaces. This applies only to the
+ numeric output formats. This flag only has an effect when the
+ field width is wider than the value to print.
+
+'''
+ A single quote or apostrophe character is a POSIX extension to ISO
+ C. It indicates that the integer part of a floating-point value, or
+ the entire part of an integer decimal value, should have a
+ thousands-separator character in it. This only works in locales
+ that support such characters. For example:
+
+ $ cat thousands.awk Show source program
+ -| BEGIN { printf "%'d\n", 1234567 }
+ $ LC_ALL=C gawk -f thousands.awk
+ -| 1234567 Results in "C" locale
+ $ LC_ALL=en_US.UTF-8 gawk -f thousands.awk
+ -| 1,234,567 Results in US English UTF locale
+
+ For more information about locales and internationalization issues,
+ see *note Locales::.
+
+ NOTE: The ''' flag is a nice feature, but its use complicates
+ things: it becomes difficult to use it in command-line
+ programs. For information on appropriate quoting tricks, see
+ *note Quoting::.
+
+WIDTH
+ This is a number specifying the desired minimum width of a field.
+ Inserting any number between the '%' sign and the format-control
+ character forces the field to expand to this width. The default
+ way to do this is to pad with spaces on the left. For example:
+
+ printf "%4s", "foo"
+
+ prints '*foo'.
+
+ The value of WIDTH is a minimum width, not a maximum. If the item
+ value requires more than WIDTH characters, it can be as wide as
+ necessary. Thus, the following:
+
+ printf "%4s", "foobar"
+
+ prints 'foobar'.
+
+ Preceding the WIDTH with a minus sign causes the output to be
+ padded with spaces on the right, instead of on the left.
+
+'.PREC'
+ A period followed by an integer constant specifies the precision to
+ use when printing. The meaning of the precision varies by control
+ letter:
+
+ '%d', '%i', '%o', '%u', '%x', '%X'
+ Minimum number of digits to print.
+
+ '%e', '%E', '%f', '%F'
+ Number of digits to the right of the decimal point.
+
+ '%g', '%G'
+ Maximum number of significant digits.
+
+ '%s'
+ Maximum number of characters from the string that should
+ print.
+
+ Thus, the following:
+
+ printf "%.4s", "foobar"
+
+ prints 'foob'.
+
+ The C library 'printf''s dynamic WIDTH and PREC capability (e.g.,
+'"%*.*s"') is supported. Instead of supplying explicit WIDTH and/or
+PREC values in the format string, they are passed in the argument list.
+For example:
+
+ w = 5
+ p = 3
+ s = "abcdefg"
+ printf "%*.*s\n", w, p, s
+
+is exactly equivalent to:
+
+ s = "abcdefg"
+ printf "%5.3s\n", s
+
+Both programs output '**abc'. Earlier versions of 'awk' did not support
+this capability. If you must use such a version, you may simulate this
+feature by using concatenation to build up the format string, like so:
+
+ w = 5
+ p = 3
+ s = "abcdefg"
+ printf "%" w "." p "s\n", s
+
+This is not particularly easy to read, but it does work.
+
+ C programmers may be used to supplying additional modifiers ('h',
+'j', 'l', 'L', 't', and 'z') in 'printf' format strings. These are not
+valid in 'awk'. Most 'awk' implementations silently ignore them. If
+'--lint' is provided on the command line (*note Options::), 'gawk' warns
+about their use. If '--posix' is supplied, their use is a fatal error.
+
+
+File: gawk.info, Node: Printf Examples, Prev: Format Modifiers, Up: Printf
+
+5.5.4 Examples Using 'printf'
+-----------------------------
+
+The following simple example shows how to use 'printf' to make an
+aligned table:
+
+ awk '{ printf "%-10s %s\n", $1, $2 }' mail-list
+
+This command prints the names of the people ('$1') in the file
+'mail-list' as a string of 10 characters that are left-justified. It
+also prints the phone numbers ('$2') next on the line. This produces an
+aligned two-column table of names and phone numbers, as shown here:
+
+ $ awk '{ printf "%-10s %s\n", $1, $2 }' mail-list
+ -| Amelia 555-5553
+ -| Anthony 555-3412
+ -| Becky 555-7685
+ -| Bill 555-1675
+ -| Broderick 555-0542
+ -| Camilla 555-2912
+ -| Fabius 555-1234
+ -| Julie 555-6699
+ -| Martin 555-6480
+ -| Samuel 555-3430
+ -| Jean-Paul 555-2127
+
+ In this case, the phone numbers had to be printed as strings because
+the numbers are separated by dashes. Printing the phone numbers as
+numbers would have produced just the first three digits: '555'. This
+would have been pretty confusing.
+
+ It wasn't necessary to specify a width for the phone numbers because
+they are last on their lines. They don't need to have spaces after
+them.
+
+ The table could be made to look even nicer by adding headings to the
+tops of the columns. This is done using a 'BEGIN' rule (*note
+BEGIN/END::) so that the headers are only printed once, at the beginning
+of the 'awk' program:
+
+ awk 'BEGIN { print "Name Number"
+ print "---- ------" }
+ { printf "%-10s %s\n", $1, $2 }' mail-list
+
+ The preceding example mixes 'print' and 'printf' statements in the
+same program. Using just 'printf' statements can produce the same
+results:
+
+ awk 'BEGIN { printf "%-10s %s\n", "Name", "Number"
+ printf "%-10s %s\n", "----", "------" }
+ { printf "%-10s %s\n", $1, $2 }' mail-list
+
+Printing each column heading with the same format specification used for
+the column elements ensures that the headings are aligned just like the
+columns.
+
+ The fact that the same format specification is used three times can
+be emphasized by storing it in a variable, like this:
+
+ awk 'BEGIN { format = "%-10s %s\n"
+ printf format, "Name", "Number"
+ printf format, "----", "------" }
+ { printf format, $1, $2 }' mail-list
+
+
+File: gawk.info, Node: Redirection, Next: Special FD, Prev: Printf, Up:
Printing
+
+5.6 Redirecting Output of 'print' and 'printf'
+==============================================
+
+So far, the output from 'print' and 'printf' has gone to the standard
+output, usually the screen. Both 'print' and 'printf' can also send
+their output to other places. This is called "redirection".
+
+ NOTE: When '--sandbox' is specified (*note Options::), redirecting
+ output to files, pipes, and coprocesses is disabled.
+
+ A redirection appears after the 'print' or 'printf' statement.
+Redirections in 'awk' are written just like redirections in shell
+commands, except that they are written inside the 'awk' program.
+
+ There are four forms of output redirection: output to a file, output
+appended to a file, output through a pipe to another command, and output
+to a coprocess. We show them all for the 'print' statement, but they
+work identically for 'printf':
+
+'print ITEMS > OUTPUT-FILE'
+ This redirection prints the items into the output file named
+ OUTPUT-FILE. The file name OUTPUT-FILE can be any expression. Its
+ value is changed to a string and then used as a file name (*note
+ Expressions::).
+
+ When this type of redirection is used, the OUTPUT-FILE is erased
+ before the first output is written to it. Subsequent writes to the
+ same OUTPUT-FILE do not erase OUTPUT-FILE, but append to it. (This
+ is different from how you use redirections in shell scripts.) If
+ OUTPUT-FILE does not exist, it is created. For example, here is
+ how an 'awk' program can write a list of peoples' names to one file
+ named 'name-list', and a list of phone numbers to another file
+ named 'phone-list':
+
+ $ awk '{ print $2 > "phone-list"
+ > print $1 > "name-list" }' mail-list
+ $ cat phone-list
+ -| 555-5553
+ -| 555-3412
+ ...
+ $ cat name-list
+ -| Amelia
+ -| Anthony
+ ...
+
+ Each output file contains one name or number per line.
+
+'print ITEMS >> OUTPUT-FILE'
+ This redirection prints the items into the preexisting output file
+ named OUTPUT-FILE. The difference between this and the single-'>'
+ redirection is that the old contents (if any) of OUTPUT-FILE are
+ not erased. Instead, the 'awk' output is appended to the file. If
+ OUTPUT-FILE does not exist, then it is created.
+
+'print ITEMS | COMMAND'
+ It is possible to send output to another program through a pipe
+ instead of into a file. This redirection opens a pipe to COMMAND,
+ and writes the values of ITEMS through this pipe to another process
+ created to execute COMMAND.
+
+ The redirection argument COMMAND is actually an 'awk' expression.
+ Its value is converted to a string whose contents give the shell
+ command to be run. For example, the following produces two files,
+ one unsorted list of peoples' names, and one list sorted in reverse
+ alphabetical order:
+
+ awk '{ print $1 > "names.unsorted"
+ command = "sort -r > names.sorted"
+ print $1 | command }' mail-list
+
+ The unsorted list is written with an ordinary redirection, while
+ the sorted list is written by piping through the 'sort' utility.
+
+ The next example uses redirection to mail a message to the mailing
+ list 'bug-system'. This might be useful when trouble is
+ encountered in an 'awk' script run periodically for system
+ maintenance:
+
+ report = "mail bug-system"
+ print("Awk script failed:", $0) | report
+ print("at record number", FNR, "of", FILENAME) | report
+ close(report)
+
+ The 'close()' function is called here because it's a good idea to
+ close the pipe as soon as all the intended output has been sent to
+ it. *Note Close Files And Pipes:: for more information.
+
+ This example also illustrates the use of a variable to represent a
+ FILE or COMMAND--it is not necessary to always use a string
+ constant. Using a variable is generally a good idea, because (if
+ you mean to refer to that same file or command) 'awk' requires that
+ the string value be written identically every time.
+
+'print ITEMS |& COMMAND'
+ This redirection prints the items to the input of COMMAND. The
+ difference between this and the single-'|' redirection is that the
+ output from COMMAND can be read with 'getline'. Thus, COMMAND is a
+ "coprocess", which works together with but is subsidiary to the
+ 'awk' program.
+
+ This feature is a 'gawk' extension, and is not available in POSIX
+ 'awk'. *Note Getline/Coprocess::, for a brief discussion. *Note
+ Two-way I/O::, for a more complete discussion.
+
+ Redirecting output using '>', '>>', '|', or '|&' asks the system to
+open a file, pipe, or coprocess only if the particular FILE or COMMAND
+you specify has not already been written to by your program or if it has
+been closed since it was last written to.
+
+ It is a common error to use '>' redirection for the first 'print' to
+a file, and then to use '>>' for subsequent output:
+
+ # clear the file
+ print "Don't panic" > "guide.txt"
+ ...
+ # append
+ print "Avoid improbability generators" >> "guide.txt"
+
+This is indeed how redirections must be used from the shell. But in
+'awk', it isn't necessary. In this kind of case, a program should use
+'>' for all the 'print' statements, because the output file is only
+opened once. (It happens that if you mix '>' and '>>' output is
+produced in the expected order. However, mixing the operators for the
+same file is definitely poor style, and is confusing to readers of your
+program.)
+
+ Many older 'awk' implementations limit the number of pipelines that
+an 'awk' program may have open to just one! In 'gawk', there is no such
+limit. 'gawk' allows a program to open as many pipelines as the
+underlying operating system permits.
+
+ Piping into 'sh'
+
+ A particularly powerful way to use redirection is to build command
+lines and pipe them into the shell, 'sh'. For example, suppose you have
+a list of files brought over from a system where all the file names are
+stored in uppercase, and you wish to rename them to have names in all
+lowercase. The following program is both simple and efficient:
+
+ { printf("mv %s %s\n", $0, tolower($0)) | "sh" }
+
+ END { close("sh") }
+
+ The 'tolower()' function returns its argument string with all
+uppercase characters converted to lowercase (*note String Functions::).
+The program builds up a list of command lines, using the 'mv' utility to
+rename the files. It then sends the list to the shell for execution.
+
+ *Note Shell Quoting:: for a function that can help in generating
+command lines to be fed to the shell.
+
+
+File: gawk.info, Node: Special FD, Next: Special Files, Prev: Redirection,
Up: Printing
+
+5.7 Special Files for Standard Preopened Data Streams
+=====================================================
+
+Running programs conventionally have three input and output streams
+already available to them for reading and writing. These are known as
+the "standard input", "standard output", and "standard error output".
+These open streams (and any other open files or pipes) are often
+referred to by the technical term "file descriptors".
+
+ These streams are, by default, connected to your keyboard and screen,
+but they are often redirected with the shell, via the '<', '<<', '>',
+'>>', '>&', and '|' operators. Standard error is typically used for
+writing error messages; the reason there are two separate streams,
+standard output and standard error, is so that they can be redirected
+separately.
+
+ In traditional implementations of 'awk', the only way to write an
+error message to standard error in an 'awk' program is as follows:
+
+ print "Serious error detected!" | "cat 1>&2"
+
+This works by opening a pipeline to a shell command that can access the
+standard error stream that it inherits from the 'awk' process. This is
+far from elegant, and it also requires a separate process. So people
+writing 'awk' programs often don't do this. Instead, they send the
+error messages to the screen, like this:
+
+ print "Serious error detected!" > "/dev/tty"
+
+('/dev/tty' is a special file supplied by the operating system that is
+connected to your keyboard and screen. It represents the "terminal,"(1)
+which on modern systems is a keyboard and screen, not a serial console.)
+This generally has the same effect, but not always: although the
+standard error stream is usually the screen, it can be redirected; when
+that happens, writing to the screen is not correct. In fact, if 'awk'
+is run from a background job, it may not have a terminal at all. Then
+opening '/dev/tty' fails.
+
+ 'gawk', BWK 'awk', and 'mawk' provide special file names for
+accessing the three standard streams. If the file name matches one of
+these special names when 'gawk' (or one of the others) redirects input
+or output, then it directly uses the descriptor that the file name
+stands for. These special file names work for all operating systems
+that 'gawk' has been ported to, not just those that are POSIX-compliant:
+
+'/dev/stdin'
+ The standard input (file descriptor 0).
+
+'/dev/stdout'
+ The standard output (file descriptor 1).
+
+'/dev/stderr'
+ The standard error output (file descriptor 2).
+
+ With these facilities, the proper way to write an error message then
+becomes:
+
+ print "Serious error detected!" > "/dev/stderr"
+
+ Note the use of quotes around the file name. Like with any other
+redirection, the value must be a string. It is a common error to omit
+the quotes, which leads to confusing results.
+
+ 'gawk' does not treat these file names as special when in
+POSIX-compatibility mode. However, because BWK 'awk' supports them,
+'gawk' does support them even when invoked with the '--traditional'
+option (*note Options::).
+
+ ---------- Footnotes ----------
+
+ (1) The "tty" in '/dev/tty' stands for "Teletype," a serial terminal.
+
+
+File: gawk.info, Node: Special Files, Next: Close Files And Pipes, Prev:
Special FD, Up: Printing
+
+5.8 Special File names in 'gawk'
+================================
+
+Besides access to standard input, standard output, and standard error,
+'gawk' provides access to any open file descriptor. Additionally, there
+are special file names reserved for TCP/IP networking.
+
+* Menu:
+
+* Other Inherited Files:: Accessing other open files with
+ 'gawk'.
+* Special Network:: Special files for network communications.
+* Special Caveats:: Things to watch out for.
+
+
+File: gawk.info, Node: Other Inherited Files, Next: Special Network, Up:
Special Files
+
+5.8.1 Accessing Other Open Files with 'gawk'
+--------------------------------------------
+
+Besides the '/dev/stdin', '/dev/stdout', and '/dev/stderr' special file
+names mentioned earlier, 'gawk' provides syntax for accessing any other
+inherited open file:
+
+'/dev/fd/N'
+ The file associated with file descriptor N. Such a file must be
+ opened by the program initiating the 'awk' execution (typically the
+ shell). Unless special pains are taken in the shell from which
+ 'gawk' is invoked, only descriptors 0, 1, and 2 are available.
+
+ The file names '/dev/stdin', '/dev/stdout', and '/dev/stderr' are
+essentially aliases for '/dev/fd/0', '/dev/fd/1', and '/dev/fd/2',
+respectively. However, those names are more self-explanatory.
+
+ Note that using 'close()' on a file name of the form '"/dev/fd/N"',
+for file descriptor numbers above two, does actually close the given
+file descriptor.
+
+
+File: gawk.info, Node: Special Network, Next: Special Caveats, Prev: Other
Inherited Files, Up: Special Files
+
+5.8.2 Special Files for Network Communications
+----------------------------------------------
+
+'gawk' programs can open a two-way TCP/IP connection, acting as either a
+client or a server. This is done using a special file name of the form:
+
+ /NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT
+
+ The NET-TYPE is one of 'inet', 'inet4', or 'inet6'. The PROTOCOL is
+one of 'tcp' or 'udp', and the other fields represent the other
+essential pieces of information for making a networking connection.
+These file names are used with the '|&' operator for communicating with
+a coprocess (*note Two-way I/O::). This is an advanced feature,
+mentioned here only for completeness. Full discussion is delayed until
+*note TCP/IP Networking::.
+
+
+File: gawk.info, Node: Special Caveats, Prev: Special Network, Up: Special
Files
+
+5.8.3 Special File name Caveats
+-------------------------------
+
+Here are some things to bear in mind when using the special file names
+that 'gawk' provides:
+
+ * Recognition of the file names for the three standard preopened
+ files is disabled only in POSIX mode.
+
+ * Recognition of the other special file names is disabled if 'gawk'
+ is in compatibility mode (either '--traditional' or '--posix';
+ *note Options::).
+
+ * 'gawk' _always_ interprets these special file names. For example,
+ using '/dev/fd/4' for output actually writes on file descriptor 4,
+ and not on a new file descriptor that is 'dup()'ed from file
+ descriptor 4. Most of the time this does not matter; however, it
+ is important to _not_ close any of the files related to file
+ descriptors 0, 1, and 2. Doing so results in unpredictable
+ behavior.
+
+
+File: gawk.info, Node: Close Files And Pipes, Next: Nonfatal, Prev: Special
Files, Up: Printing
+
+5.9 Closing Input and Output Redirections
+=========================================
+
+If the same file name or the same shell command is used with 'getline'
+more than once during the execution of an 'awk' program (*note
+Getline::), the file is opened (or the command is executed) the first
+time only. At that time, the first record of input is read from that
+file or command. The next time the same file or command is used with
+'getline', another record is read from it, and so on.
+
+ Similarly, when a file or pipe is opened for output, 'awk' remembers
+the file name or command associated with it, and subsequent writes to
+the same file or command are appended to the previous writes. The file
+or pipe stays open until 'awk' exits.
+
+ This implies that special steps are necessary in order to read the
+same file again from the beginning, or to rerun a shell command (rather
+than reading more output from the same command). The 'close()' function
+makes these things possible:
+
+ close(FILENAME)
+
+or:
+
+ close(COMMAND)
+
+ The argument FILENAME or COMMAND can be any expression. Its value
+must _exactly_ match the string that was used to open the file or start
+the command (spaces and other "irrelevant" characters included). For
+example, if you open a pipe with this:
+
+ "sort -r names" | getline foo
+
+then you must close it with this:
+
+ close("sort -r names")
+
+ Once this function call is executed, the next 'getline' from that
+file or command, or the next 'print' or 'printf' to that file or
+command, reopens the file or reruns the command. Because the expression
+that you use to close a file or pipeline must exactly match the
+expression used to open the file or run the command, it is good practice
+to use a variable to store the file name or command. The previous
+example becomes the following:
+
+ sortcom = "sort -r names"
+ sortcom | getline foo
+ ...
+ close(sortcom)
+
+This helps avoid hard-to-find typographical errors in your 'awk'
+programs. Here are some of the reasons for closing an output file:
+
+ * To write a file and read it back later on in the same 'awk'
+ program. Close the file after writing it, then begin reading it
+ with 'getline'.
+
+ * To write numerous files, successively, in the same 'awk' program.
+ If the files aren't closed, eventually 'awk' may exceed a system
+ limit on the number of open files in one process. It is best to
+ close each one when the program has finished writing it.
+
+ * To make a command finish. When output is redirected through a
+ pipe, the command reading the pipe normally continues to try to
+ read input as long as the pipe is open. Often this means the
+ command cannot really do its work until the pipe is closed. For
+ example, if output is redirected to the 'mail' program, the message
+ is not actually sent until the pipe is closed.
+
+ * To run the same program a second time, with the same arguments.
+ This is not the same thing as giving more input to the first run!
+
+ For example, suppose a program pipes output to the 'mail' program.
+ If it outputs several lines redirected to this pipe without closing
+ it, they make a single message of several lines. By contrast, if
+ the program closes the pipe after each line of output, then each
+ line makes a separate message.
+
+ If you use more files than the system allows you to have open, 'gawk'
+attempts to multiplex the available open files among your data files.
+'gawk''s ability to do this depends upon the facilities of your
+operating system, so it may not always work. It is therefore both good
+practice and good portability advice to always use 'close()' on your
+files when you are done with them. In fact, if you are using a lot of
+pipes, it is essential that you close commands when done. For example,
+consider something like this:
+
+ {
+ ...
+ command = ("grep " $1 " /some/file | my_prog -q " $3)
+ while ((command | getline) > 0) {
+ PROCESS OUTPUT OF command
+ }
+ # need close(command) here
+ }
+
+ This example creates a new pipeline based on data in _each_ record.
+Without the call to 'close()' indicated in the comment, 'awk' creates
+child processes to run the commands, until it eventually runs out of
+file descriptors for more pipelines.
+
+ Even though each command has finished (as indicated by the
+end-of-file return status from 'getline'), the child process is not
+terminated;(1) more importantly, the file descriptor for the pipe is not
+closed and released until 'close()' is called or 'awk' exits.
+
+ 'close()' silently does nothing if given an argument that does not
+represent a file, pipe, or coprocess that was opened with a redirection.
+In such a case, it returns a negative value, indicating an error. In
+addition, 'gawk' sets 'ERRNO' to a string indicating the error.
+
+ Note also that 'close(FILENAME)' has no "magic" effects on the
+implicit loop that reads through the files named on the command line.
+It is, more likely, a close of a file that was never opened with a
+redirection, so 'awk' silently does nothing, except return a negative
+value.
+
+ When using the '|&' operator to communicate with a coprocess, it is
+occasionally useful to be able to close one end of the two-way pipe
+without closing the other. This is done by supplying a second argument
+to 'close()'. As in any other call to 'close()', the first argument is
+the name of the command or special file used to start the coprocess.
+The second argument should be a string, with either of the values '"to"'
+or '"from"'. Case does not matter. As this is an advanced feature,
+discussion is delayed until *note Two-way I/O::, which describes it in
+more detail and gives an example.
+
+ Using 'close()''s Return Value
+
+ In many older versions of Unix 'awk', the 'close()' function is
+actually a statement. (d.c.) It is a syntax error to try and use the
+return value from 'close()':
+
+ command = "..."
+ command | getline info
+ retval = close(command) # syntax error in many Unix awks
+
+ 'gawk' treats 'close()' as a function. The return value is -1 if the
+argument names something that was never opened with a redirection, or if
+there is a system problem closing the file or process. In these cases,
+'gawk' sets the predefined variable 'ERRNO' to a string describing the
+problem.
+
+ In 'gawk', starting with version 4.2, when closing a pipe or
+coprocess (input or output), the return value is the exit status of the
+command, as described in *note Table 5.1:
+table-close-pipe-return-values.(2) Otherwise, it is the return value
+from the system's 'close()' or 'fclose()' C functions when closing input
+or output files, respectively. This value is zero if the close
+succeeds, or -1 if it fails.
+
+
+Situation Return value from 'close()'
+--------------------------------------------------------------------------
+Normal exit of command Command's exit status
+Death by signal of command 256 + number of murderous signal
+Death by signal of command with 512 + number of murderous signal
+core dump
+Some kind of error -1
+
+Table 5.1: Return values from 'close()' of a pipe
+
+ The POSIX standard is very vague; it says that 'close()' returns zero
+on success and a nonzero value otherwise. In general, different
+implementations vary in what they report when closing pipes; thus, the
+return value cannot be used portably. (d.c.) In POSIX mode (*note
+Options::), 'gawk' just returns zero when closing a pipe.
+
+ ---------- Footnotes ----------
+
+ (1) The technical terminology is rather morbid. The finished child
+is called a "zombie," and cleaning up after it is referred to as
+"reaping."
+
+ (2) Prior to version 4.2, the return value from closing a pipe or
+co-process was the full 16-bit exit value as defined by the 'wait()'
+system call.
+
+
+File: gawk.info, Node: Nonfatal, Next: Output Summary, Prev: Close Files
And Pipes, Up: Printing
+
+5.10 Enabling Nonfatal Output
+=============================
+
+This minor node describes a 'gawk'-specific feature.
+
+ In standard 'awk', output with 'print' or 'printf' to a nonexistent
+file, or some other I/O error (such as filling up the disk) is a fatal
+error.
+
+ $ gawk 'BEGIN { print "hi" > "/no/such/file" }'
+ error-> gawk: cmd. line:1: fatal: can't redirect to `/no/such/file' (No
+ error-> such file or directory)
+
+ 'gawk' makes it possible to detect that an error has occurred,
+allowing you to possibly recover from the error, or at least print an
+error message of your choosing before exiting. You can do this in one
+of two ways:
+
+ * For all output files, by assigning any value to
+ 'PROCINFO["NONFATAL"]'.
+
+ * On a per-file basis, by assigning any value to 'PROCINFO[FILENAME,
+ "NONFATAL"]'. Here, FILENAME is the name of the file to which you
+ wish output to be nonfatal.
+
+ Once you have enabled nonfatal output, you must check 'ERRNO' after
+every relevant 'print' or 'printf' statement to see if something went
+wrong. It is also a good idea to initialize 'ERRNO' to zero before
+attempting the output. For example:
+
+ $ gawk '
+ > BEGIN {
+ > PROCINFO["NONFATAL"] = 1
+ > ERRNO = 0
+ > print "hi" > "/no/such/file"
+ > if (ERRNO) {
+ > print("Output failed:", ERRNO) > "/dev/stderr"
+ > exit 1
+ > }
+ > }'
+ error-> Output failed: No such file or directory
+
+ Here, 'gawk' did not produce a fatal error; instead it let the 'awk'
+program code detect the problem and handle it.
+
+ This mechanism works also for standard output and standard error.
+For standard output, you may use 'PROCINFO["-", "NONFATAL"]' or
+'PROCINFO["/dev/stdout", "NONFATAL"]'. For standard error, use
+'PROCINFO["/dev/stderr", "NONFATAL"]'.
+
+ When attempting to open a TCP/IP socket (*note TCP/IP Networking::),
+'gawk' tries multiple times. The 'GAWK_SOCK_RETRIES' environment
+variable (*note Other Environment Variables::) allows you to override
+'gawk''s builtin default number of attempts. However, once nonfatal I/O
+is enabled for a given socket, 'gawk' only retries once, relying on
+'awk'-level code to notice that there was a problem.
+
+
+File: gawk.info, Node: Output Summary, Next: Output Exercises, Prev:
Nonfatal, Up: Printing
+
+5.11 Summary
+============
+
+ * The 'print' statement prints comma-separated expressions. Each
+ expression is separated by the value of 'OFS' and terminated by the
+ value of 'ORS'. 'OFMT' provides the conversion format for numeric
+ values for the 'print' statement.
+
+ * The 'printf' statement provides finer-grained control over output,
+ with format-control letters for different data types and various
+ flags that modify the behavior of the format-control letters.
+
+ * Output from both 'print' and 'printf' may be redirected to files,
+ pipes, and coprocesses.
+
+ * 'gawk' provides special file names for access to standard input,
+ output, and error, and for network communications.
+
+ * Use 'close()' to close open file, pipe, and coprocess redirections.
+ For coprocesses, it is possible to close only one direction of the
+ communications.
+
+ * Normally errors with 'print' or 'printf' are fatal. 'gawk' lets
+ you make output errors be nonfatal either for all files or on a
+ per-file basis. You must then check for errors after every
+ relevant output statement.
+
+
+File: gawk.info, Node: Output Exercises, Prev: Output Summary, Up: Printing
+
+5.12 Exercises
+==============
+
+ 1. Rewrite the program:
+
+ awk 'BEGIN { print "Month Crates"
+ print "----- ------" }
+ { print $1, " ", $2 }' inventory-shipped
+
+ from *note Output Separators::, by using a new value of 'OFS'.
+
+ 2. Use the 'printf' statement to line up the headings and table data
+ for the 'inventory-shipped' example that was covered in *note
+ Print::.
+
+ 3. What happens if you forget the double quotes when redirecting
+ output, as follows:
+
+ BEGIN { print "Serious error detected!" > /dev/stderr }
+
+
+File: gawk.info, Node: Expressions, Next: Patterns and Actions, Prev:
Printing, Up: Top
+
+6 Expressions
+*************
+
+Expressions are the basic building blocks of 'awk' patterns and actions.
+An expression evaluates to a value that you can print, test, or pass to
+a function. Additionally, an expression can assign a new value to a
+variable or a field by using an assignment operator.
+
+ An expression can serve as a pattern or action statement on its own.
+Most other kinds of statements contain one or more expressions that
+specify the data on which to operate. As in other languages,
+expressions in 'awk' can include variables, array references, constants,
+and function calls, as well as combinations of these with various
+operators.
+
+* Menu:
+
+* Values:: Constants, Variables, and Regular Expressions.
+* All Operators:: 'gawk''s operators.
+* Truth Values and Conditions:: Testing for true and false.
+* Function Calls:: A function call is an expression.
+* Precedence:: How various operators nest.
+* Locales:: How the locale affects things.
+* Expressions Summary:: Expressions summary.
+
+
+File: gawk.info, Node: Values, Next: All Operators, Up: Expressions
+
+6.1 Constants, Variables, and Conversions
+=========================================
+
+Expressions are built up from values and the operations performed upon
+them. This minor node describes the elementary objects that provide the
+values used in expressions.
+
+* Menu:
+
+* Constants:: String, numeric and regexp constants.
+* Using Constant Regexps:: When and how to use a regexp constant.
+* Variables:: Variables give names to values for later use.
+* Conversion:: The conversion of strings to numbers and vice
+ versa.
+
+
+File: gawk.info, Node: Constants, Next: Using Constant Regexps, Up: Values
+
+6.1.1 Constant Expressions
+--------------------------
+
+The simplest type of expression is the "constant", which always has the
+same value. There are three types of constants: numeric, string, and
+regular expression.
+
+ Each is used in the appropriate context when you need a data value
+that isn't going to change. Numeric constants can have different forms,
+but are internally stored in an identical manner.
+
+* Menu:
+
+* Scalar Constants:: Numeric and string constants.
+* Nondecimal-numbers:: What are octal and hex numbers.
+* Regexp Constants:: Regular Expression constants.
+
+
+File: gawk.info, Node: Scalar Constants, Next: Nondecimal-numbers, Up:
Constants
+
+6.1.1.1 Numeric and String Constants
+....................................
+
+A "numeric constant" stands for a number. This number can be an
+integer, a decimal fraction, or a number in scientific (exponential)
+notation.(1) Here are some examples of numeric constants that all have
+the same value:
+
+ 105
+ 1.05e+2
+ 1050e-1
+
+ A "string constant" consists of a sequence of characters enclosed in
+double quotation marks. For example:
+
+ "parrot"
+
+represents the string whose contents are 'parrot'. Strings in 'gawk'
+can be of any length, and they can contain any of the possible eight-bit
+ASCII characters, including ASCII NUL (character code zero). Other
+'awk' implementations may have difficulty with some character codes.
+
+ Some languages allow you to continue long strings across multiple
+lines by ending the line with a backslash. For example in C:
+
+ #include <stdio.h>
+
+ int main()
+ {
+ printf("hello, \
+ world\n");
+ return 0;
+ }
+
+In such a case, the C compiler removes both the backslash and the
+newline, producing a string as if it had been typed '"hello, world\n"'.
+This is useful when a single string needs to contain a large amount of
+text.
+
+ The POSIX standard says explicitly that newlines are not allowed
+inside string constants. And indeed, all 'awk' implementations report
+an error if you try to do so. For example:
+
+ $ gawk 'BEGIN { print "hello,
+ > world" }'
+ -| gawk: cmd. line:1: BEGIN { print "hello,
+ -| gawk: cmd. line:1: ^ unterminated string
+ -| gawk: cmd. line:1: BEGIN { print "hello,
+ -| gawk: cmd. line:1: ^ syntax error
+
+ Although POSIX doesn't define what happens if you use an escaped
+newline, as in the previous C example, all known versions of 'awk' allow
+you to do so. Unfortunately, what each one does with such a string
+varies. (d.c.) 'gawk', 'mawk', and the OpenSolaris POSIX 'awk' (*note
+Other Versions::) elide the backslash and newline, as in C:
+
+ $ gawk 'BEGIN { print "hello, \
+ > world" }'
+ -| hello, world
+
+ In POSIX mode (*note Options::), 'gawk' does not allow escaped
+newlines. Otherwise, it behaves as just described.
+
+ BWK 'awk' and BusyBox 'awk' remove the backslash but leave the
+newline intact, as part of the string:
+
+ $ nawk 'BEGIN { print "hello, \
+ > world" }'
+ -| hello,
+ -| world
+
+ ---------- Footnotes ----------
+
+ (1) The internal representation of all numbers, including integers,
+uses double-precision floating-point numbers. On most modern systems,
+these are in IEEE 754 standard format. *Note Arbitrary Precision
+Arithmetic::, for much more information.
+
+
+File: gawk.info, Node: Nondecimal-numbers, Next: Regexp Constants, Prev:
Scalar Constants, Up: Constants
+
+6.1.1.2 Octal and Hexadecimal Numbers
+.....................................
+
+In 'awk', all numbers are in decimal (i.e., base 10). Many other
+programming languages allow you to specify numbers in other bases, often
+octal (base 8) and hexadecimal (base 16). In octal, the numbers go 0,
+1, 2, 3, 4, 5, 6, 7, 10, 11, 12, and so on. Just as '11' in decimal is
+1 times 10 plus 1, so '11' in octal is 1 times 8 plus 1. This equals 9
+in decimal. In hexadecimal, there are 16 digits. Because the everyday
+decimal number system only has ten digits ('0'-'9'), the letters 'a'
+through 'f' represent the rest. (Case in the letters is usually
+irrelevant; hexadecimal 'a' and 'A' have the same value.) Thus, '11' in
+hexadecimal is 1 times 16 plus 1, which equals 17 in decimal.
+
+ Just by looking at plain '11', you can't tell what base it's in. So,
+in C, C++, and other languages derived from C, there is a special
+notation to signify the base. Octal numbers start with a leading '0',
+and hexadecimal numbers start with a leading '0x' or '0X':
+
+'11'
+ Decimal value 11
+
+'011'
+ Octal 11, decimal value 9
+
+'0x11'
+ Hexadecimal 11, decimal value 17
+
+ This example shows the difference:
+
+ $ gawk 'BEGIN { printf "%d, %d, %d\n", 011, 11, 0x11 }'
+ -| 9, 11, 17
+
+ Being able to use octal and hexadecimal constants in your programs is
+most useful when working with data that cannot be represented
+conveniently as characters or as regular numbers, such as binary data of
+various sorts.
+
+ 'gawk' allows the use of octal and hexadecimal constants in your
+program text. However, such numbers in the input data are not treated
+differently; doing so by default would break old programs. (If you
+really need to do this, use the '--non-decimal-data' command-line
+option; *note Nondecimal Data::.) If you have octal or hexadecimal
+data, you can use the 'strtonum()' function (*note String Functions::)
+to convert the data into a number. Most of the time, you will want to
+use octal or hexadecimal constants when working with the built-in
+bit-manipulation functions; see *note Bitwise Functions:: for more
+information.
+
+ Unlike in some early C implementations, '8' and '9' are not valid in
+octal constants. For example, 'gawk' treats '018' as decimal 18:
+
+ $ gawk 'BEGIN { print "021 is", 021 ; print 018 }'
+ -| 021 is 17
+ -| 18
+
+ Octal and hexadecimal source code constants are a 'gawk' extension.
+If 'gawk' is in compatibility mode (*note Options::), they are not
+available.
+
+ A Constant's Base Does Not Affect Its Value
+
+ Once a numeric constant has been converted internally into a number,
+'gawk' no longer remembers what the original form of the constant was;
+the internal value is always used. This has particular consequences for
+conversion of numbers to strings:
+
+ $ gawk 'BEGIN { printf "0x11 is <%s>\n", 0x11 }'
+ -| 0x11 is <17>
+
+
+File: gawk.info, Node: Regexp Constants, Prev: Nondecimal-numbers, Up:
Constants
+
+6.1.1.3 Regular Expression Constants
+....................................
+
+A "regexp constant" is a regular expression description enclosed in
+slashes, such as '/^beginning and end$/'. Most regexps used in 'awk'
+programs are constant, but the '~' and '!~' matching operators can also
+match computed or dynamic regexps (which are typically just ordinary
+strings or variables that contain a regexp, but could be more complex
+expressions).
+
+
+File: gawk.info, Node: Using Constant Regexps, Next: Variables, Prev:
Constants, Up: Values
+
+6.1.2 Using Regular Expression Constants
+----------------------------------------
+
+Regular expression constants consist of text describing a regular
+expression enclosed in slashes (such as '/the +answer/'). This minor
+node describes how such constants work in POSIX 'awk' and 'gawk', and
+then goes on to describe "strongly typed regexp constants", which are a
+'gawk' extension.
+
+* Menu:
+
+* Standard Regexp Constants:: Regexp constants in standard 'awk'.
+* Strong Regexp Constants:: Strongly typed regexp constants.
+
+
+File: gawk.info, Node: Standard Regexp Constants, Next: Strong Regexp
Constants, Up: Using Constant Regexps
+
+6.1.2.1 Standard Regular Expression Constants
+.............................................
+
+When used on the righthand side of the '~' or '!~' operators, a regexp
+constant merely stands for the regexp that is to be matched. However,
+regexp constants (such as '/foo/') may be used like simple expressions.
+When a regexp constant appears by itself, it has the same meaning as if
+it appeared in a pattern (i.e., '($0 ~ /foo/)'). (d.c.) *Note
+Expression Patterns::. This means that the following two code segments:
+
+ if ($0 ~ /barfly/ || $0 ~ /camelot/)
+ print "found"
+
+and:
+
+ if (/barfly/ || /camelot/)
+ print "found"
+
+are exactly equivalent. One rather bizarre consequence of this rule is
+that the following Boolean expression is valid, but does not do what its
+author probably intended:
+
+ # Note that /foo/ is on the left of the ~
+ if (/foo/ ~ $1) print "found foo"
+
+This code is "obviously" testing '$1' for a match against the regexp
+'/foo/'. But in fact, the expression '/foo/ ~ $1' really means '($0 ~
+/foo/) ~ $1'. In other words, first match the input record against the
+regexp '/foo/'. The result is either zero or one, depending upon the
+success or failure of the match. That result is then matched against
+the first field in the record. Because it is unlikely that you would
+ever really want to make this kind of test, 'gawk' issues a warning when
+it sees this construct in a program. Another consequence of this rule
+is that the assignment statement:
+
+ matches = /foo/
+
+assigns either zero or one to the variable 'matches', depending upon the
+contents of the current input record.
+
+ Constant regular expressions are also used as the first argument for
+the 'gensub()', 'sub()', and 'gsub()' functions, as the second argument
+of the 'match()' function, and as the third argument of the 'split()'
+and 'patsplit()' functions (*note String Functions::). Modern
+implementations of 'awk', including 'gawk', allow the third argument of
+'split()' to be a regexp constant, but some older implementations do
+not. (d.c.) Because some built-in functions accept regexp constants as
+arguments, confusion can arise when attempting to use regexp constants
+as arguments to user-defined functions (*note User-defined::). For
+example:
+
+ function mysub(pat, repl, str, global)
+ {
+ if (global)
+ gsub(pat, repl, str)
+ else
+ sub(pat, repl, str)
+ return str
+ }
+
+ {
+ ...
+ text = "hi! hi yourself!"
+ mysub(/hi/, "howdy", text, 1)
+ ...
+ }
+
+ In this example, the programmer wants to pass a regexp constant to
+the user-defined function 'mysub()', which in turn passes it on to
+either 'sub()' or 'gsub()'. However, what really happens is that the
+'pat' parameter is assigned a value of either one or zero, depending
+upon whether or not '$0' matches '/hi/'. 'gawk' issues a warning when
+it sees a regexp constant used as a parameter to a user-defined
+function, because passing a truth value in this way is probably not what
+was intended.
+
+
+File: gawk.info, Node: Strong Regexp Constants, Prev: Standard Regexp
Constants, Up: Using Constant Regexps
+
+6.1.2.2 Strongly Typed Regexp Constants
+.......................................
+
+This minor node describes a 'gawk'-specific feature.
+
+ As we saw in the previous minor node, regexp constants ('/.../') hold
+a strange position in the 'awk' language. In most contexts, they act
+like an expression: '$0 ~ /.../'. In other contexts, they denote only a
+regexp to be matched. In no case are they really a "first class
+citizen" of the language. That is, you cannot define a scalar variable
+whose type is "regexp" in the same sense that you can define a variable
+to be a number or a string:
+
+ num = 42 Numeric variable
+ str = "hi" String variable
+ re = /foo/ Wrong! re is the result of $0 ~ /foo/
+
+ For a number of more advanced use cases, it would be nice to have
+regexp constants that are "strongly typed"; in other words, that denote
+a regexp useful for matching, and not an expression.
+
+ 'gawk' provides this feature. A strongly typed regexp constant looks
+almost like a regular regexp constant, except that it is preceded by an
+'@' sign:
+
+ re = @/foo/ Regexp variable
+
+ Strongly typed regexp constants _cannot_ be used everywhere that a
+regular regexp constant can, because this would make the language even
+more confusing. Instead, you may use them only in certain contexts:
+
+ * On the righthand side of the '~' and '!~' operators: 'some_var ~
+ @/foo/' (*note Regexp Usage::).
+
+ * In the 'case' part of a 'switch' statement (*note Switch
+ Statement::).
+
+ * As an argument to one of the built-in functions that accept regexp
+ constants: 'gensub()', 'gsub()', 'match()', 'patsplit()',
+ 'split()', and 'sub()' (*note String Functions::).
+
+ * As a parameter in a call to a user-defined function (*note
+ User-defined::).
+
+ * As the return value of a user-defined function.
+
+ * On the righthand side of an assignment to a variable: 'some_var =
+ @/foo/'. In this case, the type of 'some_var' is regexp.
+ Additionally, 'some_var' can be used with '~' and '!~', passed to
+ one of the built-in functions listed above, or passed as a
+ parameter to a user-defined function.
+
+ You may use the '-v' option (*note Options::) to assign a
+strongly-typed regexp constant to a variable on the command line, like
+so:
+
+ gawk -v pattern='@/something(interesting)+/' ...
+
+You may also make such assignments as regular command-line arguments
+(*note Other Arguments::).
+
+ You may use the 'typeof()' built-in function (*note Type Functions::)
+to determine if a variable or function parameter is a regexp variable.
+
+ The true power of this feature comes from the ability to create
+variables that have regexp type. Such variables can be passed on to
+user-defined functions, without the confusing aspects of computed
+regular expressions created from strings or string constants. They may
+also be passed through indirect function calls (*note Indirect Calls::)
+and on to the built-in functions that accept regexp constants.
+
+ When used in numeric conversions, strongly typed regexp variables
+convert to zero. When used in string conversions, they convert to the
+string value of the original regexp text.
+
+ There is an additional, interesting corner case. When used as the
+third argument to 'sub()' or 'gsub()', they retain their type. Thus, if
+you have something like this:
+
+ re = @/don't panic/
+ sub(/don't/, "do", re)
+ print typeof(re), re
+
+then 're' retains its type, but now attempts to match the string 'do
+panic'. This provides a (very indirect) way to create regexp-typed
+variables at runtime.
+
+
+File: gawk.info, Node: Variables, Next: Conversion, Prev: Using Constant
Regexps, Up: Values
+
+6.1.3 Variables
+---------------
+
+"Variables" are ways of storing values at one point in your program for
+use later in another part of your program. They can be manipulated
+entirely within the program text, and they can also be assigned values
+on the 'awk' command line.
+
+* Menu:
+
+* Using Variables:: Using variables in your programs.
+* Assignment Options:: Setting variables on the command line and a
+ summary of command-line syntax. This is an
+ advanced method of input.
+
+
+File: gawk.info, Node: Using Variables, Next: Assignment Options, Up:
Variables
+
+6.1.3.1 Using Variables in a Program
+....................................
+
+Variables let you give names to values and refer to them later.
+Variables have already been used in many of the examples. The name of a
+variable must be a sequence of letters, digits, or underscores, and it
+may not begin with a digit. Here, a "letter" is any one of the 52
+upper- and lowercase English letters. Other characters that may be
+defined as letters in non-English locales are not valid in variable
+names. Case is significant in variable names; 'a' and 'A' are distinct
+variables.
+
+ A variable name is a valid expression by itself; it represents the
+variable's current value. Variables are given new values with
+"assignment operators", "increment operators", and "decrement operators"
+(*note Assignment Ops::). In addition, the 'sub()' and 'gsub()'
+functions can change a variable's value, and the 'match()', 'split()',
+and 'patsplit()' functions can change the contents of their array
+parameters (*note String Functions::).
+
+ A few variables have special built-in meanings, such as 'FS' (the
+field separator) and 'NF' (the number of fields in the current input
+record). *Note Built-in Variables:: for a list of the predefined
+variables. These predefined variables can be used and assigned just
+like all other variables, but their values are also used or changed
+automatically by 'awk'. All predefined variables' names are entirely
+uppercase.
+
+ Variables in 'awk' can be assigned either numeric or string values.
+The kind of value a variable holds can change over the life of a
+program. By default, variables are initialized to the empty string,
+which is zero if converted to a number. There is no need to explicitly
+initialize a variable in 'awk', which is what you would do in C and in
+most other traditional languages.
+
+
+File: gawk.info, Node: Assignment Options, Prev: Using Variables, Up:
Variables
+
+6.1.3.2 Assigning Variables on the Command Line
+...............................................
+
+Any 'awk' variable can be set by including a "variable assignment" among
+the arguments on the command line when 'awk' is invoked (*note Other
+Arguments::). Such an assignment has the following form:
+
+ VARIABLE=TEXT
+
+With it, a variable is set either at the beginning of the 'awk' run or
+in between input files. When the assignment is preceded with the '-v'
+option, as in the following:
+
+ -v VARIABLE=TEXT
+
+the variable is set at the very beginning, even before the 'BEGIN' rules
+execute. The '-v' option and its assignment must precede all the file
+name arguments, as well as the program text. (*Note Options:: for more
+information about the '-v' option.) Otherwise, the variable assignment
+is performed at a time determined by its position among the input file
+arguments--after the processing of the preceding input file argument.
+For example:
+
+ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
+
+prints the value of field number 'n' for all input records. Before the
+first file is read, the command line sets the variable 'n' equal to
+four. This causes the fourth field to be printed in lines from
+'inventory-shipped'. After the first file has finished, but before the
+second file is started, 'n' is set to two, so that the second field is
+printed in lines from 'mail-list':
+
+ $ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
+ -| 15
+ -| 24
+ ...
+ -| 555-5553
+ -| 555-3412
+ ...
+
+ Command-line arguments are made available for explicit examination by
+the 'awk' program in the 'ARGV' array (*note ARGC and ARGV::). 'awk'
+processes the values of command-line assignments for escape sequences
+(*note Escape Sequences::). (d.c.)
+
+ Normally, variables assigned on the command line (with or without the
+'-v' option) are treated as strings. When such variables are used as
+numbers, 'awk''s normal automatic conversion of strings to numbers takes
+place, and everything "just works."
+
+ However, 'gawk' supports variables whose types are "regexp". You can
+assign variables of this type using the following syntax:
+
+ gawk -v 're1=@/foo|bar/' '...' /path/to/file1 're2=@/baz|quux/'
/path/to/file2
+
+Strongly typed regexps are an advanced feature (*note Strong Regexp
+Constants::). We mention them here only for completeness.
+
+
+File: gawk.info, Node: Conversion, Prev: Variables, Up: Values
+
+6.1.4 Conversion of Strings and Numbers
+---------------------------------------
+
+Number-to-string and string-to-number conversion are generally
+straightforward. There can be subtleties to be aware of; this minor
+node discusses this important facet of 'awk'.
+
+* Menu:
+
+* Strings And Numbers:: How 'awk' Converts Between Strings And
+ Numbers.
+* Locale influences conversions:: How the locale may affect conversions.
+
+
+File: gawk.info, Node: Strings And Numbers, Next: Locale influences
conversions, Up: Conversion
+
+6.1.4.1 How 'awk' Converts Between Strings and Numbers
+......................................................
+
+Strings are converted to numbers and numbers are converted to strings,
+if the context of the 'awk' program demands it. For example, if the
+value of either 'foo' or 'bar' in the expression 'foo + bar' happens to
+be a string, it is converted to a number before the addition is
+performed. If numeric values appear in string concatenation, they are
+converted to strings. Consider the following:
+
+ two = 2; three = 3
+ print (two three) + 4
+
+This prints the (numeric) value 27. The numeric values of the variables
+'two' and 'three' are converted to strings and concatenated together.
+The resulting string is converted back to the number 23, to which 4 is
+then added.
+
+ If, for some reason, you need to force a number to be converted to a
+string, concatenate that number with the empty string, '""'. To force a
+string to be converted to a number, add zero to that string. A string
+is converted to a number by interpreting any numeric prefix of the
+string as numerals: '"2.5"' converts to 2.5, '"1e3"' converts to 1,000,
+and '"25fix"' has a numeric value of 25. Strings that can't be
+interpreted as valid numbers convert to zero.
+
+ The exact manner in which numbers are converted into strings is
+controlled by the 'awk' predefined variable 'CONVFMT' (*note Built-in
+Variables::). Numbers are converted using the 'sprintf()' function with
+'CONVFMT' as the format specifier (*note String Functions::).
+
+ 'CONVFMT''s default value is '"%.6g"', which creates a value with at
+most six significant digits. For some applications, you might want to
+change it to specify more precision. On most modern machines, 17 digits
+is usually enough to capture a floating-point number's value exactly.(1)
+
+ Strange results can occur if you set 'CONVFMT' to a string that
+doesn't tell 'sprintf()' how to format floating-point numbers in a
+useful way. For example, if you forget the '%' in the format, 'awk'
+converts all numbers to the same constant string.
+
+ As a special case, if a number is an integer, then the result of
+converting it to a string is _always_ an integer, no matter what the
+value of 'CONVFMT' may be. Given the following code fragment:
+
+ CONVFMT = "%2.2f"
+ a = 12
+ b = a ""
+
+'b' has the value '"12"', not '"12.00"'. (d.c.)
+
+ Pre-POSIX 'awk' Used 'OFMT' for String Conversion
+
+ Prior to the POSIX standard, 'awk' used the value of 'OFMT' for
+converting numbers to strings. 'OFMT' specifies the output format to
+use when printing numbers with 'print'. 'CONVFMT' was introduced in
+order to separate the semantics of conversion from the semantics of
+printing. Both 'CONVFMT' and 'OFMT' have the same default value:
+'"%.6g"'. In the vast majority of cases, old 'awk' programs do not
+change their behavior. *Note Print:: for more information on the
+'print' statement.
+
+ ---------- Footnotes ----------
+
+ (1) Pathological cases can require up to 752 digits (!), but we doubt
+that you need to worry about this.
+
+
+File: gawk.info, Node: Locale influences conversions, Prev: Strings And
Numbers, Up: Conversion
+
+6.1.4.2 Locales Can Influence Conversion
+........................................
+
+Where you are can matter when it comes to converting between numbers and
+strings. The local character set and language--the "locale"--can affect
+numeric formats. In particular, for 'awk' programs, it affects the
+decimal point character and the thousands-separator character. The
+'"C"' locale, and most English-language locales, use the period
+character ('.') as the decimal point and don't have a thousands
+separator. However, many (if not most) European and non-English locales
+use the comma (',') as the decimal point character. European locales
+often use either a space or a period as the thousands separator, if they
+have one.
+
+ The POSIX standard says that 'awk' always uses the period as the
+decimal point when reading the 'awk' program source code, and for
+command-line variable assignments (*note Other Arguments::). However,
+when interpreting input data, for 'print' and 'printf' output, and for
+number-to-string conversion, the local decimal point character is used.
+(d.c.) In all cases, numbers in source code and in input data cannot
+have a thousands separator. Here are some examples indicating the
+difference in behavior, on a GNU/Linux system:
+
+ $ export POSIXLY_CORRECT=1 Force POSIX behavior
+ $ gawk 'BEGIN { printf "%g\n", 3.1415927 }'
+ -| 3.14159
+ $ LC_ALL=en_DK.utf-8 gawk 'BEGIN { printf "%g\n", 3.1415927 }'
+ -| 3,14159
+ $ echo 4,321 | gawk '{ print $1 + 1 }'
+ -| 5
+ $ echo 4,321 | LC_ALL=en_DK.utf-8 gawk '{ print $1 + 1 }'
+ -| 5,321
+
+The 'en_DK.utf-8' locale is for English in Denmark, where the comma acts
+as the decimal point separator. In the normal '"C"' locale, 'gawk'
+treats '4,321' as 4, while in the Danish locale, it's treated as the
+full number including the fractional part, 4.321.
+
+ Some earlier versions of 'gawk' fully complied with this aspect of
+the standard. However, many users in non-English locales complained
+about this behavior, because their data used a period as the decimal
+point, so the default behavior was restored to use a period as the
+decimal point character. You can use the '--use-lc-numeric' option
+(*note Options::) to force 'gawk' to use the locale's decimal point
+character. ('gawk' also uses the locale's decimal point character when
+in POSIX mode, either via '--posix' or the 'POSIXLY_CORRECT' environment
+variable, as shown previously.)
+
+ *note Table 6.1: table-locale-affects. describes the cases in which
+the locale's decimal point character is used and when a period is used.
+Some of these features have not been described yet.
+
+
+Feature Default '--posix' or
+ '--use-lc-numeric'
+------------------------------------------------------------
+'%'g' Use locale Use locale
+'%g' Use period Use locale
+Input Use period Use locale
+'strtonum()'Use period Use locale
+
+Table 6.1: Locale decimal point versus a period
+
+ Finally, modern-day formal standards and the IEEE standard
+floating-point representation can have an unusual but important effect
+on the way 'gawk' converts some special string values to numbers. The
+details are presented in *note POSIX Floating Point Problems::.
+
+
+File: gawk.info, Node: All Operators, Next: Truth Values and Conditions,
Prev: Values, Up: Expressions
+
+6.2 Operators: Doing Something with Values
+==========================================
+
+This minor node introduces the "operators" that make use of the values
+provided by constants and variables.
+
+* Menu:
+
+* Arithmetic Ops:: Arithmetic operations ('+', '-',
+ etc.)
+* Concatenation:: Concatenating strings.
+* Assignment Ops:: Changing the value of a variable or a field.
+* Increment Ops:: Incrementing the numeric value of a variable.
+
+
+File: gawk.info, Node: Arithmetic Ops, Next: Concatenation, Up: All
Operators
+
+6.2.1 Arithmetic Operators
+--------------------------
+
+The 'awk' language uses the common arithmetic operators when evaluating
+expressions. All of these arithmetic operators follow normal precedence
+rules and work as you would expect them to.
+
+ The following example uses a file named 'grades', which contains a
+list of student names as well as three test scores per student (it's a
+small class):
+
+ Pat 100 97 58
+ Sandy 84 72 93
+ Chris 72 92 89
+
+This program takes the file 'grades' and prints the average of the
+scores:
+
+ $ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3
+ > print $1, avg }' grades
+ -| Pat 85
+ -| Sandy 83
+ -| Chris 84.3333
+
+ The following list provides the arithmetic operators in 'awk', in
+order from the highest precedence to the lowest:
+
+'X ^ Y'
+'X ** Y'
+ Exponentiation; X raised to the Y power. '2 ^ 3' has the value
+ eight; the character sequence '**' is equivalent to '^'. (c.e.)
+
+'- X'
+ Negation.
+
+'+ X'
+ Unary plus; the expression is converted to a number.
+
+'X * Y'
+ Multiplication.
+
+'X / Y'
+ Division; because all numbers in 'awk' are floating-point numbers,
+ the result is _not_ rounded to an integer--'3 / 4' has the value
+ 0.75. (It is a common mistake, especially for C programmers, to
+ forget that _all_ numbers in 'awk' are floating point, and that
+ division of integer-looking constants produces a real number, not
+ an integer.)
+
+'X % Y'
+ Remainder; further discussion is provided in the text, just after
+ this list.
+
+'X + Y'
+ Addition.
+
+'X - Y'
+ Subtraction.
+
+ Unary plus and minus have the same precedence, the multiplication
+operators all have the same precedence, and addition and subtraction
+have the same precedence.
+
+ When computing the remainder of 'X % Y', the quotient is rounded
+toward zero to an integer and multiplied by Y. This result is
+subtracted from X; this operation is sometimes known as "trunc-mod."
+The following relation always holds:
+
+ b * int(a / b) + (a % b) == a
+
+ One possibly undesirable effect of this definition of remainder is
+that 'X % Y' is negative if X is negative. Thus:
+
+ -17 % 8 = -1
+
+This definition is compliant with the POSIX standard, which says that
+the '%' operator produces results equivalent to using the standard C
+'fmod()' function, and that function in turn works as just described.
+
+ In other 'awk' implementations, the signedness of the remainder may
+be machine-dependent.
+
+ NOTE: The POSIX standard only specifies the use of '^' for
+ exponentiation. For maximum portability, do not use the '**'
+ operator.
+
+
+File: gawk.info, Node: Concatenation, Next: Assignment Ops, Prev:
Arithmetic Ops, Up: All Operators
+
+6.2.2 String Concatenation
+--------------------------
+
+ It seemed like a good idea at the time.
+ -- _Brian Kernighan_
+
+ There is only one string operation: concatenation. It does not have
+a specific operator to represent it. Instead, concatenation is
+performed by writing expressions next to one another, with no operator.
+For example:
+
+ $ awk '{ print "Field number one: " $1 }' mail-list
+ -| Field number one: Amelia
+ -| Field number one: Anthony
+ ...
+
+ Without the space in the string constant after the ':', the line runs
+together. For example:
+
+ $ awk '{ print "Field number one:" $1 }' mail-list
+ -| Field number one:Amelia
+ -| Field number one:Anthony
+ ...
+
+ Because string concatenation does not have an explicit operator, it
+is often necessary to ensure that it happens at the right time by using
+parentheses to enclose the items to concatenate. For example, you might
+expect that the following code fragment concatenates 'file' and 'name':
+
+ file = "file"
+ name = "name"
+ print "something meaningful" > file name
+
+This produces a syntax error with some versions of Unix 'awk'.(1) It is
+necessary to use the following:
+
+ print "something meaningful" > (file name)
+
+ Parentheses should be used around concatenation in all but the most
+common contexts, such as on the righthand side of '='. Be careful about
+the kinds of expressions used in string concatenation. In particular,
+the order of evaluation of expressions used for concatenation is
+undefined in the 'awk' language. Consider this example:
+
+ BEGIN {
+ a = "don't"
+ print (a " " (a = "panic"))
+ }
+
+It is not defined whether the second assignment to 'a' happens before or
+after the value of 'a' is retrieved for producing the concatenated
+value. The result could be either 'don't panic', or 'panic panic'.
+
+ The precedence of concatenation, when mixed with other operators, is
+often counter-intuitive. Consider this example:
+
+ $ awk 'BEGIN { print -12 " " -24 }'
+ -| -12-24
+
+ This "obviously" is concatenating -12, a space, and -24. But where
+did the space disappear to? The answer lies in the combination of
+operator precedences and 'awk''s automatic conversion rules. To get the
+desired result, write the program this way:
+
+ $ awk 'BEGIN { print -12 " " (-24) }'
+ -| -12 -24
+
+ This forces 'awk' to treat the '-' on the '-24' as unary. Otherwise,
+it's parsed as follows:
+
+ -12 ('" "' - 24)
+ => -12 (0 - 24)
+ => -12 (-24)
+ => -12-24
+
+ As mentioned earlier, when mixing concatenation with other operators,
+_parenthesize_. Otherwise, you're never quite sure what you'll get.
+
+ ---------- Footnotes ----------
+
+ (1) It happens that BWK 'awk', 'gawk', and 'mawk' all "get it right,"
+but you should not rely on this.
+
+
+File: gawk.info, Node: Assignment Ops, Next: Increment Ops, Prev:
Concatenation, Up: All Operators
+
+6.2.3 Assignment Expressions
+----------------------------
+
+An "assignment" is an expression that stores a (usually different) value
+into a variable. For example, let's assign the value one to the
+variable 'z':
+
+ z = 1
+
+ After this expression is executed, the variable 'z' has the value
+one. Whatever old value 'z' had before the assignment is forgotten.
+
+ Assignments can also store string values. For example, the following
+stores the value '"this food is good"' in the variable 'message':
+
+ thing = "food"
+ predicate = "good"
+ message = "this " thing " is " predicate
+
+This also illustrates string concatenation. The '=' sign is called an
+"assignment operator". It is the simplest assignment operator because
+the value of the righthand operand is stored unchanged. Most operators
+(addition, concatenation, and so on) have no effect except to compute a
+value. If the value isn't used, there's no reason to use the operator.
+An assignment operator is different; it does produce a value, but even
+if you ignore it, the assignment still makes itself felt through the
+alteration of the variable. We call this a "side effect".
+
+ The lefthand operand of an assignment need not be a variable (*note
+Variables::); it can also be a field (*note Changing Fields::) or an
+array element (*note Arrays::). These are all called "lvalues", which
+means they can appear on the lefthand side of an assignment operator.
+The righthand operand may be any expression; it produces the new value
+that the assignment stores in the specified variable, field, or array
+element. (Such values are called "rvalues".)
+
+ It is important to note that variables do _not_ have permanent types.
+A variable's type is simply the type of whatever value was last assigned
+to it. In the following program fragment, the variable 'foo' has a
+numeric value at first, and a string value later on:
+
+ foo = 1
+ print foo
+ foo = "bar"
+ print foo
+
+When the second assignment gives 'foo' a string value, the fact that it
+previously had a numeric value is forgotten.
+
+ String values that do not begin with a digit have a numeric value of
+zero. After executing the following code, the value of 'foo' is five:
+
+ foo = "a string"
+ foo = foo + 5
+
+ NOTE: Using a variable as a number and then later as a string can
+ be confusing and is poor programming style. The previous two
+ examples illustrate how 'awk' works, _not_ how you should write
+ your programs!
+
+ An assignment is an expression, so it has a value--the same value
+that is assigned. Thus, 'z = 1' is an expression with the value one.
+One consequence of this is that you can write multiple assignments
+together, such as:
+
+ x = y = z = 5
+
+This example stores the value five in all three variables ('x', 'y', and
+'z'). It does so because the value of 'z = 5', which is five, is stored
+into 'y' and then the value of 'y = z = 5', which is five, is stored
+into 'x'.
+
+ Assignments may be used anywhere an expression is called for. For
+example, it is valid to write 'x != (y = 1)' to set 'y' to one, and then
+test whether 'x' equals one. But this style tends to make programs hard
+to read; such nesting of assignments should be avoided, except perhaps
+in a one-shot program.
+
+ Aside from '=', there are several other assignment operators that do
+arithmetic with the old value of the variable. For example, the
+operator '+=' computes a new value by adding the righthand value to the
+old value of the variable. Thus, the following assignment adds five to
+the value of 'foo':
+
+ foo += 5
+
+This is equivalent to the following:
+
+ foo = foo + 5
+
+Use whichever makes the meaning of your program clearer.
+
+ There are situations where using '+=' (or any assignment operator) is
+_not_ the same as simply repeating the lefthand operand in the righthand
+expression. For example:
+
+ # Thanks to Pat Rankin for this example
+ BEGIN {
+ foo[rand()] += 5
+ for (x in foo)
+ print x, foo[x]
+
+ bar[rand()] = bar[rand()] + 5
+ for (x in bar)
+ print x, bar[x]
+ }
+
+The indices of 'bar' are practically guaranteed to be different, because
+'rand()' returns different values each time it is called. (Arrays and
+the 'rand()' function haven't been covered yet. *Note Arrays::, and
+*note Numeric Functions:: for more information.) This example
+illustrates an important fact about assignment operators: the lefthand
+expression is only evaluated _once_.
+
+ It is up to the implementation as to which expression is evaluated
+first, the lefthand or the righthand. Consider this example:
+
+ i = 1
+ a[i += 2] = i + 1
+
+The value of 'a[3]' could be either two or four.
+
+ *note Table 6.2: table-assign-ops. lists the arithmetic assignment
+operators. In each case, the righthand operand is an expression whose
+value is converted to a number.
+
+
+Operator Effect
+--------------------------------------------------------------------------
+LVALUE '+=' Add INCREMENT to the value of LVALUE.
+INCREMENT
+LVALUE '-=' Subtract DECREMENT from the value of LVALUE.
+DECREMENT
+LVALUE '*=' Multiply the value of LVALUE by COEFFICIENT.
+COEFFICIENT
+LVALUE '/=' DIVISOR Divide the value of LVALUE by DIVISOR.
+LVALUE '%=' MODULUS Set LVALUE to its remainder by MODULUS.
+LVALUE '^=' POWER Raise LVALUE to the power POWER.
+LVALUE '**=' POWER Raise LVALUE to the power POWER. (c.e.)
+
+Table 6.2: Arithmetic assignment operators
+
+ NOTE: Only the '^=' operator is specified by POSIX. For maximum
+ portability, do not use the '**=' operator.
+
+ Syntactic Ambiguities Between '/=' and Regular Expressions
+
+ There is a syntactic ambiguity between the '/=' assignment operator
+and regexp constants whose first character is an '='. (d.c.) This is
+most notable in some commercial 'awk' versions. For example:
+
+ $ awk /==/ /dev/null
+ error-> awk: syntax error at source line 1
+ error-> context is
+ error-> >>> /= <<<
+ error-> awk: bailing out at source line 1
+
+A workaround is:
+
+ awk '/[=]=/' /dev/null
+
+ 'gawk' does not have this problem; BWK 'awk' and 'mawk' also do not.
+
+
+File: gawk.info, Node: Increment Ops, Prev: Assignment Ops, Up: All
Operators
+
+6.2.4 Increment and Decrement Operators
+---------------------------------------
+
+"Increment" and "decrement operators" increase or decrease the value of
+a variable by one. An assignment operator can do the same thing, so the
+increment operators add no power to the 'awk' language; however, they
+are convenient abbreviations for very common operations.
+
+ The operator used for adding one is written '++'. It can be used to
+increment a variable either before or after taking its value. To
+"pre-increment" a variable 'v', write '++v'. This adds one to the value
+of 'v'--that new value is also the value of the expression. (The
+assignment expression 'v += 1' is completely equivalent.) Writing the
+'++' after the variable specifies "post-increment". This increments the
+variable value just the same; the difference is that the value of the
+increment expression itself is the variable's _old_ value. Thus, if
+'foo' has the value four, then the expression 'foo++' has the value
+four, but it changes the value of 'foo' to five. In other words, the
+operator returns the old value of the variable, but with the side effect
+of incrementing it.
+
+ The post-increment 'foo++' is nearly the same as writing '(foo += 1)
+- 1'. It is not perfectly equivalent because all numbers in 'awk' are
+floating point--in floating point, 'foo + 1 - 1' does not necessarily
+equal 'foo'. But the difference is minute as long as you stick to
+numbers that are fairly small (less than 10e12).
+
+ Fields and array elements are incremented just like variables. (Use
+'$(i++)' when you want to do a field reference and a variable increment
+at the same time. The parentheses are necessary because of the
+precedence of the field reference operator '$'.)
+
+ The decrement operator '--' works just like '++', except that it
+subtracts one instead of adding it. As with '++', it can be used before
+the lvalue to pre-decrement or after it to post-decrement. Following is
+a summary of increment and decrement expressions:
+
+'++LVALUE'
+ Increment LVALUE, returning the new value as the value of the
+ expression.
+
+'LVALUE++'
+ Increment LVALUE, returning the _old_ value of LVALUE as the value
+ of the expression.
+
+'--LVALUE'
+ Decrement LVALUE, returning the new value as the value of the
+ expression. (This expression is like '++LVALUE', but instead of
+ adding, it subtracts.)
+
+'LVALUE--'
+ Decrement LVALUE, returning the _old_ value of LVALUE as the value
+ of the expression. (This expression is like 'LVALUE++', but
+ instead of adding, it subtracts.)
+
+ Operator Evaluation Order
+
+ Doctor, it hurts when I do this!
+ Then don't do that!
+ -- _Groucho Marx_
+
+What happens for something like the following?
+
+ b = 6
+ print b += b++
+
+Or something even stranger?
+
+ b = 6
+ b += ++b + b++
+ print b
+
+ In other words, when do the various side effects prescribed by the
+postfix operators ('b++') take effect? When side effects happen is
+"implementation-defined". In other words, it is up to the particular
+version of 'awk'. The result for the first example may be 12 or 13, and
+for the second, it may be 22 or 23.
+
+ In short, doing things like this is not recommended and definitely
+not anything that you can rely upon for portability. You should avoid
+such things in your own programs.
+
+
+File: gawk.info, Node: Truth Values and Conditions, Next: Function Calls,
Prev: All Operators, Up: Expressions
+
+6.3 Truth Values and Conditions
+===============================
+
+In certain contexts, expression values also serve as "truth values";
+i.e., they determine what should happen next as the program runs. This
+minor node describes how 'awk' defines "true" and "false" and how values
+are compared.
+
+* Menu:
+
+* Truth Values:: What is "true" and what is "false".
+* Typing and Comparison:: How variables acquire types and how this
+ affects comparison of numbers and strings with
+ '<', etc.
+* Boolean Ops:: Combining comparison expressions using boolean
+ operators '||' ("or"), '&&'
+ ("and") and '!' ("not").
+* Conditional Exp:: Conditional expressions select between two
+ subexpressions under control of a third
+ subexpression.
+
+
+File: gawk.info, Node: Truth Values, Next: Typing and Comparison, Up: Truth
Values and Conditions
+
+6.3.1 True and False in 'awk'
+-----------------------------
+
+Many programming languages have a special representation for the
+concepts of "true" and "false." Such languages usually use the special
+constants 'true' and 'false', or perhaps their uppercase equivalents.
+However, 'awk' is different. It borrows a very simple concept of true
+and false from C. In 'awk', any nonzero numeric value _or_ any nonempty
+string value is true. Any other value (zero or the null string, '""')
+is false. The following program prints 'A strange truth value' three
+times:
+
+ BEGIN {
+ if (3.1415927)
+ print "A strange truth value"
+ if ("Four Score And Seven Years Ago")
+ print "A strange truth value"
+ if (j = 57)
+ print "A strange truth value"
+ }
+
+ There is a surprising consequence of the "nonzero or non-null" rule:
+the string constant '"0"' is actually true, because it is non-null.
+(d.c.)
+
+
+File: gawk.info, Node: Typing and Comparison, Next: Boolean Ops, Prev:
Truth Values, Up: Truth Values and Conditions
+
+6.3.2 Variable Typing and Comparison Expressions
+------------------------------------------------
+
+ The Guide is definitive. Reality is frequently inaccurate.
+ -- _Douglas Adams, 'The Hitchhiker's Guide to the Galaxy'_
+
+ Unlike in other programming languages, in 'awk' variables do not have
+a fixed type. Instead, they can be either a number or a string,
+depending upon the value that is assigned to them. We look now at how
+variables are typed, and how 'awk' compares variables.
+
+* Menu:
+
+* Variable Typing:: String type versus numeric type.
+* Comparison Operators:: The comparison operators.
+* POSIX String Comparison:: String comparison with POSIX rules.
+
+
+File: gawk.info, Node: Variable Typing, Next: Comparison Operators, Up:
Typing and Comparison
+
+6.3.2.1 String Type versus Numeric Type
+.......................................
+
+Scalar objects in 'awk' (variables, array elements, and fields) are
+_dynamically_ typed. This means their type can change as the program
+runs, from "untyped" before any use,(1) to string or number, and then
+from string to number or number to string, as the program progresses.
+('gawk' also provides regexp-typed scalars, but let's ignore that for
+now; *note Strong Regexp Constants::.)
+
+ You can't do much with untyped variables, other than tell that they
+are untyped. The following program tests 'a' against '""' and '0'; the
+test succeeds when 'a' has never been assigned a value. It also uses
+the built-in 'typeof()' function (not presented yet; *note Type
+Functions::) to show 'a''s type:
+
+ $ gawk 'BEGIN { print (a == "" && a == 0 ?
+ > "a is untyped" : "a has a type!") ; print typeof(a) }'
+ -| a is untyped
+ -| unassigned
+
+ A scalar has numeric type when assigned a numeric value, such as from
+a numeric constant, or from another scalar with numeric type:
+
+ $ gawk 'BEGIN { a = 42 ; print typeof(a)
+ > b = a ; print typeof(b) }'
+ number
+ number
+
+ Similarly, a scalar has string type when assigned a string value,
+such as from a string constant, or from another scalar with string type:
+
+ $ gawk 'BEGIN { a = "forty two" ; print typeof(a)
+ > b = a ; print typeof(b) }'
+ string
+ string
+
+ So far, this is all simple and straightforward. What happens,
+though, when 'awk' has to process data from a user? Let's start with
+field data. What should the following command produce as output?
+
+ echo hello | awk '{ printf("%s %s < 42\n", $1,
+ ($1 < 42 ? "is" : "is not")) }'
+
+Since 'hello' is alphabetic data, 'awk' can only do a string comparison.
+Internally, it converts '42' into '"42"' and compares the two string
+values '"hello"' and '"42"'. Here's the result:
+
+ $ echo hello | awk '{ printf("%s %s < 42\n", $1,
+ > ($1 < 42 ? "is" : "is not")) }'
+ -| hello is not < 42
+
+ However, what happens when data from a user _looks like_ a number?
+On the one hand, in reality, the input data consists of characters, not
+binary numeric values. But, on the other hand, the data looks numeric,
+and 'awk' really ought to treat it as such. And indeed, it does:
+
+ $ echo 37 | awk '{ printf("%s %s < 42\n", $1,
+ > ($1 < 42 ? "is" : "is not")) }'
+ -| 37 is < 42
+
+ Here are the rules for when 'awk' treats data as a number, and for
+when it treats data as a string.
+
+ The POSIX standard uses the term "numeric string" for input data that
+looks numeric. The '37' in the previous example is a numeric string.
+So what is the type of a numeric string? Answer: numeric.
+
+ The type of a variable is important because the types of two
+variables determine how they are compared. Variable typing follows
+these definitions and rules:
+
+ * A numeric constant or the result of a numeric operation has the
+ "numeric" attribute.
+
+ * A string constant or the result of a string operation has the
+ "string" attribute.
+
+ * Fields, 'getline' input, 'FILENAME', 'ARGV' elements, 'ENVIRON'
+ elements, and the elements of an array created by 'match()',
+ 'split()', and 'patsplit()' that are numeric strings have the
+ "strnum" attribute.(2) Otherwise, they have the "string"
+ attribute. Uninitialized variables also have the "strnum"
+ attribute.
+
+ * Attributes propagate across assignments but are not changed by any
+ use.
+
+ The last rule is particularly important. In the following program,
+'a' has numeric type, even though it is later used in a string
+operation:
+
+ BEGIN {
+ a = 12.345
+ b = a " is a cute number"
+ print b
+ }
+
+ When two operands are compared, either string comparison or numeric
+comparison may be used. This depends upon the attributes of the
+operands, according to the following symmetric matrix:
+
+ +----------------------------------------------
+ | STRING NUMERIC STRNUM
+--------+----------------------------------------------
+ |
+STRING | string string string
+ |
+NUMERIC | string numeric numeric
+ |
+STRNUM | string numeric numeric
+--------+----------------------------------------------
+
+ The basic idea is that user input that looks numeric--and _only_ user
+input--should be treated as numeric, even though it is actually made of
+characters and is therefore also a string. Thus, for example, the
+string constant '" +3.14"', when it appears in program source code, is a
+string--even though it looks numeric--and is _never_ treated as a number
+for comparison purposes.
+
+ In short, when one operand is a "pure" string, such as a string
+constant, then a string comparison is performed. Otherwise, a numeric
+comparison is performed. (The primary difference between a number and a
+strnum is that for strnums 'gawk' preserves the original string value
+that the scalar had when it came in.)
+
+ This point bears additional emphasis: Input that looks numeric _is_
+numeric. All other input is treated as strings.
+
+ Thus, the six-character input string ' +3.14' receives the strnum
+attribute. In contrast, the eight characters '" +3.14"' appearing in
+program text comprise a string constant. The following examples print
+'1' when the comparison between the two different constants is true, and
+'0' otherwise:
+
+ $ echo ' +3.14' | awk '{ print($0 == " +3.14") }' True
+ -| 1
+ $ echo ' +3.14' | awk '{ print($0 == "+3.14") }' False
+ -| 0
+ $ echo ' +3.14' | awk '{ print($0 == "3.14") }' False
+ -| 0
+ $ echo ' +3.14' | awk '{ print($0 == 3.14) }' True
+ -| 1
+ $ echo ' +3.14' | awk '{ print($1 == " +3.14") }' False
+ -| 0
+ $ echo ' +3.14' | awk '{ print($1 == "+3.14") }' True
+ -| 1
+ $ echo ' +3.14' | awk '{ print($1 == "3.14") }' False
+ -| 0
+ $ echo ' +3.14' | awk '{ print($1 == 3.14) }' True
+ -| 1
+
+ You can see the type of an input field (or other user input) using
+'typeof()':
+
+ $ echo hello 37 | gawk '{ print typeof($1), typeof($2) }'
+ -| string strnum
+
+ ---------- Footnotes ----------
+
+ (1) 'gawk' calls this "unassigned", as the following example shows.
+
+ (2) Thus, a POSIX numeric string and 'gawk''s strnum are the same
+thing.
+
+
+File: gawk.info, Node: Comparison Operators, Next: POSIX String Comparison,
Prev: Variable Typing, Up: Typing and Comparison
+
+6.3.2.2 Comparison Operators
+............................
+
+"Comparison expressions" compare strings or numbers for relationships
+such as equality. They are written using "relational operators", which
+are a superset of those in C. *note Table 6.3: table-relational-ops.
+describes them.
+
+
+Expression Result
+--------------------------------------------------------------------------
+X '<' Y True if X is less than Y
+X '<=' Y True if X is less than or equal to Y
+X '>' Y True if X is greater than Y
+X '>=' Y True if X is greater than or equal to Y
+X '==' Y True if X is equal to Y
+X '!=' Y True if X is not equal to Y
+X '~' Y True if the string X matches the regexp denoted by Y
+X '!~' Y True if the string X does not match the regexp
+ denoted by Y
+SUBSCRIPT 'in' True if the array ARRAY has an element with the
+ARRAY subscript SUBSCRIPT
+
+Table 6.3: Relational operators
+
+ Comparison expressions have the value one if true and zero if false.
+When comparing operands of mixed types, numeric operands are converted
+to strings using the value of 'CONVFMT' (*note Conversion::).
+
+ Strings are compared by comparing the first character of each, then
+the second character of each, and so on. Thus, '"10"' is less than
+'"9"'. If there are two strings where one is a prefix of the other, the
+shorter string is less than the longer one. Thus, '"abc"' is less than
+'"abcd"'.
+
+ It is very easy to accidentally mistype the '==' operator and leave
+off one of the '=' characters. The result is still valid 'awk' code,
+but the program does not do what is intended:
+
+ if (a = b) # oops! should be a == b
+ ...
+ else
+ ...
+
+Unless 'b' happens to be zero or the null string, the 'if' part of the
+test always succeeds. Because the operators are so similar, this kind
+of error is very difficult to spot when scanning the source code.
+
+ The following list of expressions illustrates the kinds of
+comparisons 'awk' performs, as well as what the result of each
+comparison is:
+
+'1.5 <= 2.0'
+ Numeric comparison (true)
+
+'"abc" >= "xyz"'
+ String comparison (false)
+
+'1.5 != " +2"'
+ String comparison (true)
+
+'"1e2" < "3"'
+ String comparison (true)
+
+'a = 2; b = "2"'
+'a == b'
+ String comparison (true)
+
+'a = 2; b = " +2"'
+'a == b'
+ String comparison (false)
+
+ In this example:
+
+ $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
+ -| false
+
+the result is 'false' because both '$1' and '$2' are user input. They
+are numeric strings--therefore both have the strnum attribute, dictating
+a numeric comparison. The purpose of the comparison rules and the use
+of numeric strings is to attempt to produce the behavior that is "least
+surprising," while still "doing the right thing."
+
+ String comparisons and regular expression comparisons are very
+different. For example:
+
+ x == "foo"
+
+has the value one, or is true if the variable 'x' is precisely 'foo'.
+By contrast:
+
+ x ~ /foo/
+
+has the value one if 'x' contains 'foo', such as '"Oh, what a fool am
+I!"'.
+
+ The righthand operand of the '~' and '!~' operators may be either a
+regexp constant ('/'...'/') or an ordinary expression. In the latter
+case, the value of the expression as a string is used as a dynamic
+regexp (*note Regexp Usage::; also *note Computed Regexps::).
+
+ A constant regular expression in slashes by itself is also an
+expression. '/REGEXP/' is an abbreviation for the following comparison
+expression:
+
+ $0 ~ /REGEXP/
+
+ One special place where '/foo/' is _not_ an abbreviation for '$0 ~
+/foo/' is when it is the righthand operand of '~' or '!~'. *Note Using
+Constant Regexps::, where this is discussed in more detail.
+
+
+File: gawk.info, Node: POSIX String Comparison, Prev: Comparison Operators,
Up: Typing and Comparison
+
+6.3.2.3 String Comparison Based on Locale Collating Order
+.........................................................
+
+The POSIX standard used to say that all string comparisons are performed
+based on the locale's "collating order". This is the order in which
+characters sort, as defined by the locale (for more discussion, *note
+Locales::). This order is usually very different from the results
+obtained when doing straight byte-by-byte comparison.(1)
+
+ Because this behavior differs considerably from existing practice,
+'gawk' only implemented it when in POSIX mode (*note Options::). Here
+is an example to illustrate the difference, in an 'en_US.UTF-8' locale:
+
+ $ gawk 'BEGIN { printf("ABC < abc = %s\n",
+ > ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
+ -| ABC < abc = TRUE
+ $ gawk --posix 'BEGIN { printf("ABC < abc = %s\n",
+ > ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
+ -| ABC < abc = FALSE
+
+ Fortunately, as of August 2016, comparison based on locale collating
+order is no longer required for the '==' and '!=' operators.(2)
+However, comparison based on locales is still required for '<', '<=',
+'>', and '>='. POSIX thus recommends as follows:
+
+ Since the '==' operator checks whether strings are identical, not
+ whether they collate equally, applications needing to check whether
+ strings collate equally can use:
+
+ a <= b && a >= b
+
+ As of version 4.2, 'gawk' continues to use locale collating order for
+'<', '<=', '>', and '>=' only in POSIX mode.
+
+ ---------- Footnotes ----------
+
+ (1) Technically, string comparison is supposed to behave the same way
+as if the strings were compared with the C 'strcoll()' function.
+
+ (2) See the Austin Group website
+(http://austingroupbugs.net/view.php?id=1070).
+
+
+File: gawk.info, Node: Boolean Ops, Next: Conditional Exp, Prev: Typing and
Comparison, Up: Truth Values and Conditions
+
+6.3.3 Boolean Expressions
+-------------------------
+
+A "Boolean expression" is a combination of comparison expressions or
+matching expressions, using the Boolean operators "or" ('||'), "and"
+('&&'), and "not" ('!'), along with parentheses to control nesting. The
+truth value of the Boolean expression is computed by combining the truth
+values of the component expressions. Boolean expressions are also
+referred to as "logical expressions". The terms are equivalent.
+
+ Boolean expressions can be used wherever comparison and matching
+expressions can be used. They can be used in 'if', 'while', 'do', and
+'for' statements (*note Statements::). They have numeric values (one if
+true, zero if false) that come into play if the result of the Boolean
+expression is stored in a variable or used in arithmetic.
+
+ In addition, every Boolean expression is also a valid pattern, so you
+can use one as a pattern to control the execution of rules. The Boolean
+operators are:
+
+'BOOLEAN1 && BOOLEAN2'
+ True if both BOOLEAN1 and BOOLEAN2 are true. For example, the
+ following statement prints the current input record if it contains
+ both 'edu' and 'li':
+
+ if ($0 ~ /edu/ && $0 ~ /li/) print
+
+ The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
+ This can make a difference when BOOLEAN2 contains expressions that
+ have side effects. In the case of '$0 ~ /foo/ && ($2 == bar++)',
+ the variable 'bar' is not incremented if there is no substring
+ 'foo' in the record.
+
+'BOOLEAN1 || BOOLEAN2'
+ True if at least one of BOOLEAN1 or BOOLEAN2 is true. For example,
+ the following statement prints all records in the input that
+ contain _either_ 'edu' or 'li':
+
+ if ($0 ~ /edu/ || $0 ~ /li/) print
+
+ The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
+ This can make a difference when BOOLEAN2 contains expressions that
+ have side effects. (Thus, this test never really distinguishes
+ records that contain both 'edu' and 'li'--as soon as 'edu' is
+ matched, the full test succeeds.)
+
+'! BOOLEAN'
+ True if BOOLEAN is false. For example, the following program
+ prints 'no home!' in the unusual event that the 'HOME' environment
+ variable is not defined:
+
+ BEGIN { if (! ("HOME" in ENVIRON))
+ print "no home!" }
+
+ (The 'in' operator is described in *note Reference to Elements::.)
+
+ The '&&' and '||' operators are called "short-circuit" operators
+because of the way they work. Evaluation of the full expression is
+"short-circuited" if the result can be determined partway through its
+evaluation.
+
+ Statements that end with '&&' or '||' can be continued simply by
+putting a newline after them. But you cannot put a newline in front of
+either of these operators without using backslash continuation (*note
+Statements/Lines::).
+
+ The actual value of an expression using the '!' operator is either
+one or zero, depending upon the truth value of the expression it is
+applied to. The '!' operator is often useful for changing the sense of
+a flag variable from false to true and back again. For example, the
+following program is one way to print lines in between special
+bracketing lines:
+
+ $1 == "START" { interested = ! interested; next }
+ interested { print }
+ $1 == "END" { interested = ! interested; next }
+
+The variable 'interested', as with all 'awk' variables, starts out
+initialized to zero, which is also false. When a line is seen whose
+first field is 'START', the value of 'interested' is toggled to true,
+using '!'. The next rule prints lines as long as 'interested' is true.
+When a line is seen whose first field is 'END', 'interested' is toggled
+back to false.(1)
+
+ Most commonly, the '!' operator is used in the conditions of 'if' and
+'while' statements, where it often makes more sense to phrase the logic
+in the negative:
+
+ if (! SOME CONDITION || SOME OTHER CONDITION) {
+ ... DO WHATEVER PROCESSING ...
+ }
+
+ NOTE: The 'next' statement is discussed in *note Next Statement::.
+ 'next' tells 'awk' to skip the rest of the rules, get the next
+ record, and start processing the rules over again at the top. The
+ reason it's there is to avoid printing the bracketing 'START' and
+ 'END' lines.
+
+ ---------- Footnotes ----------
+
+ (1) This program has a bug; it prints lines starting with 'END'. How
+would you fix it?
+
+
+File: gawk.info, Node: Conditional Exp, Prev: Boolean Ops, Up: Truth Values
and Conditions
+
+6.3.4 Conditional Expressions
+-----------------------------
+
+A "conditional expression" is a special kind of expression that has
+three operands. It allows you to use one expression's value to select
+one of two other expressions. The conditional expression in 'awk' is
+the same as in the C language, as shown here:
+
+ SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP
+
+There are three subexpressions. The first, SELECTOR, is always computed
+first. If it is "true" (not zero or not null), then IF-TRUE-EXP is
+computed next, and its value becomes the value of the whole expression.
+Otherwise, IF-FALSE-EXP is computed next, and its value becomes the
+value of the whole expression. For example, the following expression
+produces the absolute value of 'x':
+
+ x >= 0 ? x : -x
+
+ Each time the conditional expression is computed, only one of
+IF-TRUE-EXP and IF-FALSE-EXP is used; the other is ignored. This is
+important when the expressions have side effects. For example, this
+conditional expression examines element 'i' of either array 'a' or array
+'b', and increments 'i':
+
+ x == y ? a[i++] : b[i++]
+
+This is guaranteed to increment 'i' exactly once, because each time only
+one of the two increment expressions is executed and the other is not.
+*Note Arrays::, for more information about arrays.
+
+ As a minor 'gawk' extension, a statement that uses '?:' can be
+continued simply by putting a newline after either character. However,
+putting a newline in front of either character does not work without
+using backslash continuation (*note Statements/Lines::). If '--posix'
+is specified (*note Options::), this extension is disabled.
+
+
+File: gawk.info, Node: Function Calls, Next: Precedence, Prev: Truth Values
and Conditions, Up: Expressions
+
+6.4 Function Calls
+==================
+
+A "function" is a name for a particular calculation. This enables you
+to ask for it by name at any point in the program. For example, the
+function 'sqrt()' computes the square root of a number.
+
+ A fixed set of functions are "built in", which means they are
+available in every 'awk' program. The 'sqrt()' function is one of
+these. *Note Built-in:: for a list of built-in functions and their
+descriptions. In addition, you can define functions for use in your
+program. *Note User-defined:: for instructions on how to do this.
+Finally, 'gawk' lets you write functions in C or C++ that may be called
+from your program (*note Dynamic Extensions::).
+
+ The way to use a function is with a "function call" expression, which
+consists of the function name followed immediately by a list of
+"arguments" in parentheses. The arguments are expressions that provide
+the raw materials for the function's calculations. When there is more
+than one argument, they are separated by commas. If there are no
+arguments, just write '()' after the function name. The following
+examples show function calls with and without arguments:
+
+ sqrt(x^2 + y^2) one argument
+ atan2(y, x) two arguments
+ rand() no arguments
+
+ CAUTION: Do not put any space between the function name and the
+ opening parenthesis! A user-defined function name looks just like
+ the name of a variable--a space would make the expression look like
+ concatenation of a variable with an expression inside parentheses.
+ With built-in functions, space before the parenthesis is harmless,
+ but it is best not to get into the habit of using space to avoid
+ mistakes with user-defined functions.
+
+ Each function expects a particular number of arguments. For example,
+the 'sqrt()' function must be called with a single argument, the number
+of which to take the square root:
+
+ sqrt(ARGUMENT)
+
+ Some of the built-in functions have one or more optional arguments.
+If those arguments are not supplied, the functions use a reasonable
+default value. *Note Built-in:: for full details. If arguments are
+omitted in calls to user-defined functions, then those arguments are
+treated as local variables. Such local variables act like the empty
+string if referenced where a string value is required, and like zero if
+referenced where a numeric value is required (*note User-defined::).
+
+ As an advanced feature, 'gawk' provides indirect function calls,
+which is a way to choose the function to call at runtime, instead of
+when you write the source code to your program. We defer discussion of
+this feature until later; see *note Indirect Calls::.
+
+ Like every other expression, the function call has a value, often
+called the "return value", which is computed by the function based on
+the arguments you give it. In this example, the return value of
+'sqrt(ARGUMENT)' is the square root of ARGUMENT. The following program
+reads numbers, one number per line, and prints the square root of each
+one:
+
+ $ awk '{ print "The square root of", $1, "is", sqrt($1) }'
+ 1
+ -| The square root of 1 is 1
+ 3
+ -| The square root of 3 is 1.73205
+ 5
+ -| The square root of 5 is 2.23607
+ Ctrl-d
+
+ A function can also have side effects, such as assigning values to
+certain variables or doing I/O. This program shows how the 'match()'
+function (*note String Functions::) changes the variables 'RSTART' and
+'RLENGTH':
+
+ {
+ if (match($1, $2))
+ print RSTART, RLENGTH
+ else
+ print "no match"
+ }
+
+Here is a sample run:
+
+ $ awk -f matchit.awk
+ aaccdd c+
+ -| 3 2
+ foo bar
+ -| no match
+ abcdefg e
+ -| 5 1
+
+
+File: gawk.info, Node: Precedence, Next: Locales, Prev: Function Calls,
Up: Expressions
+
+6.5 Operator Precedence (How Operators Nest)
+============================================
+
+"Operator precedence" determines how operators are grouped when
+different operators appear close by in one expression. For example, '*'
+has higher precedence than '+'; thus, 'a + b * c' means to multiply 'b'
+and 'c', and then add 'a' to the product (i.e., 'a + (b * c)').
+
+ The normal precedence of the operators can be overruled by using
+parentheses. Think of the precedence rules as saying where the
+parentheses are assumed to be. In fact, it is wise to always use
+parentheses whenever there is an unusual combination of operators,
+because other people who read the program may not remember what the
+precedence is in this case. Even experienced programmers occasionally
+forget the exact rules, which leads to mistakes. Explicit parentheses
+help prevent any such mistakes.
+
+ When operators of equal precedence are used together, the leftmost
+operator groups first, except for the assignment, conditional, and
+exponentiation operators, which group in the opposite order. Thus, 'a -
+b + c' groups as '(a - b) + c' and 'a = b = c' groups as 'a = (b = c)'.
+
+ Normally the precedence of prefix unary operators does not matter,
+because there is only one way to interpret them: innermost first. Thus,
+'$++i' means '$(++i)' and '++$x' means '++($x)'. However, when another
+operator follows the operand, then the precedence of the unary operators
+can matter. '$x^2' means '($x)^2', but '-x^2' means '-(x^2)', because
+'-' has lower precedence than '^', whereas '$' has higher precedence.
+Also, operators cannot be combined in a way that violates the precedence
+rules; for example, '$$0++--' is not a valid expression because the
+first '$' has higher precedence than the '++'; to avoid the problem the
+expression can be rewritten as '$($0++)--'.
+
+ This list presents 'awk''s operators, in order of highest to lowest
+precedence:
+
+'('...')'
+ Grouping.
+
+'$'
+ Field reference.
+
+'++ --'
+ Increment, decrement.
+
+'^ **'
+ Exponentiation. These operators group right to left.
+
+'+ - !'
+ Unary plus, minus, logical "not."
+
+'* / %'
+ Multiplication, division, remainder.
+
+'+ -'
+ Addition, subtraction.
+
+String concatenation
+ There is no special symbol for concatenation. The operands are
+ simply written side by side (*note Concatenation::).
+
+'< <= == != > >= >> | |&'
+ Relational and redirection. The relational operators and the
+ redirections have the same precedence level. Characters such as
+ '>' serve both as relationals and as redirections; the context
+ distinguishes between the two meanings.
+
+ Note that the I/O redirection operators in 'print' and 'printf'
+ statements belong to the statement level, not to expressions. The
+ redirection does not produce an expression that could be the
+ operand of another operator. As a result, it does not make sense
+ to use a redirection operator near another operator of lower
+ precedence without parentheses. Such combinations (e.g., 'print
+ foo > a ? b : c') result in syntax errors. The correct way to
+ write this statement is 'print foo > (a ? b : c)'.
+
+'~ !~'
+ Matching, nonmatching.
+
+'in'
+ Array membership.
+
+'&&'
+ Logical "and."
+
+'||'
+ Logical "or."
+
+'?:'
+ Conditional. This operator groups right to left.
+
+'= += -= *= /= %= ^= **='
+ Assignment. These operators group right to left.
+
+ NOTE: The '|&', '**', and '**=' operators are not specified by
+ POSIX. For maximum portability, do not use them.
+
+
+File: gawk.info, Node: Locales, Next: Expressions Summary, Prev:
Precedence, Up: Expressions
+
+6.6 Where You Are Makes a Difference
+====================================
+
+Modern systems support the notion of "locales": a way to tell the system
+about the local character set and language. The ISO C standard defines
+a default '"C"' locale, which is an environment that is typical of what
+many C programmers are used to.
+
+ Once upon a time, the locale setting used to affect regexp matching,
+but this is no longer true (*note Ranges and Locales::).
+
+ Locales can affect record splitting. For the normal case of 'RS =
+"\n"', the locale is largely irrelevant. For other single-character
+record separators, setting 'LC_ALL=C' in the environment will give you
+much better performance when reading records. Otherwise, 'gawk' has to
+make several function calls, _per input character_, to find the record
+terminator.
+
+ Locales can affect how dates and times are formatted (*note Time
+Functions::). For example, a common way to abbreviate the date
+September 4, 2015, in the United States is "9/4/15." In many countries
+in Europe, however, it is abbreviated "4.9.15." Thus, the '%x'
+specification in a '"US"' locale might produce '9/4/15', while in a
+'"EUROPE"' locale, it might produce '4.9.15'.
+
+ According to POSIX, string comparison is also affected by locales
+(similar to regular expressions). The details are presented in *note
+POSIX String Comparison::.
+
+ Finally, the locale affects the value of the decimal point character
+used when 'gawk' parses input data. This is discussed in detail in
+*note Conversion::.
+
+
+File: gawk.info, Node: Expressions Summary, Prev: Locales, Up: Expressions
+
+6.7 Summary
+===========
+
+ * Expressions are the basic elements of computation in programs.
+ They are built from constants, variables, function calls, and
+ combinations of the various kinds of values with operators.
+
+ * 'awk' supplies three kinds of constants: numeric, string, and
+ regexp. 'gawk' lets you specify numeric constants in octal and
+ hexadecimal (bases 8 and 16) as well as decimal (base 10). In
+ certain contexts, a standalone regexp constant such as '/foo/' has
+ the same meaning as '$0 ~ /foo/'.
+
+ * Variables hold values between uses in computations. A number of
+ built-in variables provide information to your 'awk' program, and a
+ number of others let you control how 'awk' behaves.
+
+ * Numbers are automatically converted to strings, and strings to
+ numbers, as needed by 'awk'. Numeric values are converted as if
+ they were formatted with 'sprintf()' using the format in 'CONVFMT'.
+ Locales can influence the conversions.
+
+ * 'awk' provides the usual arithmetic operators (addition,
+ subtraction, multiplication, division, modulus), and unary plus and
+ minus. It also provides comparison operators, Boolean operators,
+ an array membership testing operator, and regexp matching
+ operators. String concatenation is accomplished by placing two
+ expressions next to each other; there is no explicit operator. The
+ three-operand '?:' operator provides an "if-else" test within
+ expressions.
+
+ * Assignment operators provide convenient shorthands for common
+ arithmetic operations.
+
+ * In 'awk', a value is considered to be true if it is nonzero _or_
+ non-null. Otherwise, the value is false.
+
+ * A variable's type is set upon each assignment and may change over
+ its lifetime. The type determines how it behaves in comparisons
+ (string or numeric).
+
+ * Function calls return a value that may be used as part of a larger
+ expression. Expressions used to pass parameter values are fully
+ evaluated before the function is called. 'awk' provides built-in
+ and user-defined functions; this is described in *note Functions::.
+
+ * Operator precedence specifies the order in which operations are
+ performed, unless explicitly overridden by parentheses. 'awk''s
+ operator precedence is compatible with that of C.
+
+ * Locales can affect the format of data as output by an 'awk'
+ program, and occasionally the format for data read as input.
+
+
+File: gawk.info, Node: Patterns and Actions, Next: Arrays, Prev:
Expressions, Up: Top
+
+7 Patterns, Actions, and Variables
+**********************************
+
+As you have already seen, each 'awk' statement consists of a pattern
+with an associated action. This major node describes how you build
+patterns and actions, what kinds of things you can do within actions,
+and 'awk''s predefined variables.
+
+ The pattern-action rules and the statements available for use within
+actions form the core of 'awk' programming. In a sense, everything
+covered up to here has been the foundation that programs are built on
+top of. Now it's time to start building something useful.
+
+* Menu:
+
+* Pattern Overview:: What goes into a pattern.
+* Using Shell Variables:: How to use shell variables with 'awk'.
+* Action Overview:: What goes into an action.
+* Statements:: Describes the various control statements in
+ detail.
+* Built-in Variables:: Summarizes the predefined variables.
+* Pattern Action Summary:: Patterns and Actions summary.
+
+
+File: gawk.info, Node: Pattern Overview, Next: Using Shell Variables, Up:
Patterns and Actions
+
+7.1 Pattern Elements
+====================
+
+* Menu:
+
+* Regexp Patterns:: Using regexps as patterns.
+* Expression Patterns:: Any expression can be used as a pattern.
+* Ranges:: Pairs of patterns specify record ranges.
+* BEGIN/END:: Specifying initialization and cleanup rules.
+* BEGINFILE/ENDFILE:: Two special patterns for advanced control.
+* Empty:: The empty pattern, which matches every record.
+
+Patterns in 'awk' control the execution of rules--a rule is executed
+when its pattern matches the current input record. The following is a
+summary of the types of 'awk' patterns:
+
+'/REGULAR EXPRESSION/'
+ A regular expression. It matches when the text of the input record
+ fits the regular expression. (*Note Regexp::.)
+
+'EXPRESSION'
+ A single expression. It matches when its value is nonzero (if a
+ number) or non-null (if a string). (*Note Expression Patterns::.)
+
+'BEGPAT, ENDPAT'
+ A pair of patterns separated by a comma, specifying a "range" of
+ records. The range includes both the initial record that matches
+ BEGPAT and the final record that matches ENDPAT. (*Note Ranges::.)
+
+'BEGIN'
+'END'
+ Special patterns for you to supply startup or cleanup actions for
+ your 'awk' program. (*Note BEGIN/END::.)
+
+'BEGINFILE'
+'ENDFILE'
+ Special patterns for you to supply startup or cleanup actions to be
+ done on a per-file basis. (*Note BEGINFILE/ENDFILE::.)
+
+'EMPTY'
+ The empty pattern matches every input record. (*Note Empty::.)
+
+
+File: gawk.info, Node: Regexp Patterns, Next: Expression Patterns, Up:
Pattern Overview
+
+7.1.1 Regular Expressions as Patterns
+-------------------------------------
+
+Regular expressions are one of the first kinds of patterns presented in
+this book. This kind of pattern is simply a regexp constant in the
+pattern part of a rule. Its meaning is '$0 ~ /PATTERN/'. The pattern
+matches when the input record matches the regexp. For example:
+
+ /foo|bar|baz/ { buzzwords++ }
+ END { print buzzwords, "buzzwords seen" }
+
+
+File: gawk.info, Node: Expression Patterns, Next: Ranges, Prev: Regexp
Patterns, Up: Pattern Overview
+
+7.1.2 Expressions as Patterns
+-----------------------------
+
+Any 'awk' expression is valid as an 'awk' pattern. The pattern matches
+if the expression's value is nonzero (if a number) or non-null (if a
+string). The expression is reevaluated each time the rule is tested
+against a new input record. If the expression uses fields such as '$1',
+the value depends directly on the new input record's text; otherwise, it
+depends on only what has happened so far in the execution of the 'awk'
+program.
+
+ Comparison expressions, using the comparison operators described in
+*note Typing and Comparison::, are a very common kind of pattern.
+Regexp matching and nonmatching are also very common expressions. The
+left operand of the '~' and '!~' operators is a string. The right
+operand is either a constant regular expression enclosed in slashes
+('/REGEXP/'), or any expression whose string value is used as a dynamic
+regular expression (*note Computed Regexps::). The following example
+prints the second field of each input record whose first field is
+precisely 'li':
+
+ $ awk '$1 == "li" { print $2 }' mail-list
+
+(There is no output, because there is no person with the exact name
+'li'.) Contrast this with the following regular expression match, which
+accepts any record with a first field that contains 'li':
+
+ $ awk '$1 ~ /li/ { print $2 }' mail-list
+ -| 555-5553
+ -| 555-6699
+
+ A regexp constant as a pattern is also a special case of an
+expression pattern. The expression '/li/' has the value one if 'li'
+appears in the current input record. Thus, as a pattern, '/li/' matches
+any record containing 'li'.
+
+ Boolean expressions are also commonly used as patterns. Whether the
+pattern matches an input record depends on whether its subexpressions
+match. For example, the following command prints all the records in
+'mail-list' that contain both 'edu' and 'li':
+
+ $ awk '/edu/ && /li/' mail-list
+ -| Samuel 555-3430 samuel.lanceolis@shu.edu A
+
+ The following command prints all records in 'mail-list' that contain
+_either_ 'edu' or 'li' (or both, of course):
+
+ $ awk '/edu/ || /li/' mail-list
+ -| Amelia 555-5553 amelia.zodiacusque@gmail.com F
+ -| Broderick 555-0542 broderick.aliquotiens@yahoo.com R
+ -| Fabius 555-1234 fabius.undevicesimus@ucb.edu F
+ -| Julie 555-6699 julie.perscrutabor@skeeve.com F
+ -| Samuel 555-3430 samuel.lanceolis@shu.edu A
+ -| Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+
+ The following command prints all records in 'mail-list' that do _not_
+contain the string 'li':
+
+ $ awk '! /li/' mail-list
+ -| Anthony 555-3412 anthony.asserturo@hotmail.com A
+ -| Becky 555-7685 becky.algebrarum@gmail.com A
+ -| Bill 555-1675 bill.drowning@hotmail.com A
+ -| Camilla 555-2912 camilla.infusarum@skynet.be R
+ -| Fabius 555-1234 fabius.undevicesimus@ucb.edu F
+ -| Martin 555-6480 martin.codicibus@hotmail.com A
+ -| Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R
+
+ The subexpressions of a Boolean operator in a pattern can be constant
+regular expressions, comparisons, or any other 'awk' expressions. Range
+patterns are not expressions, so they cannot appear inside Boolean
+patterns. Likewise, the special patterns 'BEGIN', 'END', 'BEGINFILE',
+and 'ENDFILE', which never match any input record, are not expressions
+and cannot appear inside Boolean patterns.
+
+ The precedence of the different operators that can appear in patterns
+is described in *note Precedence::.
+
+
+File: gawk.info, Node: Ranges, Next: BEGIN/END, Prev: Expression Patterns,
Up: Pattern Overview
+
+7.1.3 Specifying Record Ranges with Patterns
+--------------------------------------------
+
+A "range pattern" is made of two patterns separated by a comma, in the
+form 'BEGPAT, ENDPAT'. It is used to match ranges of consecutive input
+records. The first pattern, BEGPAT, controls where the range begins,
+while ENDPAT controls where the pattern ends. For example, the
+following:
+
+ awk '$1 == "on", $1 == "off"' myfile
+
+prints every record in 'myfile' between 'on'/'off' pairs, inclusive.
+
+ A range pattern starts out by matching BEGPAT against every input
+record. When a record matches BEGPAT, the range pattern is "turned on",
+and the range pattern matches this record as well. As long as the range
+pattern stays turned on, it automatically matches every input record
+read. The range pattern also matches ENDPAT against every input record;
+when this succeeds, the range pattern is "turned off" again for the
+following record. Then the range pattern goes back to checking BEGPAT
+against each record.
+
+ The record that turns on the range pattern and the one that turns it
+off both match the range pattern. If you don't want to operate on these
+records, you can write 'if' statements in the rule's action to
+distinguish them from the records you are interested in.
+
+ It is possible for a pattern to be turned on and off by the same
+record. If the record satisfies both conditions, then the action is
+executed for just that record. For example, suppose there is text
+between two identical markers (e.g., the '%' symbol), each on its own
+line, that should be ignored. A first attempt would be to combine a
+range pattern that describes the delimited text with the 'next'
+statement (not discussed yet, *note Next Statement::). This causes
+'awk' to skip any further processing of the current record and start
+over again with the next input record. Such a program looks like this:
+
+ /^%$/,/^%$/ { next }
+ { print }
+
+This program fails because the range pattern is both turned on and
+turned off by the first line, which just has a '%' on it. To accomplish
+this task, write the program in the following manner, using a flag:
+
+ /^%$/ { skip = ! skip; next }
+ skip == 1 { next } # skip lines with `skip' set
+
+ In a range pattern, the comma (',') has the lowest precedence of all
+the operators (i.e., it is evaluated last). Thus, the following program
+attempts to combine a range pattern with another, simpler test:
+
+ echo Yes | awk '/1/,/2/ || /Yes/'
+
+ The intent of this program is '(/1/,/2/) || /Yes/'. However, 'awk'
+interprets this as '/1/, (/2/ || /Yes/)'. This cannot be changed or
+worked around; range patterns do not combine with other patterns:
+
+ $ echo Yes | gawk '(/1/,/2/) || /Yes/'
+ error-> gawk: cmd. line:1: (/1/,/2/) || /Yes/
+ error-> gawk: cmd. line:1: ^ syntax error
+
+ As a minor point of interest, although it is poor style, POSIX allows
+you to put a newline after the comma in a range pattern. (d.c.)
+
+
+File: gawk.info, Node: BEGIN/END, Next: BEGINFILE/ENDFILE, Prev: Ranges,
Up: Pattern Overview
+
+7.1.4 The 'BEGIN' and 'END' Special Patterns
+--------------------------------------------
+
+All the patterns described so far are for matching input records. The
+'BEGIN' and 'END' special patterns are different. They supply startup
+and cleanup actions for 'awk' programs. 'BEGIN' and 'END' rules must
+have actions; there is no default action for these rules because there
+is no current record when they run. 'BEGIN' and 'END' rules are often
+referred to as "'BEGIN' and 'END' blocks" by longtime 'awk' programmers.
+
+* Menu:
+
+* Using BEGIN/END:: How and why to use BEGIN/END rules.
+* I/O And BEGIN/END:: I/O issues in BEGIN/END rules.
+
+
+File: gawk.info, Node: Using BEGIN/END, Next: I/O And BEGIN/END, Up:
BEGIN/END
+
+7.1.4.1 Startup and Cleanup Actions
+...................................
+
+A 'BEGIN' rule is executed once only, before the first input record is
+read. Likewise, an 'END' rule is executed once only, after all the
+input is read. For example:
+
+ $ awk '
+ > BEGIN { print "Analysis of \"li\"" }
+ > /li/ { ++n }
+ > END { print "\"li\" appears in", n, "records." }' mail-list
+ -| Analysis of "li"
+ -| "li" appears in 4 records.
+
+ This program finds the number of records in the input file
+'mail-list' that contain the string 'li'. The 'BEGIN' rule prints a
+title for the report. There is no need to use the 'BEGIN' rule to
+initialize the counter 'n' to zero, as 'awk' does this automatically
+(*note Variables::). The second rule increments the variable 'n' every
+time a record containing the pattern 'li' is read. The 'END' rule
+prints the value of 'n' at the end of the run.
+
+ The special patterns 'BEGIN' and 'END' cannot be used in ranges or
+with Boolean operators (indeed, they cannot be used with any operators).
+An 'awk' program may have multiple 'BEGIN' and/or 'END' rules. They are
+executed in the order in which they appear: all the 'BEGIN' rules at
+startup and all the 'END' rules at termination.
+
+ 'BEGIN' and 'END' rules may be intermixed with other rules. This
+feature was added in the 1987 version of 'awk' and is included in the
+POSIX standard. The original (1978) version of 'awk' required the
+'BEGIN' rule to be placed at the beginning of the program, the 'END'
+rule to be placed at the end, and only allowed one of each. This is no
+longer required, but it is a good idea to follow this template in terms
+of program organization and readability.
+
+ Multiple 'BEGIN' and 'END' rules are useful for writing library
+functions, because each library file can have its own 'BEGIN' and/or
+'END' rule to do its own initialization and/or cleanup. The order in
+which library functions are named on the command line controls the order
+in which their 'BEGIN' and 'END' rules are executed. Therefore, you
+have to be careful when writing such rules in library files so that the
+order in which they are executed doesn't matter. *Note Options:: for
+more information on using library functions. *Note Library Functions::,
+for a number of useful library functions.
+
+ If an 'awk' program has only 'BEGIN' rules and no other rules, then
+the program exits after the 'BEGIN' rules are run.(1) However, if an
+'END' rule exists, then the input is read, even if there are no other
+rules in the program. This is necessary in case the 'END' rule checks
+the 'FNR' and 'NR' variables, or the fields.
+
+ ---------- Footnotes ----------
+
+ (1) The original version of 'awk' kept reading and ignoring input
+until the end of the file was seen.
+
+
+File: gawk.info, Node: I/O And BEGIN/END, Prev: Using BEGIN/END, Up:
BEGIN/END
+
+7.1.4.2 Input/Output from 'BEGIN' and 'END' Rules
+.................................................
+
+There are several (sometimes subtle) points to be aware of when doing
+I/O from a 'BEGIN' or 'END' rule. The first has to do with the value of
+'$0' in a 'BEGIN' rule. Because 'BEGIN' rules are executed before any
+input is read, there simply is no input record, and therefore no fields,
+when executing 'BEGIN' rules. References to '$0' and the fields yield a
+null string or zero, depending upon the context. One way to give '$0' a
+real value is to execute a 'getline' command without a variable (*note
+Getline::). Another way is simply to assign a value to '$0'.
+
+ The second point is similar to the first, but from the other
+direction. Traditionally, due largely to implementation issues, '$0'
+and 'NF' were _undefined_ inside an 'END' rule. The POSIX standard
+specifies that 'NF' is available in an 'END' rule. It contains the
+number of fields from the last input record. Most probably due to an
+oversight, the standard does not say that '$0' is also preserved,
+although logically one would think that it should be. In fact, all of
+BWK 'awk', 'mawk', and 'gawk' preserve the value of '$0' for use in
+'END' rules. Be aware, however, that some other implementations and
+many older versions of Unix 'awk' do not.
+
+ The third point follows from the first two. The meaning of 'print'
+inside a 'BEGIN' or 'END' rule is the same as always: 'print $0'. If
+'$0' is the null string, then this prints an empty record. Many
+longtime 'awk' programmers use an unadorned 'print' in 'BEGIN' and 'END'
+rules to mean 'print ""', relying on '$0' being null. Although one
+might generally get away with this in 'BEGIN' rules, it is a very bad
+idea in 'END' rules, at least in 'gawk'. It is also poor style, because
+if an empty line is needed in the output, the program should print one
+explicitly.
+
+ Finally, the 'next' and 'nextfile' statements are not allowed in a
+'BEGIN' rule, because the implicit
+read-a-record-and-match-against-the-rules loop has not started yet.
+Similarly, those statements are not valid in an 'END' rule, because all
+the input has been read. (*Note Next Statement:: and *note Nextfile
+Statement::.)
+
+
+File: gawk.info, Node: BEGINFILE/ENDFILE, Next: Empty, Prev: BEGIN/END,
Up: Pattern Overview
+
+7.1.5 The 'BEGINFILE' and 'ENDFILE' Special Patterns
+----------------------------------------------------
+
+This minor node describes a 'gawk'-specific feature.
+
+ Two special kinds of rule, 'BEGINFILE' and 'ENDFILE', give you
+"hooks" into 'gawk''s command-line file processing loop. As with the
+'BEGIN' and 'END' rules (*note BEGIN/END::), 'BEGINFILE' rules in a
+program execute in the order they are read by 'gawk'. Similarly, all
+'ENDFILE' rules also execute in the order they are read.
+
+ The bodies of the 'BEGINFILE' rules execute just before 'gawk' reads
+the first record from a file. 'FILENAME' is set to the name of the
+current file, and 'FNR' is set to zero.
+
+ Prior to version 5.1.1 of 'gawk', as an accident of the
+implementation, '$0' and the fields retained any previous values they
+had in 'BEGINFILE' rules. Starting with version 5.1.1, '$0' and the
+fields are cleared, since no record has been read yet from the file that
+is about to be processed.
+
+ The 'BEGINFILE' rule provides you the opportunity to accomplish two
+tasks that would otherwise be difficult or impossible to perform:
+
+ * You can test if the file is readable. Normally, it is a fatal
+ error if a file named on the command line cannot be opened for
+ reading. However, you can bypass the fatal error and move on to
+ the next file on the command line.
+
+ You do this by checking if the 'ERRNO' variable is not the empty
+ string; if so, then 'gawk' was not able to open the file. In this
+ case, your program can execute the 'nextfile' statement (*note
+ Nextfile Statement::). This causes 'gawk' to skip the file
+ entirely. Otherwise, 'gawk' exits with the usual fatal error.
+
+ * If you have written extensions that modify the record handling (by
+ inserting an "input parser"; *note Input Parsers::), you can invoke
+ them at this point, before 'gawk' has started processing the file.
+ (This is a _very_ advanced feature, currently used only by the
+ 'gawkextlib' project
+ (https://sourceforge.net/projects/gawkextlib).)
+
+ The 'ENDFILE' rule is called when 'gawk' has finished processing the
+last record in an input file. For the last input file, it will be
+called before any 'END' rules. The 'ENDFILE' rule is executed even for
+empty input files.
+
+ Normally, when an error occurs when reading input in the normal
+input-processing loop, the error is fatal. However, if a 'BEGINFILE'
+rule is present, the error becomes non-fatal, and instead 'ERRNO' is
+set. This makes it possible to catch and process I/O errors at the
+level of the 'awk' program.
+
+ The 'next' statement (*note Next Statement::) is not allowed inside
+either a 'BEGINFILE' or an 'ENDFILE' rule. The 'nextfile' statement is
+allowed only inside a 'BEGINFILE' rule, not inside an 'ENDFILE' rule.
+
+ The 'getline' statement (*note Getline::) is restricted inside both
+'BEGINFILE' and 'ENDFILE': only redirected forms of 'getline' are
+allowed.
+
+ 'BEGINFILE' and 'ENDFILE' are 'gawk' extensions. In most other 'awk'
+implementations, or if 'gawk' is in compatibility mode (*note
+Options::), they are not special.
+
+
+File: gawk.info, Node: Empty, Prev: BEGINFILE/ENDFILE, Up: Pattern Overview
+
+7.1.6 The Empty Pattern
+-----------------------
+
+An empty (i.e., nonexistent) pattern is considered to match _every_
+input record. For example, the program:
+
+ awk '{ print $1 }' mail-list
+
+prints the first field of every record.
+
+
+File: gawk.info, Node: Using Shell Variables, Next: Action Overview, Prev:
Pattern Overview, Up: Patterns and Actions
+
+7.2 Using Shell Variables in Programs
+=====================================
+
+'awk' programs are often used as components in larger programs written
+in shell. For example, it is very common to use a shell variable to
+hold a pattern that the 'awk' program searches for. There are two ways
+to get the value of the shell variable into the body of the 'awk'
+program.
+
+ A common method is to use shell quoting to substitute the variable's
+value into the program inside the script. For example, consider the
+following program:
+
+ printf "Enter search pattern: "
+ read pattern
+ awk "/$pattern/ "'{ nmatches++ }
+ END { print nmatches, "found" }' /path/to/data
+
+The 'awk' program consists of two pieces of quoted text that are
+concatenated together to form the program. The first part is
+double-quoted, which allows substitution of the 'pattern' shell variable
+inside the quotes. The second part is single-quoted.
+
+ Variable substitution via quoting works, but can potentially be
+messy. It requires a good understanding of the shell's quoting rules
+(*note Quoting::), and it's often difficult to correctly match up the
+quotes when reading the program.
+
+ A better method is to use 'awk''s variable assignment feature (*note
+Assignment Options::) to assign the shell variable's value to an 'awk'
+variable. Then use dynamic regexps to match the pattern (*note Computed
+Regexps::). The following shows how to redo the previous example using
+this technique:
+
+ printf "Enter search pattern: "
+ read pattern
+ awk -v pat="$pattern" '$0 ~ pat { nmatches++ }
+ END { print nmatches, "found" }' /path/to/data
+
+Now, the 'awk' program is just one single-quoted string. The assignment
+'-v pat="$pattern"' still requires double quotes, in case there is
+whitespace in the value of '$pattern'. The 'awk' variable 'pat' could
+be named 'pattern' too, but that would be more confusing. Using a
+variable also provides more flexibility, as the variable can be used
+anywhere inside the program--for printing, as an array subscript, or for
+any other use--without requiring the quoting tricks at every point in
+the program.
+
+
+File: gawk.info, Node: Action Overview, Next: Statements, Prev: Using Shell
Variables, Up: Patterns and Actions
+
+7.3 Actions
+===========
+
+An 'awk' program or script consists of a series of rules and function
+definitions interspersed. (Functions are described later. *Note
+User-defined::.) A rule contains a pattern and an action, either of
+which (but not both) may be omitted. The purpose of the "action" is to
+tell 'awk' what to do once a match for the pattern is found. Thus, in
+outline, an 'awk' program generally looks like this:
+
+ [PATTERN] '{ ACTION }'
+ PATTERN ['{ ACTION }']
+ ...
+ 'function NAME(ARGS) { ... }'
+ ...
+
+ An action consists of one or more 'awk' "statements", enclosed in
+braces ('{...}'). Each statement specifies one thing to do. The
+statements are separated by newlines or semicolons. The braces around
+an action must be used even if the action contains only one statement,
+or if it contains no statements at all. However, if you omit the action
+entirely, omit the braces as well. An omitted action is equivalent to
+'{ print $0 }':
+
+ /foo/ { } match 'foo', do nothing -- empty action
+ /foo/ match 'foo', print the record -- omitted action
+
+ The following types of statements are supported in 'awk':
+
+Expressions
+ Call functions or assign values to variables (*note Expressions::).
+ Executing this kind of statement simply computes the value of the
+ expression. This is useful when the expression has side effects
+ (*note Assignment Ops::).
+
+Control statements
+ Specify the control flow of 'awk' programs. The 'awk' language
+ gives you C-like constructs ('if', 'for', 'while', and 'do') as
+ well as a few special ones (*note Statements::).
+
+Compound statements
+ Enclose one or more statements in braces. A compound statement is
+ used in order to put several statements together in the body of an
+ 'if', 'while', 'do', or 'for' statement.
+
+Input statements
+ Use the 'getline' command (*note Getline::). Also supplied in
+ 'awk' are the 'next' statement (*note Next Statement::) and the
+ 'nextfile' statement (*note Nextfile Statement::).
+
+Output statements
+ Such as 'print' and 'printf'. *Note Printing::.
+
+Deletion statements
+ For deleting array elements. *Note Delete::.
+
+
+File: gawk.info, Node: Statements, Next: Built-in Variables, Prev: Action
Overview, Up: Patterns and Actions
+
+7.4 Control Statements in Actions
+=================================
+
+"Control statements", such as 'if', 'while', and so on, control the flow
+of execution in 'awk' programs. Most of 'awk''s control statements are
+patterned after similar statements in C.
+
+ All the control statements start with special keywords, such as 'if'
+and 'while', to distinguish them from simple expressions. Many control
+statements contain other statements. For example, the 'if' statement
+contains another statement that may or may not be executed. The
+contained statement is called the "body". To include more than one
+statement in the body, group them into a single "compound statement"
+with braces, separating them with newlines or semicolons.
+
+* Menu:
+
+* If Statement:: Conditionally execute some 'awk'
+ statements.
+* While Statement:: Loop until some condition is satisfied.
+* Do Statement:: Do specified action while looping until some
+ condition is satisfied.
+* For Statement:: Another looping statement, that provides
+ initialization and increment clauses.
+* Switch Statement:: Switch/case evaluation for conditional
+ execution of statements based on a value.
+* Break Statement:: Immediately exit the innermost enclosing loop.
+* Continue Statement:: Skip to the end of the innermost enclosing
+ loop.
+* Next Statement:: Stop processing the current input record.
+* Nextfile Statement:: Stop processing the current file.
+* Exit Statement:: Stop execution of 'awk'.
+
+
+File: gawk.info, Node: If Statement, Next: While Statement, Up: Statements
+
+7.4.1 The 'if'-'else' Statement
+-------------------------------
+
+The 'if'-'else' statement is 'awk''s decision-making statement. It
+looks like this:
+
+ 'if (CONDITION) THEN-BODY' ['else ELSE-BODY']
+
+The CONDITION is an expression that controls what the rest of the
+statement does. If the CONDITION is true, THEN-BODY is executed;
+otherwise, ELSE-BODY is executed. The 'else' part of the statement is
+optional. The condition is considered false if its value is zero or the
+null string; otherwise, the condition is true. Refer to the following:
+
+ if (x % 2 == 0)
+ print "x is even"
+ else
+ print "x is odd"
+
+ In this example, if the expression 'x % 2 == 0' is true (i.e., if the
+value of 'x' is evenly divisible by two), then the first 'print'
+statement is executed; otherwise, the second 'print' statement is
+executed. If the 'else' keyword appears on the same line as THEN-BODY
+and THEN-BODY is not a compound statement (i.e., not surrounded by
+braces), then a semicolon must separate THEN-BODY from the 'else'. To
+illustrate this, the previous example can be rewritten as:
+
+ if (x % 2 == 0) print "x is even"; else
+ print "x is odd"
+
+If the ';' is left out, 'awk' can't interpret the statement and it
+produces a syntax error. Don't actually write programs this way,
+because a human reader might fail to see the 'else' if it is not the
+first thing on its line.
+
+
+File: gawk.info, Node: While Statement, Next: Do Statement, Prev: If
Statement, Up: Statements
+
+7.4.2 The 'while' Statement
+---------------------------
+
+In programming, a "loop" is a part of a program that can be executed two
+or more times in succession. The 'while' statement is the simplest
+looping statement in 'awk'. It repeatedly executes a statement as long
+as a condition is true. For example:
+
+ while (CONDITION)
+ BODY
+
+BODY is a statement called the "body" of the loop, and CONDITION is an
+expression that controls how long the loop keeps running. The first
+thing the 'while' statement does is test the CONDITION. If the
+CONDITION is true, it executes the statement BODY. (The CONDITION is
+true when the value is not zero and not a null string.) After BODY has
+been executed, CONDITION is tested again, and if it is still true, BODY
+executes again. This process repeats until the CONDITION is no longer
+true. If the CONDITION is initially false, the body of the loop never
+executes and 'awk' continues with the statement following the loop.
+This example prints the first three fields of each record, one per line:
+
+ awk '
+ {
+ i = 1
+ while (i <= 3) {
+ print $i
+ i++
+ }
+ }' inventory-shipped
+
+The body of this loop is a compound statement enclosed in braces,
+containing two statements. The loop works in the following manner:
+first, the value of 'i' is set to one. Then, the 'while' statement
+tests whether 'i' is less than or equal to three. This is true when 'i'
+equals one, so the 'i'th field is printed. Then the 'i++' increments
+the value of 'i' and the loop repeats. The loop terminates when 'i'
+reaches four.
+
+ A newline is not required between the condition and the body;
+however, using one makes the program clearer unless the body is a
+compound statement or else is very simple. The newline after the open
+brace that begins the compound statement is not required either, but the
+program is harder to read without it.
+
+
+File: gawk.info, Node: Do Statement, Next: For Statement, Prev: While
Statement, Up: Statements
+
+7.4.3 The 'do'-'while' Statement
+--------------------------------
+
+The 'do' loop is a variation of the 'while' looping statement. The 'do'
+loop executes the BODY once and then repeats the BODY as long as the
+CONDITION is true. It looks like this:
+
+ do
+ BODY
+ while (CONDITION)
+
+ Even if the CONDITION is false at the start, the BODY executes at
+least once (and only once, unless executing BODY makes CONDITION true).
+Contrast this with the corresponding 'while' statement:
+
+ while (CONDITION)
+ BODY
+
+This statement does not execute the BODY even once if the CONDITION is
+false to begin with. The following is an example of a 'do' statement:
+
+ {
+ i = 1
+ do {
+ print $0
+ i++
+ } while (i <= 10)
+ }
+
+This program prints each input record 10 times. However, it isn't a
+very realistic example, because in this case an ordinary 'while' would
+do just as well. This situation reflects actual experience; only
+occasionally is there a real use for a 'do' statement.
+
+
+File: gawk.info, Node: For Statement, Next: Switch Statement, Prev: Do
Statement, Up: Statements
+
+7.4.4 The 'for' Statement
+-------------------------
+
+The 'for' statement makes it more convenient to count iterations of a
+loop. The general form of the 'for' statement looks like this:
+
+ for (INITIALIZATION; CONDITION; INCREMENT)
+ BODY
+
+The INITIALIZATION, CONDITION, and INCREMENT parts are arbitrary 'awk'
+expressions, and BODY stands for any 'awk' statement.
+
+ The 'for' statement starts by executing INITIALIZATION. Then, as
+long as the CONDITION is true, it repeatedly executes BODY and then
+INCREMENT. Typically, INITIALIZATION sets a variable to either zero or
+one, INCREMENT adds one to it, and CONDITION compares it against the
+desired number of iterations. For example:
+
+ awk '
+ {
+ for (i = 1; i <= 3; i++)
+ print $i
+ }' inventory-shipped
+
+This prints the first three fields of each input record, with one input
+field per output line.
+
+ C and C++ programmers might expect to be able to use the comma
+operator to set more than one variable in the INITIALIZATION part of the
+'for' loop, or to increment multiple variables in the INCREMENT part of
+the loop, like so:
+
+ for (i = 0, j = length(a); i < j; i++, j--) ... C/C++, not awk!
+
+You cannot do this; the comma operator is not supported in 'awk'. There
+are workarounds, but they are nonobvious and can lead to code that is
+difficult to read and understand. It is best, therefore, to simply
+write additional initializations as separate statements preceding the
+'for' loop and to place additional increment statements at the end of
+the loop's body.
+
+ Most often, INCREMENT is an increment expression, as in the earlier
+example. But this is not required; it can be any expression whatsoever.
+For example, the following statement prints all the powers of two
+between 1 and 100:
+
+ for (i = 1; i <= 100; i *= 2)
+ print i
+
+ If there is nothing to be done, any of the three expressions in the
+parentheses following the 'for' keyword may be omitted. Thus,
+'for (; x > 0;)' is equivalent to 'while (x > 0)'. If the CONDITION is
+omitted, it is treated as true, effectively yielding an "infinite loop"
+(i.e., a loop that never terminates).
+
+ In most cases, a 'for' loop is an abbreviation for a 'while' loop, as
+shown here:
+
+ INITIALIZATION
+ while (CONDITION) {
+ BODY
+ INCREMENT
+ }
+
+The only exception is when the 'continue' statement (*note Continue
+Statement::) is used inside the loop. Changing a 'for' statement to a
+'while' statement in this way can change the effect of the 'continue'
+statement inside the loop.
+
+ The 'awk' language has a 'for' statement in addition to a 'while'
+statement because a 'for' loop is often both less work to type and more
+natural to think of. Counting the number of iterations is very common
+in loops. It can be easier to think of this counting as part of looping
+rather than as something to do inside the loop.
+
+ There is an alternative version of the 'for' loop, for iterating over
+all the indices of an array:
+
+ for (i in array)
+ DO SOMETHING WITH array[i]
+
+*Note Scanning an Array:: for more information on this version of the
+'for' loop.
+
+
+File: gawk.info, Node: Switch Statement, Next: Break Statement, Prev: For
Statement, Up: Statements
+
+7.4.5 The 'switch' Statement
+----------------------------
+
+This minor node describes a 'gawk'-specific feature. If 'gawk' is in
+compatibility mode (*note Options::), it is not available.
+
+ The 'switch' statement allows the evaluation of an expression and the
+execution of statements based on a 'case' match. Case statements are
+checked for a match in the order they are defined. If no suitable
+'case' is found, the 'default' section is executed, if supplied.
+
+ Each 'case' contains a single constant, be it numeric, string, or
+regexp. The 'switch' expression is evaluated, and then each 'case''s
+constant is compared against the result in turn. The type of constant
+determines the comparison: numeric or string do the usual comparisons.
+A regexp constant (either regular, '/foo/', or strongly typed, '@/foo/')
+does a regular expression match against the string value of the original
+expression. The general form of the 'switch' statement looks like this:
+
+ switch (EXPRESSION) {
+ case VALUE OR REGULAR EXPRESSION:
+ CASE-BODY
+ default:
+ DEFAULT-BODY
+ }
+
+ Control flow in the 'switch' statement works as it does in C. Once a
+match to a given case is made, the case statement bodies execute until a
+'break', 'continue', 'next', 'nextfile', or 'exit' is encountered, or
+the end of the 'switch' statement itself. For example:
+
+ while ((c = getopt(ARGC, ARGV, "aksx")) != -1) {
+ switch (c) {
+ case "a":
+ # report size of all files
+ all_files = TRUE;
+ break
+ case "k":
+ BLOCK_SIZE = 1024 # 1K block size
+ break
+ case "s":
+ # do sums only
+ sum_only = TRUE
+ break
+ case "x":
+ # don't cross filesystems
+ fts_flags = or(fts_flags, FTS_XDEV)
+ break
+ case "?":
+ default:
+ usage()
+ break
+ }
+ }
+
+ Note that if none of the statements specified here halt execution of
+a matched 'case' statement, execution falls through to the next 'case'
+until execution halts. In this example, the 'case' for '"?"' falls
+through to the 'default' case, which is to call a function named
+'usage()'. (The 'getopt()' function being called here is described in
+*note Getopt Function::.)
+
+
+File: gawk.info, Node: Break Statement, Next: Continue Statement, Prev:
Switch Statement, Up: Statements
+
+7.4.6 The 'break' Statement
+---------------------------
+
+The 'break' statement jumps out of the innermost 'for', 'while', or 'do'
+loop that encloses it. The following example finds the smallest divisor
+of any integer, and also identifies prime numbers:
+
+ # find smallest divisor of num
+ {
+ num = $1
+ for (divisor = 2; divisor * divisor <= num; divisor++) {
+ if (num % divisor == 0)
+ break
+ }
+ if (num % divisor == 0)
+ printf "Smallest divisor of %d is %d\n", num, divisor
+ else
+ printf "%d is prime\n", num
+ }
+
+ When the remainder is zero in the first 'if' statement, 'awk'
+immediately "breaks out" of the containing 'for' loop. This means that
+'awk' proceeds immediately to the statement following the loop and
+continues processing. (This is very different from the 'exit'
+statement, which stops the entire 'awk' program. *Note Exit
+Statement::.)
+
+ The following program illustrates how the CONDITION of a 'for' or
+'while' statement could be replaced with a 'break' inside an 'if':
+
+ # find smallest divisor of num
+ {
+ num = $1
+ for (divisor = 2; ; divisor++) {
+ if (num % divisor == 0) {
+ printf "Smallest divisor of %d is %d\n", num, divisor
+ break
+ }
+ if (divisor * divisor > num) {
+ printf "%d is prime\n", num
+ break
+ }
+ }
+ }
+
+ The 'break' statement is also used to break out of the 'switch'
+statement. This is discussed in *note Switch Statement::.
+
+ The 'break' statement has no meaning when used outside the body of a
+loop or 'switch'. However, although it was never documented, historical
+implementations of 'awk' treated the 'break' statement outside of a loop
+as if it were a 'next' statement (*note Next Statement::). (d.c.)
+Recent versions of BWK 'awk' no longer allow this usage, nor does
+'gawk'.
+
+
+File: gawk.info, Node: Continue Statement, Next: Next Statement, Prev:
Break Statement, Up: Statements
+
+7.4.7 The 'continue' Statement
+------------------------------
+
+Similar to 'break', the 'continue' statement is used only inside 'for',
+'while', and 'do' loops. It skips over the rest of the loop body,
+causing the next cycle around the loop to begin immediately. Contrast
+this with 'break', which jumps out of the loop altogether.
+
+ The 'continue' statement in a 'for' loop directs 'awk' to skip the
+rest of the body of the loop and resume execution with the
+increment-expression of the 'for' statement. The following program
+illustrates this fact:
+
+ BEGIN {
+ for (x = 0; x <= 20; x++) {
+ if (x == 5)
+ continue
+ printf "%d ", x
+ }
+ print ""
+ }
+
+This program prints all the numbers from 0 to 20--except for 5, for
+which the 'printf' is skipped. Because the increment 'x++' is not
+skipped, 'x' does not remain stuck at 5. Contrast the 'for' loop from
+the previous example with the following 'while' loop:
+
+ BEGIN {
+ x = 0
+ while (x <= 20) {
+ if (x == 5)
+ continue
+ printf "%d ", x
+ x++
+ }
+ print ""
+ }
+
+This program loops forever once 'x' reaches 5, because the increment
+('x++') is never reached.
+
+ The 'continue' statement has no special meaning with respect to the
+'switch' statement, nor does it have any meaning when used outside the
+body of a loop. Historical versions of 'awk' treated a 'continue'
+statement outside a loop the same way they treated a 'break' statement
+outside a loop: as if it were a 'next' statement (*note Next
+Statement::). (d.c.) Recent versions of BWK 'awk' no longer work this
+way, nor does 'gawk'.
+
+
+File: gawk.info, Node: Next Statement, Next: Nextfile Statement, Prev:
Continue Statement, Up: Statements
+
+7.4.8 The 'next' Statement
+--------------------------
+
+The 'next' statement forces 'awk' to immediately stop processing the
+current record and go on to the next record. This means that no further
+rules are executed for the current record, and the rest of the current
+rule's action isn't executed.
+
+ Contrast this with the effect of the 'getline' function (*note
+Getline::). That also causes 'awk' to read the next record immediately,
+but it does not alter the flow of control in any way (i.e., the rest of
+the current action executes with a new input record).
+
+ At the highest level, 'awk' program execution is a loop that reads an
+input record and then tests each rule's pattern against it. If you
+think of this loop as a 'for' statement whose body contains the rules,
+then the 'next' statement is analogous to a 'continue' statement. It
+skips to the end of the body of this implicit loop and executes the
+increment (which reads another record).
+
+ For example, suppose an 'awk' program works only on records with four
+fields, and it shouldn't fail when given bad input. To avoid
+complicating the rest of the program, write a "weed out" rule near the
+beginning, in the following manner:
+
+ NF != 4 {
+ printf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) > "/dev/stderr"
+ next
+ }
+
+Because of the 'next' statement, the program's subsequent rules won't
+see the bad record. The error message is redirected to the standard
+error output stream, as error messages should be. For more detail, see
+*note Special Files::.
+
+ If the 'next' statement causes the end of the input to be reached,
+then the code in any 'END' rules is executed. *Note BEGIN/END::.
+
+ The 'next' statement is not allowed inside 'BEGINFILE' and 'ENDFILE'
+rules. *Note BEGINFILE/ENDFILE::.
+
+ According to the POSIX standard, the behavior is undefined if the
+'next' statement is used in a 'BEGIN' or 'END' rule. 'gawk' treats it
+as a syntax error. Although POSIX does not disallow it, most other
+'awk' implementations don't allow the 'next' statement inside function
+bodies (*note User-defined::). Just as with any other 'next' statement,
+a 'next' statement inside a function body reads the next record and
+starts processing it with the first rule in the program.
+
+
+File: gawk.info, Node: Nextfile Statement, Next: Exit Statement, Prev: Next
Statement, Up: Statements
+
+7.4.9 The 'nextfile' Statement
+------------------------------
+
+The 'nextfile' statement is similar to the 'next' statement. However,
+instead of abandoning processing of the current record, the 'nextfile'
+statement instructs 'awk' to stop processing the current data file.
+
+ Upon execution of the 'nextfile' statement, 'FILENAME' is updated to
+the name of the next data file listed on the command line, 'FNR' is
+reset to one, and processing starts over with the first rule in the
+program. If the 'nextfile' statement causes the end of the input to be
+reached, then the code in any 'END' rules is executed. An exception to
+this is when 'nextfile' is invoked during execution of any statement in
+an 'END' rule; in this case, it causes the program to stop immediately.
+*Note BEGIN/END::.
+
+ The 'nextfile' statement is useful when there are many data files to
+process but it isn't necessary to process every record in every file.
+Without 'nextfile', in order to move on to the next data file, a program
+would have to continue scanning the unwanted records. The 'nextfile'
+statement accomplishes this much more efficiently.
+
+ In 'gawk', execution of 'nextfile' causes additional things to
+happen: any 'ENDFILE' rules are executed if 'gawk' is not currently in
+an 'END' rule, 'ARGIND' is incremented, and any 'BEGINFILE' rules are
+executed. ('ARGIND' hasn't been introduced yet. *Note Built-in
+Variables::.)
+
+ There is an additional, special, use case with 'gawk'. 'nextfile' is
+useful inside a 'BEGINFILE' rule to skip over a file that would
+otherwise cause 'gawk' to exit with a fatal error. In this special
+case, 'ENDFILE' rules are not executed. *Note BEGINFILE/ENDFILE::.
+
+ Although it might seem that 'close(FILENAME)' would accomplish the
+same as 'nextfile', this isn't true. 'close()' is reserved for closing
+files, pipes, and coprocesses that are opened with redirections. It is
+not related to the main processing that 'awk' does with the files listed
+in 'ARGV'.
+
+ NOTE: For many years, 'nextfile' was a common extension. In
+ September 2012, it was accepted for inclusion into the POSIX
+ standard. See the Austin Group website
+ (http://austingroupbugs.net/view.php?id=607).
+
+ The current version of BWK 'awk' and 'mawk' also support 'nextfile'.
+However, they don't allow the 'nextfile' statement inside function
+bodies (*note User-defined::). 'gawk' does; a 'nextfile' inside a
+function body reads the first record from the next file and starts
+processing it with the first rule in the program, just as any other
+'nextfile' statement.
+
+
+File: gawk.info, Node: Exit Statement, Prev: Nextfile Statement, Up:
Statements
+
+7.4.10 The 'exit' Statement
+---------------------------
+
+The 'exit' statement causes 'awk' to immediately stop executing the
+current rule and to stop processing input; any remaining input is
+ignored. The 'exit' statement is written as follows:
+
+ 'exit' [RETURN CODE]
+
+ When an 'exit' statement is executed from a 'BEGIN' rule, the program
+stops processing everything immediately. No input records are read.
+However, if an 'END' rule is present, as part of executing the 'exit'
+statement, the 'END' rule is executed (*note BEGIN/END::). If 'exit' is
+used in the body of an 'END' rule, it causes the program to stop
+immediately.
+
+ An 'exit' statement that is not part of a 'BEGIN' or 'END' rule stops
+the execution of any further automatic rules for the current record,
+skips reading any remaining input records, and executes the 'END' rule
+if there is one. 'gawk' also skips any 'ENDFILE' rules; they do not
+execute.
+
+ In such a case, if you don't want the 'END' rule to do its job, set a
+variable to a nonzero value before the 'exit' statement and check that
+variable in the 'END' rule. *Note Assert Function:: for an example that
+does this.
+
+ If an argument is supplied to 'exit', its value is used as the exit
+status code for the 'awk' process. If no argument is supplied, 'exit'
+causes 'awk' to return a "success" status. In the case where an
+argument is supplied to a first 'exit' statement, and then 'exit' is
+called a second time from an 'END' rule with no argument, 'awk' uses the
+previously supplied exit value. (d.c.) *Note Exit Status:: for more
+information.
+
+ For example, suppose an error condition occurs that is difficult or
+impossible to handle. Conventionally, programs report this by exiting
+with a nonzero status. An 'awk' program can do this using an 'exit'
+statement with a nonzero argument, as shown in the following example:
+
+ BEGIN {
+ if (("date" | getline date_now) <= 0) {
+ print "Can't get system date" > "/dev/stderr"
+ exit 1
+ }
+ print "current date is", date_now
+ close("date")
+ }
+
+ NOTE: For full portability, exit values should be between zero and
+ 126, inclusive. Negative values, and values of 127 or greater, may
+ not produce consistent results across different operating systems.
+
+
+File: gawk.info, Node: Built-in Variables, Next: Pattern Action Summary,
Prev: Statements, Up: Patterns and Actions
+
+7.5 Predefined Variables
+========================
+
+Most 'awk' variables are available to use for your own purposes; they
+never change unless your program assigns values to them, and they never
+affect anything unless your program examines them. However, a few
+variables in 'awk' have special built-in meanings. 'awk' examines some
+of these automatically, so that they enable you to tell 'awk' how to do
+certain things. Others are set automatically by 'awk', so that they
+carry information from the internal workings of 'awk' to your program.
+
+ This minor node documents all of 'gawk''s predefined variables, most
+of which are also documented in the major nodes describing their areas
+of activity.
+
+* Menu:
+
+* User-modified:: Built-in variables that you change to control
+ 'awk'.
+* Auto-set:: Built-in variables where 'awk' gives
+ you information.
+* ARGC and ARGV:: Ways to use 'ARGC' and 'ARGV'.
+
+
+File: gawk.info, Node: User-modified, Next: Auto-set, Up: Built-in Variables
+
+7.5.1 Built-in Variables That Control 'awk'
+-------------------------------------------
+
+The following is an alphabetical list of variables that you can change
+to control how 'awk' does certain things.
+
+ The variables that are specific to 'gawk' are marked with a pound
+sign ('#'). These variables are 'gawk' extensions. In other 'awk'
+implementations or if 'gawk' is in compatibility mode (*note Options::),
+they are not special. (Any exceptions are noted in the description of
+each variable.)
+
+'BINMODE #'
+ On non-POSIX systems, this variable specifies use of binary mode
+ for all I/O. Numeric values of one, two, or three specify that
+ input files, output files, or all files, respectively, should use
+ binary I/O. A numeric value less than zero is treated as zero, and
+ a numeric value greater than three is treated as three.
+ Alternatively, string values of '"r"' or '"w"' specify that input
+ files and output files, respectively, should use binary I/O. A
+ string value of '"rw"' or '"wr"' indicates that all files should
+ use binary I/O. Any other string value is treated the same as
+ '"rw"', but causes 'gawk' to generate a warning message. 'BINMODE'
+ is described in more detail in *note PC Using::. 'mawk' (*note
+ Other Versions::) also supports this variable, but only using
+ numeric values.
+
+'CONVFMT'
+ A string that controls the conversion of numbers to strings (*note
+ Conversion::). It works by being passed, in effect, as the first
+ argument to the 'sprintf()' function (*note String Functions::).
+ Its default value is '"%.6g"'. 'CONVFMT' was introduced by the
+ POSIX standard.
+
+'FIELDWIDTHS #'
+ A space-separated list of columns that tells 'gawk' how to split
+ input with fixed columnar boundaries. Starting in version 4.2,
+ each field width may optionally be preceded by a colon-separated
+ value specifying the number of characters to skip before the field
+ starts. Assigning a value to 'FIELDWIDTHS' overrides the use of
+ 'FS' and 'FPAT' for field splitting. *Note Constant Size:: for
+ more information.
+
+'FPAT #'
+ A regular expression (as a string) that tells 'gawk' to create the
+ fields based on text that matches the regular expression.
+ Assigning a value to 'FPAT' overrides the use of 'FS' and
+ 'FIELDWIDTHS' for field splitting. *Note Splitting By Content::
+ for more information.
+
+'FS'
+ The input field separator (*note Field Separators::). The value is
+ a single-character string or a multicharacter regular expression
+ that matches the separations between fields in an input record. If
+ the value is the null string ('""'), then each character in the
+ record becomes a separate field. (This behavior is a 'gawk'
+ extension. POSIX 'awk' does not specify the behavior when 'FS' is
+ the null string. Nonetheless, some other versions of 'awk' also
+ treat '""' specially.)
+
+ The default value is '" "', a string consisting of a single space.
+ As a special exception, this value means that any sequence of
+ spaces, TABs, and/or newlines is a single separator. It also
+ causes spaces, TABs, and newlines at the beginning and end of a
+ record to be ignored.
+
+ You can set the value of 'FS' on the command line using the '-F'
+ option:
+
+ awk -F, 'PROGRAM' INPUT-FILES
+
+ If 'gawk' is using 'FIELDWIDTHS' or 'FPAT' for field splitting,
+ assigning a value to 'FS' causes 'gawk' to return to the normal,
+ 'FS'-based field splitting. An easy way to do this is to simply
+ say 'FS = FS', perhaps with an explanatory comment.
+
+'IGNORECASE #'
+ If 'IGNORECASE' is nonzero or non-null, then all string comparisons
+ and all regular expression matching are case-independent. This
+ applies to regexp matching with '~' and '!~', the 'gensub()',
+ 'gsub()', 'index()', 'match()', 'patsplit()', 'split()', and
+ 'sub()' functions, record termination with 'RS', and field
+ splitting with 'FS' and 'FPAT'. However, the value of 'IGNORECASE'
+ does _not_ affect array subscripting and it does not affect field
+ splitting when using a single-character field separator. *Note
+ Case-sensitivity::.
+
+'LINT #'
+ When this variable is true (nonzero or non-null), 'gawk' behaves as
+ if the '--lint' command-line option is in effect (*note Options::).
+ With a value of '"fatal"', lint warnings become fatal errors. With
+ a value of '"invalid"', only warnings about things that are
+ actually invalid are issued. (This is not fully implemented yet.)
+ Any other true value prints nonfatal warnings. Assigning a false
+ value to 'LINT' turns off the lint warnings.
+
+ This variable is a 'gawk' extension. It is not special in other
+ 'awk' implementations. Unlike with the other special variables,
+ changing 'LINT' does affect the production of lint warnings, even
+ if 'gawk' is in compatibility mode. Much as the '--lint' and
+ '--traditional' options independently control different aspects of
+ 'gawk''s behavior, the control of lint warnings during program
+ execution is independent of the flavor of 'awk' being executed.
+
+'OFMT'
+ A string that controls conversion of numbers to strings (*note
+ Conversion::) for printing with the 'print' statement. It works by
+ being passed as the first argument to the 'sprintf()' function
+ (*note String Functions::). Its default value is '"%.6g"'.
+ Earlier versions of 'awk' used 'OFMT' to specify the format for
+ converting numbers to strings in general expressions; this is now
+ done by 'CONVFMT'.
+
+'OFS'
+ The output field separator (*note Output Separators::). It is
+ output between the fields printed by a 'print' statement. Its
+ default value is '" "', a string consisting of a single space.
+
+'ORS'
+ The output record separator. It is output at the end of every
+ 'print' statement. Its default value is '"\n"', the newline
+ character. (*Note Output Separators::.)
+
+'PREC #'
+ The working precision of arbitrary-precision floating-point
+ numbers, 53 bits by default (*note Setting precision::).
+
+'ROUNDMODE #'
+ The rounding mode to use for arbitrary-precision arithmetic on
+ numbers, by default '"N"' ('roundTiesToEven' in the IEEE 754
+ standard; *note Setting the rounding mode::).
+
+'RS'
+ The input record separator. Its default value is a string
+ containing a single newline character, which means that an input
+ record consists of a single line of text. It can also be the null
+ string, in which case records are separated by runs of blank lines.
+ If it is a regexp, records are separated by matches of the regexp
+ in the input text. (*Note Records::.)
+
+ The ability for 'RS' to be a regular expression is a 'gawk'
+ extension. In most other 'awk' implementations, or if 'gawk' is in
+ compatibility mode (*note Options::), just the first character of
+ 'RS''s value is used.
+
+'SUBSEP'
+ The subscript separator. It has the default value of '"\034"' and
+ is used to separate the parts of the indices of a multidimensional
+ array. Thus, the expression 'foo["A", "B"]' really accesses
+ 'foo["A\034B"]' (*note Multidimensional::).
+
+'TEXTDOMAIN #'
+ Used for internationalization of programs at the 'awk' level. It
+ sets the default text domain for specially marked string constants
+ in the source text, as well as for the 'dcgettext()',
+ 'dcngettext()', and 'bindtextdomain()' functions (*note
+ Internationalization::). The default value of 'TEXTDOMAIN' is
+ '"messages"'.
+
+
+File: gawk.info, Node: Auto-set, Next: ARGC and ARGV, Prev: User-modified,
Up: Built-in Variables
+
+7.5.2 Built-in Variables That Convey Information
+------------------------------------------------
+
+The following is an alphabetical list of variables that 'awk' sets
+automatically on certain occasions in order to provide information to
+your program.
+
+ The variables that are specific to 'gawk' are marked with a pound
+sign ('#'). These variables are 'gawk' extensions. In other 'awk'
+implementations or if 'gawk' is in compatibility mode (*note Options::),
+they are not special:
+
+'ARGC', 'ARGV'
+ The command-line arguments available to 'awk' programs are stored
+ in an array called 'ARGV'. 'ARGC' is the number of command-line
+ arguments present. *Note Other Arguments::. Unlike most 'awk'
+ arrays, 'ARGV' is indexed from 0 to 'ARGC' - 1. In the following
+ example:
+
+ $ awk 'BEGIN {
+ > for (i = 0; i < ARGC; i++)
+ > print ARGV[i]
+ > }' inventory-shipped mail-list
+ -| awk
+ -| inventory-shipped
+ -| mail-list
+
+ 'ARGV[0]' contains 'awk', 'ARGV[1]' contains 'inventory-shipped',
+ and 'ARGV[2]' contains 'mail-list'. The value of 'ARGC' is three,
+ one more than the index of the last element in 'ARGV', because the
+ elements are numbered from zero.
+
+ The names 'ARGC' and 'ARGV', as well as the convention of indexing
+ the array from 0 to 'ARGC' - 1, are derived from the C language's
+ method of accessing command-line arguments.
+
+ The value of 'ARGV[0]' can vary from system to system. Also, you
+ should note that the program text is _not_ included in 'ARGV', nor
+ are any of 'awk''s command-line options. *Note ARGC and ARGV:: for
+ information about how 'awk' uses these variables. (d.c.)
+
+'ARGIND #'
+ The index in 'ARGV' of the current file being processed. Every
+ time 'gawk' opens a new data file for processing, it sets 'ARGIND'
+ to the index in 'ARGV' of the file name. When 'gawk' is processing
+ the input files, 'FILENAME == ARGV[ARGIND]' is always true.
+
+ This variable is useful in file processing; it allows you to tell
+ how far along you are in the list of data files as well as to
+ distinguish between successive instances of the same file name on
+ the command line.
+
+ While you can change the value of 'ARGIND' within your 'awk'
+ program, 'gawk' automatically sets it to a new value when it opens
+ the next file.
+
+'ENVIRON'
+ An associative array containing the values of the environment. The
+ array indices are the environment variable names; the elements are
+ the values of the particular environment variables. For example,
+ 'ENVIRON["HOME"]' might be '/home/arnold'.
+
+ For POSIX 'awk', changing this array does not affect the
+ environment passed on to any programs that 'awk' may spawn via
+ redirection or the 'system()' function.
+
+ However, beginning with version 4.2, if not in POSIX compatibility
+ mode, 'gawk' does update its own environment when 'ENVIRON' is
+ changed, thus changing the environment seen by programs that it
+ creates. You should therefore be especially careful if you modify
+ 'ENVIRON["PATH"]', which is the search path for finding executable
+ programs.
+
+ This can also affect the running 'gawk' program, since some of the
+ built-in functions may pay attention to certain environment
+ variables. The most notable instance of this is 'mktime()' (*note
+ Time Functions::), which pays attention the value of the 'TZ'
+ environment variable on many systems.
+
+ Some operating systems may not have environment variables. On such
+ systems, the 'ENVIRON' array is empty (except for
+ 'ENVIRON["AWKPATH"]' and 'ENVIRON["AWKLIBPATH"]'; *note AWKPATH
+ Variable:: and *note AWKLIBPATH Variable::).
+
+'ERRNO #'
+ If a system error occurs during a redirection for 'getline', during
+ a read for 'getline', or during a 'close()' operation, then 'ERRNO'
+ contains a string describing the error.
+
+ In addition, 'gawk' clears 'ERRNO' before opening each command-line
+ input file. This enables checking if the file is readable inside a
+ 'BEGINFILE' pattern (*note BEGINFILE/ENDFILE::).
+
+ Otherwise, 'ERRNO' works similarly to the C variable 'errno'.
+ Except for the case just mentioned, 'gawk' _never_ clears it (sets
+ it to zero or '""'). Thus, you should only expect its value to be
+ meaningful when an I/O operation returns a failure value, such as
+ 'getline' returning -1. You are, of course, free to clear it
+ yourself before doing an I/O operation.
+
+ If the value of 'ERRNO' corresponds to a system error in the C
+ 'errno' variable, then 'PROCINFO["errno"]' will be set to the value
+ of 'errno'. For non-system errors, 'PROCINFO["errno"]' will be
+ zero.
+
+'FILENAME'
+ The name of the current input file. When no data files are listed
+ on the command line, 'awk' reads from the standard input and
+ 'FILENAME' is set to '"-"'. 'FILENAME' changes each time a new
+ file is read (*note Reading Files::). Inside a 'BEGIN' rule, the
+ value of 'FILENAME' is '""', because there are no input files being
+ processed yet.(1) (d.c.) Note, though, that using 'getline'
+ (*note Getline::) inside a 'BEGIN' rule can give 'FILENAME' a
+ value.
+
+'FNR'
+ The current record number in the current file. 'awk' increments
+ 'FNR' each time it reads a new record (*note Records::). 'awk'
+ resets 'FNR' to zero each time it starts a new input file.
+
+'NF'
+ The number of fields in the current input record. 'NF' is set each
+ time a new record is read, when a new field is created, or when
+ '$0' changes (*note Fields::).
+
+ Unlike most of the variables described in this node, assigning a
+ value to 'NF' has the potential to affect 'awk''s internal
+ workings. In particular, assignments to 'NF' can be used to create
+ fields in or remove fields from the current record. *Note Changing
+ Fields::.
+
+'FUNCTAB #'
+ An array whose indices and corresponding values are the names of
+ all the built-in, user-defined, and extension functions in the
+ program.
+
+ NOTE: Attempting to use the 'delete' statement with the
+ 'FUNCTAB' array causes a fatal error. Any attempt to assign
+ to an element of 'FUNCTAB' also causes a fatal error.
+
+'NR'
+ The number of input records 'awk' has processed since the beginning
+ of the program's execution (*note Records::). 'awk' increments
+ 'NR' each time it reads a new record.
+
+'PROCINFO #'
+ The elements of this array provide access to information about the
+ running 'awk' program. The following elements (listed
+ alphabetically) are guaranteed to be available:
+
+ 'PROCINFO["argv"]'
+ The 'PROCINFO["argv"]' array contains all of the command-line
+ arguments (after glob expansion and redirection processing on
+ platforms where that must be done manually by the program)
+ with subscripts ranging from 0 through 'argc' - 1. For
+ example, 'PROCINFO["argv"][0]' will contain the name by which
+ 'gawk' was invoked. Here is an example of how this feature
+ may be used:
+
+ gawk '
+ BEGIN {
+ for (i = 0; i < length(PROCINFO["argv"]); i++)
+ print i, PROCINFO["argv"][i]
+ }'
+
+ Please note that this differs from the standard 'ARGV' array
+ which does not include command-line arguments that have
+ already been processed by 'gawk' (*note ARGC and ARGV::).
+
+ 'PROCINFO["egid"]'
+ The value of the 'getegid()' system call.
+
+ 'PROCINFO["errno"]'
+ The value of the C 'errno' variable when 'ERRNO' is set to the
+ associated error message.
+
+ 'PROCINFO["euid"]'
+ The value of the 'geteuid()' system call.
+
+ 'PROCINFO["FS"]'
+ This is '"FS"' if field splitting with 'FS' is in effect,
+ '"FIELDWIDTHS"' if field splitting with 'FIELDWIDTHS' is in
+ effect, '"FPAT"' if field matching with 'FPAT' is in effect,
+ or '"API"' if field splitting is controlled by an API input
+ parser.
+
+ 'PROCINFO["gid"]'
+ The value of the 'getgid()' system call.
+
+ 'PROCINFO["identifiers"]'
+ A subarray, indexed by the names of all identifiers used in
+ the text of the 'awk' program. An "identifier" is simply the
+ name of a variable (be it scalar or array), built-in function,
+ user-defined function, or extension function. For each
+ identifier, the value of the element is one of the following:
+
+ '"array"'
+ The identifier is an array.
+
+ '"builtin"'
+ The identifier is a built-in function.
+
+ '"extension"'
+ The identifier is an extension function loaded via
+ '@load' or '-l'.
+
+ '"scalar"'
+ The identifier is a scalar.
+
+ '"untyped"'
+ The identifier is untyped (could be used as a scalar or
+ an array; 'gawk' doesn't know yet).
+
+ '"user"'
+ The identifier is a user-defined function.
+
+ The values indicate what 'gawk' knows about the identifiers
+ after it has finished parsing the program; they are _not_
+ updated while the program runs.
+
+ 'PROCINFO["platform"]'
+ This element gives a string indicating the platform for which
+ 'gawk' was compiled. The value will be one of the following:
+
+ '"djgpp"'
+ '"mingw"'
+ Microsoft Windows, using either DJGPP or MinGW,
+ respectively.
+
+ '"os2"'
+ OS/2.
+
+ '"os390"'
+ OS/390.
+
+ '"posix"'
+ GNU/Linux, Cygwin, Mac OS X, and legacy Unix systems.
+
+ '"vms"'
+ OpenVMS or Vax/VMS.
+
+ 'PROCINFO["pgrpid"]'
+ The process group ID of the current process.
+
+ 'PROCINFO["pid"]'
+ The process ID of the current process.
+
+ 'PROCINFO["ppid"]'
+ The parent process ID of the current process.
+
+ 'PROCINFO["strftime"]'
+ The default time format string for 'strftime()'. Assigning a
+ new value to this element changes the default. *Note Time
+ Functions::.
+
+ 'PROCINFO["uid"]'
+ The value of the 'getuid()' system call.
+
+ 'PROCINFO["version"]'
+ The version of 'gawk'.
+
+ The following additional elements in the array are available to
+ provide information about the MPFR and GMP libraries if your
+ version of 'gawk' supports arbitrary-precision arithmetic (*note
+ Arbitrary Precision Arithmetic::):
+
+ 'PROCINFO["gmp_version"]'
+ The version of the GNU MP library.
+
+ 'PROCINFO["mpfr_version"]'
+ The version of the GNU MPFR library.
+
+ 'PROCINFO["prec_max"]'
+ The maximum precision supported by MPFR.
+
+ 'PROCINFO["prec_min"]'
+ The minimum precision required by MPFR.
+
+ The following additional elements in the array are available to
+ provide information about the version of the extension API, if your
+ version of 'gawk' supports dynamic loading of extension functions
+ (*note Dynamic Extensions::):
+
+ 'PROCINFO["api_major"]'
+ The major version of the extension API.
+
+ 'PROCINFO["api_minor"]'
+ The minor version of the extension API.
+
+ On some systems, there may be elements in the array, '"group1"'
+ through '"groupN"' for some N. N is the number of supplementary
+ groups that the process has. Use the 'in' operator to test for
+ these elements (*note Reference to Elements::).
+
+ The following elements allow you to change 'gawk''s behavior:
+
+ 'PROCINFO["NONFATAL"]'
+ If this element exists, then I/O errors for all redirections
+ become nonfatal. *Note Nonfatal::.
+
+ 'PROCINFO["NAME", "NONFATAL"]'
+ Make I/O errors for NAME be nonfatal. *Note Nonfatal::.
+
+ 'PROCINFO["COMMAND", "pty"]'
+ For two-way communication to COMMAND, use a pseudo-tty instead
+ of setting up a two-way pipe. *Note Two-way I/O:: for more
+ information.
+
+ 'PROCINFO["INPUT_NAME", "READ_TIMEOUT"]'
+ Set a timeout for reading from input redirection INPUT_NAME.
+ *Note Read Timeout:: for more information.
+
+ 'PROCINFO["INPUT_NAME", "RETRY"]'
+ If an I/O error that may be retried occurs when reading data
+ from INPUT_NAME, and this array entry exists, then 'getline'
+ returns -2 instead of following the default behavior of
+ returning -1 and configuring INPUT_NAME to return no further
+ data. An I/O error that may be retried is one where 'errno'
+ has the value 'EAGAIN', 'EWOULDBLOCK', 'EINTR', or
+ 'ETIMEDOUT'. This may be useful in conjunction with
+ 'PROCINFO["INPUT_NAME", "READ_TIMEOUT"]' or situations where a
+ file descriptor has been configured to behave in a
+ non-blocking fashion. *Note Retrying Input:: for more
+ information.
+
+ 'PROCINFO["sorted_in"]'
+ If this element exists in 'PROCINFO', its value controls the
+ order in which array indices will be processed by 'for (INDX
+ in ARRAY)' loops. This is an advanced feature, so we defer
+ the full description until later; see *note Controlling
+ Scanning::.
+
+'RLENGTH'
+ The length of the substring matched by the 'match()' function
+ (*note String Functions::). 'RLENGTH' is set by invoking the
+ 'match()' function. Its value is the length of the matched string,
+ or -1 if no match is found.
+
+'RSTART'
+ The start index in characters of the substring that is matched by
+ the 'match()' function (*note String Functions::). 'RSTART' is set
+ by invoking the 'match()' function. Its value is the position of
+ the string where the matched substring starts, or zero if no match
+ was found.
+
+'RT #'
+ The input text that matched the text denoted by 'RS', the record
+ separator. It is set every time a record is read.
+
+'SYMTAB #'
+ An array whose indices are the names of all defined global
+ variables and arrays in the program. 'SYMTAB' makes 'gawk''s
+ symbol table visible to the 'awk' programmer. It is built as
+ 'gawk' parses the program and is complete before the program starts
+ to run.
+
+ The array may be used for indirect access to read or write the
+ value of a variable:
+
+ foo = 5
+ SYMTAB["foo"] = 4
+ print foo # prints 4
+
+ The 'isarray()' function (*note Type Functions::) may be used to
+ test if an element in 'SYMTAB' is an array. Also, you may not use
+ the 'delete' statement with the 'SYMTAB' array.
+
+ Prior to version 5.0 of 'gawk', you could use an index for 'SYMTAB'
+ that was not a predefined identifier:
+
+ SYMTAB["xxx"] = 5
+ print SYMTAB["xxx"]
+
+ This no longer works, instead producing a fatal error, as it led to
+ rampant confusion.
+
+ The 'SYMTAB' array is more interesting than it looks. Andrew
+ Schorr points out that it effectively gives 'awk' data pointers.
+ Consider his example:
+
+ # Indirect multiply of any variable by amount, return result
+
+ function multiply(variable, amount)
+ {
+ return SYMTAB[variable] *= amount
+ }
+
+ You would use it like this:
+
+ BEGIN {
+ answer = 10.5
+ multiply("answer", 4)
+ print "The answer is", answer
+ }
+
+ When run, this produces:
+
+ $ gawk -f answer.awk
+ -| The answer is 42
+
+ NOTE: In order to avoid severe time-travel paradoxes,(2)
+ neither 'FUNCTAB' nor 'SYMTAB' is available as an element
+ within the 'SYMTAB' array.
+
+ Changing 'NR' and 'FNR'
+
+ 'awk' increments 'NR' and 'FNR' each time it reads a record, instead
+of setting them to the absolute value of the number of records read.
+This means that a program can change these variables and their new
+values are incremented for each record. (d.c.) The following example
+shows this:
+
+ $ echo '1
+ > 2
+ > 3
+ > 4' | awk 'NR == 2 { NR = 17 }
+ > { print NR }'
+ -| 1
+ -| 17
+ -| 18
+ -| 19
+
+Before 'FNR' was added to the 'awk' language (*note V7/SVR3.1::), many
+'awk' programs used this feature to track the number of records in a
+file by resetting 'NR' to zero when 'FILENAME' changed.
+
+ ---------- Footnotes ----------
+
+ (1) Some early implementations of Unix 'awk' initialized 'FILENAME'
+to '"-"', even if there were data files to be processed. This behavior
+was incorrect and should not be relied upon in your programs.
+
+ (2) Not to mention difficult implementation issues.
+
+
+File: gawk.info, Node: ARGC and ARGV, Prev: Auto-set, Up: Built-in Variables
+
+7.5.3 Using 'ARGC' and 'ARGV'
+-----------------------------
+
+*note Auto-set:: presented the following program describing the
+information contained in 'ARGC' and 'ARGV':
+
+ $ awk 'BEGIN {
+ > for (i = 0; i < ARGC; i++)
+ > print ARGV[i]
+ > }' inventory-shipped mail-list
+ -| awk
+ -| inventory-shipped
+ -| mail-list
+
+In this example, 'ARGV[0]' contains 'awk', 'ARGV[1]' contains
+'inventory-shipped', and 'ARGV[2]' contains 'mail-list'. Notice that
+the 'awk' program is not entered in 'ARGV'. The other command-line
+options, with their arguments, are also not entered. This includes
+variable assignments done with the '-v' option (*note Options::).
+Normal variable assignments on the command line _are_ treated as
+arguments and do show up in the 'ARGV' array. Given the following
+program in a file named 'showargs.awk':
+
+ BEGIN {
+ printf "A=%d, B=%d\n", A, B
+ for (i = 0; i < ARGC; i++)
+ printf "\tARGV[%d] = %s\n", i, ARGV[i]
+ }
+ END { printf "A=%d, B=%d\n", A, B }
+
+Running it produces the following:
+
+ $ awk -v A=1 -f showargs.awk B=2 /dev/null
+ -| A=1, B=0
+ -| ARGV[0] = awk
+ -| ARGV[1] = B=2
+ -| ARGV[2] = /dev/null
+ -| A=1, B=2
+
+ A program can alter 'ARGC' and the elements of 'ARGV'. Each time
+'awk' reaches the end of an input file, it uses the next element of
+'ARGV' as the name of the next input file. By storing a different
+string there, a program can change which files are read. Use '"-"' to
+represent the standard input. Storing additional elements and
+incrementing 'ARGC' causes additional files to be read.
+
+ If the value of 'ARGC' is decreased, that eliminates input files from
+the end of the list. By recording the old value of 'ARGC' elsewhere, a
+program can treat the eliminated arguments as something other than file
+names.
+
+ To eliminate a file from the middle of the list, store the null
+string ('""') into 'ARGV' in place of the file's name. As a special
+feature, 'awk' ignores file names that have been replaced with the null
+string. Another option is to use the 'delete' statement to remove
+elements from 'ARGV' (*note Delete::).
+
+ All of these actions are typically done in the 'BEGIN' rule, before
+actual processing of the input begins. *Note Split Program:: and *note
+Tee Program:: for examples of each way of removing elements from 'ARGV'.
+
+ To actually get options into an 'awk' program, end the 'awk' options
+with '--' and then supply the 'awk' program's options, in the following
+manner:
+
+ awk -f myprog.awk -- -v -q file1 file2 ...
+
+ The following fragment processes 'ARGV' in order to examine, and then
+remove, the previously mentioned command-line options:
+
+ BEGIN {
+ for (i = 1; i < ARGC; i++) {
+ if (ARGV[i] == "-v")
+ verbose = 1
+ else if (ARGV[i] == "-q")
+ debug = 1
+ else if (ARGV[i] ~ /^-./) {
+ e = sprintf("%s: unrecognized option -- %c",
+ ARGV[0], substr(ARGV[i], 2, 1))
+ print e > "/dev/stderr"
+ } else
+ break
+ delete ARGV[i]
+ }
+ }
+
+ Ending the 'awk' options with '--' isn't necessary in 'gawk'. Unless
+'--posix' has been specified, 'gawk' silently puts any unrecognized
+options into 'ARGV' for the 'awk' program to deal with. As soon as it
+sees an unknown option, 'gawk' stops looking for other options that it
+might otherwise recognize. The previous command line with 'gawk' would
+be:
+
+ gawk -f myprog.awk -q -v file1 file2 ...
+
+Because '-q' is not a valid 'gawk' option, it and the following '-v' are
+passed on to the 'awk' program. (*Note Getopt Function:: for an 'awk'
+library function that parses command-line options.)
+
+ When designing your program, you should choose options that don't
+conflict with 'gawk''s, because it will process any options that it
+accepts before passing the rest of the command line on to your program.
+Using '#!' with the '-E' option may help (*note Executable Scripts:: and
+*note Options::).
+
+
+File: gawk.info, Node: Pattern Action Summary, Prev: Built-in Variables,
Up: Patterns and Actions
+
+7.6 Summary
+===========
+
+ * Pattern-action pairs make up the basic elements of an 'awk'
+ program. Patterns are either normal expressions, range
+ expressions, or regexp constants; one of the special keywords
+ 'BEGIN', 'END', 'BEGINFILE', or 'ENDFILE'; or empty. The action
+ executes if the current record matches the pattern. Empty
+ (missing) patterns match all records.
+
+ * I/O from 'BEGIN' and 'END' rules has certain constraints. This is
+ also true, only more so, for 'BEGINFILE' and 'ENDFILE' rules. The
+ latter two give you "hooks" into 'gawk''s file processing, allowing
+ you to recover from a file that otherwise would cause a fatal error
+ (such as a file that cannot be opened).
+
+ * Shell variables can be used in 'awk' programs by careful use of
+ shell quoting. It is easier to pass a shell variable into 'awk' by
+ using the '-v' option and an 'awk' variable.
+
+ * Actions consist of statements enclosed in curly braces. Statements
+ are built up from expressions, control statements, compound
+ statements, input and output statements, and deletion statements.
+
+ * The control statements in 'awk' are 'if'-'else', 'while', 'for',
+ and 'do'-'while'. 'gawk' adds the 'switch' statement. There are
+ two flavors of 'for' statement: one for performing general looping,
+ and the other for iterating through an array.
+
+ * 'break' and 'continue' let you exit early or start the next
+ iteration of a loop (or get out of a 'switch').
+
+ * 'next' and 'nextfile' let you read the next record and start over
+ at the top of your program or skip to the next input file and start
+ over, respectively.
+
+ * The 'exit' statement terminates your program. When executed from
+ an action (or function body), it transfers control to the 'END'
+ statements. From an 'END' statement body, it exits immediately.
+ You may pass an optional numeric value to be used as 'awk''s exit
+ status.
+
+ * Some predefined variables provide control over 'awk', mainly for
+ I/O. Other variables convey information from 'awk' to your program.
+
+ * 'ARGC' and 'ARGV' make the command-line arguments available to your
+ program. Manipulating them from a 'BEGIN' rule lets you control
+ how 'awk' will process the provided data files.
+
+
+File: gawk.info, Node: Arrays, Next: Functions, Prev: Patterns and Actions,
Up: Top
+
+8 Arrays in 'awk'
+*****************
+
+An "array" is a table of values called "elements". The elements of an
+array are distinguished by their "indices". Indices may be either
+numbers or strings.
+
+ This major node describes how arrays work in 'awk', how to use array
+elements, how to scan through every element in an array, and how to
+remove array elements. It also describes how 'awk' simulates
+multidimensional arrays, as well as some of the less obvious points
+about array usage. The major node moves on to discuss 'gawk''s facility
+for sorting arrays, and ends with a brief description of 'gawk''s
+ability to support true arrays of arrays.
+
+* Menu:
+
+* Array Basics:: The basics of arrays.
+* Numeric Array Subscripts:: How to use numbers as subscripts in
+ 'awk'.
+* Uninitialized Subscripts:: Using Uninitialized variables as subscripts.
+* Delete:: The 'delete' statement removes an element
+ from an array.
+* Multidimensional:: Emulating multidimensional arrays in
+ 'awk'.
+* Arrays of Arrays:: True multidimensional arrays.
+* Arrays Summary:: Summary of arrays.
+
+
+File: gawk.info, Node: Array Basics, Next: Numeric Array Subscripts, Up:
Arrays
+
+8.1 The Basics of Arrays
+========================
+
+This minor node presents the basics: working with elements in arrays one
+at a time, and traversing all of the elements in an array.
+
+* Menu:
+
+* Array Intro:: Introduction to Arrays
+* Reference to Elements:: How to examine one element of an array.
+* Assigning Elements:: How to change an element of an array.
+* Array Example:: Basic Example of an Array
+* Scanning an Array:: A variation of the 'for' statement. It
+ loops through the indices of an array's
+ existing elements.
+* Controlling Scanning:: Controlling the order in which arrays are
+ scanned.
+
+
+File: gawk.info, Node: Array Intro, Next: Reference to Elements, Up: Array
Basics
+
+8.1.1 Introduction to Arrays
+----------------------------
+
+ Doing linear scans over an associative array is like trying to club
+ someone to death with a loaded Uzi.
+ -- _Larry Wall_
+
+ The 'awk' language provides one-dimensional arrays for storing groups
+of related strings or numbers. Every 'awk' array must have a name.
+Array names have the same syntax as variable names; any valid variable
+name would also be a valid array name. But one name cannot be used in
+both ways (as an array and as a variable) in the same 'awk' program.
+
+ Arrays in 'awk' superficially resemble arrays in other programming
+languages, but there are fundamental differences. In 'awk', it isn't
+necessary to specify the size of an array before starting to use it.
+Additionally, any number or string, not just consecutive integers, may
+be used as an array index.
+
+ In most other languages, arrays must be "declared" before use,
+including a specification of how many elements or components they
+contain. In such languages, the declaration causes a contiguous block
+of memory to be allocated for that many elements. Usually, an index in
+the array must be a nonnegative integer. For example, the index zero
+specifies the first element in the array, which is actually stored at
+the beginning of the block of memory. Index one specifies the second
+element, which is stored in memory right after the first element, and so
+on. It is impossible to add more elements to the array, because it has
+room only for as many elements as given in the declaration. (Some
+languages allow arbitrary starting and ending indices--e.g., '15 ..
+27'--but the size of the array is still fixed when the array is
+declared.)
+
+ A contiguous array of four elements might look like *note Figure 8.1:
+figure-array-elements, conceptually, if the element values are eight,
+'"foo"', '""', and 30.
+
+
+ [image src="array-elements.png" alt="A Contiguous Array"
text="+---------+---------+--------+---------+
+| 8 | \"foo\" | \"\" | 30 | Value
++---------+---------+--------+---------+
+ 0 1 2 3 Index" ]
+
+Figure 8.1: A contiguous array
+
+Only the values are stored; the indices are implicit from the order of
+the values. Here, eight is the value at index zero, because eight
+appears in the position with zero elements before it.
+
+ Arrays in 'awk' are different--they are "associative". This means
+that each array is a collection of pairs--an index and its corresponding
+array element value:
+
+ Index Value
+------------------------
+ '3' '30'
+ '1' '"foo"'
+ '0' '8'
+ '2' '""'
+
+The pairs are shown in jumbled order because their order is
+irrelevant.(1)
+
+ One advantage of associative arrays is that new pairs can be added at
+any time. For example, suppose a tenth element is added to the array
+whose value is '"number ten"'. The result is:
+
+ Index Value
+-------------------------------
+ '10' '"number
+ ten"'
+ '3' '30'
+ '1' '"foo"'
+ '0' '8'
+ '2' '""'
+
+Now the array is "sparse", which just means some indices are missing.
+It has elements 0-3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or
+9.
+
+ Another consequence of associative arrays is that the indices don't
+have to be nonnegative integers. Any number, or even a string, can be
+an index. For example, the following is an array that translates words
+from English to French:
+
+ Index Value
+------------------------
+ '"dog"' '"chien"'
+ '"cat"' '"chat"'
+ '"one"' '"un"'
+ '1' '"un"'
+
+Here we decided to translate the number one in both spelled-out and
+numeric form--thus illustrating that a single array can have both
+numbers and strings as indices. (In fact, array subscripts are always
+strings. There are some subtleties to how numbers work when used as
+array subscripts; this is discussed in more detail in *note Numeric
+Array Subscripts::.) Here, the number '1' isn't double-quoted, because
+'awk' automatically converts it to a string.
+
+ The value of 'IGNORECASE' has no effect upon array subscripting. The
+identical string value used to store an array element must be used to
+retrieve it. When 'awk' creates an array (e.g., with the 'split()'
+built-in function), that array's indices are consecutive integers
+starting at one. (*Note String Functions::.)
+
+ 'awk''s arrays are efficient--the time to access an element is
+independent of the number of elements in the array.
+
+ ---------- Footnotes ----------
+
+ (1) The ordering will vary among 'awk' implementations, which
+typically use hash tables to store array elements and values.
+
+
+File: gawk.info, Node: Reference to Elements, Next: Assigning Elements,
Prev: Array Intro, Up: Array Basics
+
+8.1.2 Referring to an Array Element
+-----------------------------------
+
+The principal way to use an array is to refer to one of its elements.
+An "array reference" is an expression as follows:
+
+ ARRAY[INDEX-EXPRESSION]
+
+Here, ARRAY is the name of an array. The expression INDEX-EXPRESSION is
+the index of the desired element of the array.
+
+ The value of the array reference is the current value of that array
+element. For example, 'foo[4.3]' is an expression referencing the
+element of array 'foo' at index '4.3'.
+
+ A reference to an array element that has no recorded value yields a
+value of '""', the null string. This includes elements that have not
+been assigned any value as well as elements that have been deleted
+(*note Delete::).
+
+ NOTE: A reference to an element that does not exist _automatically_
+ creates that array element, with the null string as its value. (In
+ some cases, this is unfortunate, because it might waste memory
+ inside 'awk'.)
+
+ Novice 'awk' programmers often make the mistake of checking if an
+ element exists by checking if the value is empty:
+
+ # Check if "foo" exists in a: Incorrect!
+ if (a["foo"] != "") ...
+
+ This is incorrect for two reasons. First, it _creates_ 'a["foo"]'
+ if it didn't exist before! Second, it is valid (if a bit unusual)
+ to set an array element equal to the empty string.
+
+ To determine whether an element exists in an array at a certain
+index, use the following expression:
+
+ INDX in ARRAY
+
+This expression tests whether the particular index INDX exists, without
+the side effect of creating that element if it is not present. The
+expression has the value one (true) if 'ARRAY[INDX]' exists and zero
+(false) if it does not exist. (We use INDX here, because 'index' is the
+name of a built-in function.) For example, this statement tests whether
+the array 'frequencies' contains the index '2':
+
+ if (2 in frequencies)
+ print "Subscript 2 is present."
+
+ Note that this is _not_ a test of whether the array 'frequencies'
+contains an element whose _value_ is two. There is no way to do that
+except to scan all the elements. Also, this _does not_ create
+'frequencies[2]', while the following (incorrect) alternative does:
+
+ if (frequencies[2] != "")
+ print "Subscript 2 is present."
+
+
+File: gawk.info, Node: Assigning Elements, Next: Array Example, Prev:
Reference to Elements, Up: Array Basics
+
+8.1.3 Assigning Array Elements
+------------------------------
+
+Array elements can be assigned values just like 'awk' variables:
+
+ ARRAY[INDEX-EXPRESSION] = VALUE
+
+ARRAY is the name of an array. The expression INDEX-EXPRESSION is the
+index of the element of the array that is assigned a value. The
+expression VALUE is the value to assign to that element of the array.
+
+
+File: gawk.info, Node: Array Example, Next: Scanning an Array, Prev:
Assigning Elements, Up: Array Basics
+
+8.1.4 Basic Array Example
+-------------------------
+
+The following program takes a list of lines, each beginning with a line
+number, and prints them out in order of line number. The line numbers
+are not in order when they are first read--instead, they are scrambled.
+This program sorts the lines by making an array using the line numbers
+as subscripts. The program then prints out the lines in sorted order of
+their numbers. It is a very simple program and gets confused upon
+encountering repeated numbers, gaps, or lines that don't begin with a
+number:
+
+ {
+ if ($1 > max)
+ max = $1
+ arr[$1] = $0
+ }
+
+ END {
+ for (x = 1; x <= max; x++)
+ print arr[x]
+ }
+
+ The first rule keeps track of the largest line number seen so far; it
+also stores each line into the array 'arr', at an index that is the
+line's number. The second rule runs after all the input has been read,
+to print out all the lines. When this program is run with the following
+input:
+
+ 5 I am the Five man
+ 2 Who are you? The new number two!
+ 4 . . . And four on the floor
+ 1 Who is number one?
+ 3 I three you.
+
+Its output is:
+
+ 1 Who is number one?
+ 2 Who are you? The new number two!
+ 3 I three you.
+ 4 . . . And four on the floor
+ 5 I am the Five man
+
+ If a line number is repeated, the last line with a given number
+overrides the others. Gaps in the line numbers can be handled with an
+easy improvement to the program's 'END' rule, as follows:
+
+ END {
+ for (x = 1; x <= max; x++)
+ if (x in arr)
+ print arr[x]
+ }
+
+
+File: gawk.info, Node: Scanning an Array, Next: Controlling Scanning, Prev:
Array Example, Up: Array Basics
+
+8.1.5 Scanning All Elements of an Array
+---------------------------------------
+
+In programs that use arrays, it is often necessary to use a loop that
+executes once for each element of an array. In other languages, where
+arrays are contiguous and indices are limited to nonnegative integers,
+this is easy: all the valid indices can be found by counting from the
+lowest index up to the highest. This technique won't do the job in
+'awk', because any number or string can be an array index. So 'awk' has
+a special kind of 'for' statement for scanning an array:
+
+ for (VAR in ARRAY)
+ BODY
+
+This loop executes BODY once for each index in ARRAY that the program
+has previously used, with the variable VAR set to that index.
+
+ The following program uses this form of the 'for' statement. The
+first rule scans the input records and notes which words appear (at
+least once) in the input, by storing a one into the array 'used' with
+the word as the index. The second rule scans the elements of 'used' to
+find all the distinct words that appear in the input. It prints each
+word that is more than 10 characters long and also prints the number of
+such words. *Note String Functions:: for more information on the
+built-in function 'length()'.
+
+ # Record a 1 for each word that is used at least once
+ {
+ for (i = 1; i <= NF; i++)
+ used[$i] = 1
+ }
+
+ # Find number of distinct words more than 10 characters long
+ END {
+ for (x in used) {
+ if (length(x) > 10) {
+ ++num_long_words
+ print x
+ }
+ }
+ print num_long_words, "words longer than 10 characters"
+ }
+
+*Note Word Sorting:: for a more detailed example of this type.
+
+ The order in which elements of the array are accessed by this
+statement is determined by the internal arrangement of the array
+elements within 'awk' and in standard 'awk' cannot be controlled or
+changed. This can lead to problems if new elements are added to ARRAY
+by statements in the loop body; it is not predictable whether the 'for'
+loop will reach them. Similarly, changing VAR inside the loop may
+produce strange results. It is best to avoid such things.
+
+ As a point of information, 'gawk' sets up the list of elements to be
+iterated over before the loop starts, and does not change it. But not
+all 'awk' versions do so. Consider this program, named 'loopcheck.awk':
+
+ BEGIN {
+ a["here"] = "here"
+ a["is"] = "is"
+ a["a"] = "a"
+ a["loop"] = "loop"
+ for (i in a) {
+ j++
+ a[j] = j
+ print i
+ }
+ }
+
+ Here is what happens when run with 'gawk' (and 'mawk'):
+
+ $ gawk -f loopcheck.awk
+ -| here
+ -| loop
+ -| a
+ -| is
+
+ Contrast this to BWK 'awk':
+
+ $ nawk -f loopcheck.awk
+ -| loop
+ -| here
+ -| is
+ -| a
+ -| 1
+
+
+File: gawk.info, Node: Controlling Scanning, Prev: Scanning an Array, Up:
Array Basics
+
+8.1.6 Using Predefined Array Scanning Orders with 'gawk'
+--------------------------------------------------------
+
+This node describes a feature that is specific to 'gawk'.
+
+ By default, when a 'for' loop traverses an array, the order is
+undefined, meaning that the 'awk' implementation determines the order in
+which the array is traversed. This order is usually based on the
+internal implementation of arrays and will vary from one version of
+'awk' to the next.
+
+ Often, though, you may wish to do something simple, such as "traverse
+the array by comparing the indices in ascending order," or "traverse the
+array by comparing the values in descending order." 'gawk' provides two
+mechanisms that give you this control:
+
+ * Set 'PROCINFO["sorted_in"]' to one of a set of predefined values.
+ We describe this now.
+
+ * Set 'PROCINFO["sorted_in"]' to the name of a user-defined function
+ to use for comparison of array elements. This advanced feature is
+ described later in *note Array Sorting::.
+
+ The following special values for 'PROCINFO["sorted_in"]' are
+available:
+
+'"@unsorted"'
+ Array elements are processed in arbitrary order, which is the
+ default 'awk' behavior.
+
+'"@ind_str_asc"'
+ Order by indices in ascending order compared as strings; this is
+ the most basic sort. (Internally, array indices are always
+ strings, so with 'a[2*5] = 1' the index is '"10"' rather than
+ numeric 10.)
+
+'"@ind_num_asc"'
+ Order by indices in ascending order but force them to be treated as
+ numbers in the process. Any index with a non-numeric value will
+ end up positioned as if it were zero.
+
+'"@val_type_asc"'
+ Order by element values in ascending order (rather than by
+ indices). Ordering is by the type assigned to the element (*note
+ Typing and Comparison::). All numeric values come before all
+ string values, which in turn come before all subarrays. (Subarrays
+ have not been described yet; *note Arrays of Arrays::.)
+
+ If you choose to use this feature in traversing 'FUNCTAB' (*note
+ Auto-set::), then the order is built-in functions first (*note
+ Built-in::), then user-defined functions (*note User-defined::)
+ next, and finally functions loaded from an extension (*note Dynamic
+ Extensions::).
+
+'"@val_str_asc"'
+ Order by element values in ascending order (rather than by
+ indices). Scalar values are compared as strings. If the string
+ values are identical, the index string values are compared instead.
+ When comparing non-scalar values, '"@val_type_asc"' sort ordering
+ is used, so subarrays, if present, come out last.
+
+'"@val_num_asc"'
+ Order by element values in ascending order (rather than by
+ indices). Scalar values are compared as numbers. Non-scalar
+ values are compared using '"@val_type_asc"' sort ordering, so
+ subarrays, if present, come out last. When numeric values are
+ equal, the string values are used to provide an ordering: this
+ guarantees consistent results across different versions of the C
+ 'qsort()' function,(1) which 'gawk' uses internally to perform the
+ sorting. If the string values are also identical, the index string
+ values are compared instead.
+
+'"@ind_str_desc"'
+ Like '"@ind_str_asc"', but the string indices are ordered from high
+ to low.
+
+'"@ind_num_desc"'
+ Like '"@ind_num_asc"', but the numeric indices are ordered from
+ high to low.
+
+'"@val_type_desc"'
+ Like '"@val_type_asc"', but the element values, based on type, are
+ ordered from high to low. Subarrays, if present, come out first.
+
+'"@val_str_desc"'
+ Like '"@val_str_asc"', but the element values, treated as strings,
+ are ordered from high to low. If the string values are identical,
+ the index string values are compared instead. When comparing
+ non-scalar values, '"@val_type_desc"' sort ordering is used, so
+ subarrays, if present, come out first.
+
+'"@val_num_desc"'
+ Like '"@val_num_asc"', but the element values, treated as numbers,
+ are ordered from high to low. If the numeric values are equal, the
+ string values are compared instead. If they are also identical,
+ the index string values are compared instead. Non-scalar values
+ are compared using '"@val_type_desc"' sort ordering, so subarrays,
+ if present, come out first.
+
+ The array traversal order is determined before the 'for' loop starts
+to run. Changing 'PROCINFO["sorted_in"]' in the loop body does not
+affect the loop. For example:
+
+ $ gawk '
+ > BEGIN {
+ > a[4] = 4
+ > a[3] = 3
+ > for (i in a)
+ > print i, a[i]
+ > }'
+ -| 4 4
+ -| 3 3
+ $ gawk '
+ > BEGIN {
+ > PROCINFO["sorted_in"] = "@ind_str_asc"
+ > a[4] = 4
+ > a[3] = 3
+ > for (i in a)
+ > print i, a[i]
+ > }'
+ -| 3 3
+ -| 4 4
+
+ When sorting an array by element values, if a value happens to be a
+subarray then it is considered to be greater than any string or numeric
+value, regardless of what the subarray itself contains, and all
+subarrays are treated as being equal to each other. Their order
+relative to each other is determined by their index strings.
+
+ Here are some additional things to bear in mind about sorted array
+traversal:
+
+ * The value of 'PROCINFO["sorted_in"]' is global. That is, it
+ affects all array traversal 'for' loops. If you need to change it
+ within your own code, you should see if it's defined and save and
+ restore the value:
+
+ ...
+ if ("sorted_in" in PROCINFO) {
+ save_sorted = PROCINFO["sorted_in"]
+ PROCINFO["sorted_in"] = "@val_str_desc" # or whatever
+ }
+ ...
+ if (save_sorted)
+ PROCINFO["sorted_in"] = save_sorted
+
+ * As already mentioned, the default array traversal order is
+ represented by '"@unsorted"'. You can also get the default
+ behavior by assigning the null string to 'PROCINFO["sorted_in"]' or
+ by just deleting the '"sorted_in"' element from the 'PROCINFO'
+ array with the 'delete' statement. (The 'delete' statement hasn't
+ been described yet; *note Delete::.)
+
+ In addition, 'gawk' provides built-in functions for sorting arrays;
+see *note Array Sorting Functions::.
+
+ ---------- Footnotes ----------
+
+ (1) When two elements compare as equal, the C 'qsort()' function does
+not guarantee that they will maintain their original relative order
+after sorting. Using the string value to provide a unique ordering when
+the numeric values are equal ensures that 'gawk' behaves consistently
+across different environments.
+
+
+File: gawk.info, Node: Numeric Array Subscripts, Next: Uninitialized
Subscripts, Prev: Array Basics, Up: Arrays
+
+8.2 Using Numbers to Subscript Arrays
+=====================================
+
+An important aspect to remember about arrays is that _array subscripts
+are always strings_. When a numeric value is used as a subscript, it is
+converted to a string value before being used for subscripting (*note
+Conversion::). This means that the value of the predefined variable
+'CONVFMT' can affect how your program accesses elements of an array.
+For example:
+
+ xyz = 12.153
+ data[xyz] = 1
+ CONVFMT = "%2.2f"
+ if (xyz in data)
+ printf "%s is in data\n", xyz
+ else
+ printf "%s is not in data\n", xyz
+
+This prints '12.15 is not in data'. The first statement gives 'xyz' a
+numeric value. Assigning to 'data[xyz]' subscripts 'data' with the
+string value '"12.153"' (using the default conversion value of
+'CONVFMT', '"%.6g"'). Thus, the array element 'data["12.153"]' is
+assigned the value one. The program then changes the value of
+'CONVFMT'. The test '(xyz in data)' generates a new string value from
+'xyz'--this time '"12.15"'--because the value of 'CONVFMT' only allows
+two significant digits. This test fails, because '"12.15"' is different
+from '"12.153"'.
+
+ According to the rules for conversions (*note Conversion::), integer
+values always convert to strings as integers, no matter what the value
+of 'CONVFMT' may happen to be. So the usual case of the following
+works:
+
+ for (i = 1; i <= maxsub; i++)
+ do something with array[i]
+
+ The "integer values always convert to strings as integers" rule has
+an additional consequence for array indexing. Octal and hexadecimal
+constants (*note Nondecimal-numbers::) are converted internally into
+numbers, and their original form is forgotten. This means, for example,
+that 'array[17]', 'array[021]', and 'array[0x11]' all refer to the same
+element!
+
+ As with many things in 'awk', the majority of the time things work as
+you would expect them to. But it is useful to have a precise knowledge
+of the actual rules, as they can sometimes have a subtle effect on your
+programs.
+
+
+File: gawk.info, Node: Uninitialized Subscripts, Next: Delete, Prev:
Numeric Array Subscripts, Up: Arrays
+
+8.3 Using Uninitialized Variables as Subscripts
+===============================================
+
+Suppose it's necessary to write a program to print the input data in
+reverse order. A reasonable attempt to do so (with some test data)
+might look like this:
+
+ $ echo 'line 1
+ > line 2
+ > line 3' | awk '{ l[lines] = $0; ++lines }
+ > END {
+ > for (i = lines - 1; i >= 0; i--)
+ > print l[i]
+ > }'
+ -| line 3
+ -| line 2
+
+ Unfortunately, the very first line of input data did not appear in
+the output!
+
+ Upon first glance, we would think that this program should have
+worked. The variable 'lines' is uninitialized, and uninitialized
+variables have the numeric value zero. So, 'awk' should have printed
+the value of 'l[0]'.
+
+ The issue here is that subscripts for 'awk' arrays are _always_
+strings. Uninitialized variables, when used as strings, have the value
+'""', not zero. Thus, 'line 1' ends up stored in 'l[""]'. The
+following version of the program works correctly:
+
+ { l[lines++] = $0 }
+ END {
+ for (i = lines - 1; i >= 0; i--)
+ print l[i]
+ }
+
+ Here, the '++' forces 'lines' to be numeric, thus making the "old
+value" numeric zero. This is then converted to '"0"' as the array
+subscript.
+
+ Even though it is somewhat unusual, the null string ('""') is a valid
+array subscript. (d.c.) 'gawk' warns about the use of the null string
+as a subscript if '--lint' is provided on the command line (*note
+Options::).
+
+
+File: gawk.info, Node: Delete, Next: Multidimensional, Prev: Uninitialized
Subscripts, Up: Arrays
+
+8.4 The 'delete' Statement
+==========================
+
+To remove an individual element of an array, use the 'delete' statement:
+
+ delete ARRAY[INDEX-EXPRESSION]
+
+ Once an array element has been deleted, any value the element once
+had is no longer available. It is as if the element had never been
+referred to or been given a value. The following is an example of
+deleting elements in an array:
+
+ for (i in frequencies)
+ delete frequencies[i]
+
+This example removes all the elements from the array 'frequencies'.
+Once an element is deleted, a subsequent 'for' statement to scan the
+array does not report that element and using the 'in' operator to check
+for the presence of that element returns zero (i.e., false):
+
+ delete foo[4]
+ if (4 in foo)
+ print "This will never be printed"
+
+ It is important to note that deleting an element is _not_ the same as
+assigning it a null value (the empty string, '""'). For example:
+
+ foo[4] = ""
+ if (4 in foo)
+ print "This is printed, even though foo[4] is empty"
+
+ It is not an error to delete an element that does not exist.
+However, if '--lint' is provided on the command line (*note Options::),
+'gawk' issues a warning message when an element that is not in the array
+is deleted.
+
+ All the elements of an array may be deleted with a single statement
+by leaving off the subscript in the 'delete' statement, as follows:
+
+ delete ARRAY
+
+ Using this version of the 'delete' statement is about three times
+more efficient than the equivalent loop that deletes each element one at
+a time.
+
+ This form of the 'delete' statement is also supported by BWK 'awk'
+and 'mawk', as well as by a number of other implementations.
+
+ NOTE: For many years, using 'delete' without a subscript was a
+ common extension. In September 2012, it was accepted for inclusion
+ into the POSIX standard. See the Austin Group website
+ (http://austingroupbugs.net/view.php?id=544).
+
+ The following statement provides a portable but nonobvious way to
+clear out an array:(1)
+
+ split("", array)
+
+ The 'split()' function (*note String Functions::) clears out the
+target array first. This call asks it to split apart the null string.
+Because there is no data to split out, the function simply clears the
+array and then returns.
+
+ CAUTION: Deleting all the elements from an array does not change
+ its type; you cannot clear an array and then use the array's name
+ as a scalar (i.e., a regular variable). For example, the following
+ does not work:
+
+ a[1] = 3
+ delete a
+ a = 3
+
+ ---------- Footnotes ----------
+
+ (1) Thanks to Michael Brennan for pointing this out.
+
+
+File: gawk.info, Node: Multidimensional, Next: Arrays of Arrays, Prev:
Delete, Up: Arrays
+
+8.5 Multidimensional Arrays
+===========================
+
+* Menu:
+
+* Multiscanning:: Scanning multidimensional arrays.
+
+A "multidimensional array" is an array in which an element is identified
+by a sequence of indices instead of a single index. For example, a
+two-dimensional array requires two indices. The usual way (in many
+languages, including 'awk') to refer to an element of a two-dimensional
+array named 'grid' is with 'grid[X,Y]'.
+
+ Multidimensional arrays are supported in 'awk' through concatenation
+of indices into one string. 'awk' converts the indices into strings
+(*note Conversion::) and concatenates them together, with a separator
+between them. This creates a single string that describes the values of
+the separate indices. The combined string is used as a single index
+into an ordinary, one-dimensional array. The separator used is the
+value of the built-in variable 'SUBSEP'.
+
+ For example, suppose we evaluate the expression 'foo[5,12] = "value"'
+when the value of 'SUBSEP' is '"@"'. The numbers 5 and 12 are converted
+to strings and concatenated with an '@' between them, yielding '"5@12"';
+thus, the array element 'foo["5@12"]' is set to '"value"'.
+
+ Once the element's value is stored, 'awk' has no record of whether it
+was stored with a single index or a sequence of indices. The two
+expressions 'foo[5,12]' and 'foo[5 SUBSEP 12]' are always equivalent.
+
+ The default value of 'SUBSEP' is the string '"\034"', which contains
+a nonprinting character that is unlikely to appear in an 'awk' program
+or in most input data. The usefulness of choosing an unlikely character
+comes from the fact that index values that contain a string matching
+'SUBSEP' can lead to combined strings that are ambiguous. Suppose that
+'SUBSEP' is '"@"'; then 'foo["a@b", "c"]' and 'foo["a", "b@c"]' are
+indistinguishable because both are actually stored as 'foo["a@b@c"]'.
+
+ To test whether a particular index sequence exists in a
+multidimensional array, use the same operator ('in') that is used for
+single-dimensional arrays. Write the whole sequence of indices in
+parentheses, separated by commas, as the left operand:
+
+ if ((SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY)
+ ...
+
+ Here is an example that treats its input as a two-dimensional array
+of fields; it rotates this array 90 degrees clockwise and prints the
+result. It assumes that all lines have the same number of elements:
+
+ {
+ if (max_nf < NF)
+ max_nf = NF
+ max_nr = NR
+ for (x = 1; x <= NF; x++)
+ vector[x, NR] = $x
+ }
+
+ END {
+ for (x = 1; x <= max_nf; x++) {
+ for (y = max_nr; y >= 1; --y)
+ printf("%s ", vector[x, y])
+ printf("\n")
+ }
+ }
+
+When given the input:
+
+ 1 2 3 4 5 6
+ 2 3 4 5 6 1
+ 3 4 5 6 1 2
+ 4 5 6 1 2 3
+
+the program produces the following output:
+
+ 4 3 2 1
+ 5 4 3 2
+ 6 5 4 3
+ 1 6 5 4
+ 2 1 6 5
+ 3 2 1 6
+
+
+File: gawk.info, Node: Multiscanning, Up: Multidimensional
+
+8.5.1 Scanning Multidimensional Arrays
+--------------------------------------
+
+There is no special 'for' statement for scanning a "multidimensional"
+array. There cannot be one, because, in truth, 'awk' does not have
+multidimensional arrays or elements--there is only a multidimensional
+_way of accessing_ an array.
+
+ However, if your program has an array that is always accessed as
+multidimensional, you can get the effect of scanning it by combining the
+scanning 'for' statement (*note Scanning an Array::) with the built-in
+'split()' function (*note String Functions::). It works in the
+following manner:
+
+ for (combined in array) {
+ split(combined, separate, SUBSEP)
+ ...
+ }
+
+This sets the variable 'combined' to each concatenated combined index in
+the array, and splits it into the individual indices by breaking it
+apart where the value of 'SUBSEP' appears. The individual indices then
+become the elements of the array 'separate'.
+
+ Thus, if a value is previously stored in 'array[1, "foo"]', then an
+element with index '"1\034foo"' exists in 'array'. (Recall that the
+default value of 'SUBSEP' is the character with code 034.) Sooner or
+later, the 'for' statement finds that index and does an iteration with
+the variable 'combined' set to '"1\034foo"'. Then the 'split()'
+function is called as follows:
+
+ split("1\034foo", separate, "\034")
+
+The result is to set 'separate[1]' to '"1"' and 'separate[2]' to
+'"foo"'. Presto! The original sequence of separate indices is
+recovered.
+
+
+File: gawk.info, Node: Arrays of Arrays, Next: Arrays Summary, Prev:
Multidimensional, Up: Arrays
+
+8.6 Arrays of Arrays
+====================
+
+'gawk' goes beyond standard 'awk''s multidimensional array access and
+provides true arrays of arrays. Elements of a subarray are referred to
+by their own indices enclosed in square brackets, just like the elements
+of the main array. For example, the following creates a two-element
+subarray at index '1' of the main array 'a':
+
+ a[1][1] = 1
+ a[1][2] = 2
+
+ This simulates a true two-dimensional array. Each subarray element
+can contain another subarray as a value, which in turn can hold other
+arrays as well. In this way, you can create arrays of three or more
+dimensions. The indices can be any 'awk' expressions, including scalars
+separated by commas (i.e., a regular 'awk' simulated multidimensional
+subscript). So the following is valid in 'gawk':
+
+ a[1][3][1, "name"] = "barney"
+
+ Each subarray and the main array can be of different length. In
+fact, the elements of an array or its subarray do not all have to have
+the same type. This means that the main array and any of its subarrays
+can be nonrectangular, or jagged in structure. You can assign a scalar
+value to the index '4' of the main array 'a', even though 'a[1]' is
+itself an array and not a scalar:
+
+ a[4] = "An element in a jagged array"
+
+ The terms "dimension", "row", and "column" are meaningless when
+applied to such an array, but we will use "dimension" henceforth to
+imply the maximum number of indices needed to refer to an existing
+element. The type of any element that has already been assigned cannot
+be changed by assigning a value of a different type. You have to first
+delete the current element, which effectively makes 'gawk' forget about
+the element at that index:
+
+ delete a[4]
+ a[4][5][6][7] = "An element in a four-dimensional array"
+
+This removes the scalar value from index '4' and then inserts a
+three-level nested subarray containing a scalar. You can also delete an
+entire subarray or subarray of subarrays:
+
+ delete a[4][5]
+ a[4][5] = "An element in subarray a[4]"
+
+ But recall that you can not delete the main array 'a' and then use it
+as a scalar.
+
+ The built-in functions that take array arguments can also be used
+with subarrays. For example, the following code fragment uses
+'length()' (*note String Functions::) to determine the number of
+elements in the main array 'a' and its subarrays:
+
+ print length(a), length(a[1]), length(a[1][3])
+
+This results in the following output for our main array 'a':
+
+ 2, 3, 1
+
+The 'SUBSCRIPT in ARRAY' expression (*note Reference to Elements::)
+works similarly for both regular 'awk'-style arrays and arrays of
+arrays. For example, the tests '1 in a', '3 in a[1]', and '(1, "name")
+in a[1][3]' all evaluate to one (true) for our array 'a'.
+
+ The 'for (item in array)' statement (*note Scanning an Array::) can
+be nested to scan all the elements of an array of arrays if it is
+rectangular in structure. In order to print the contents (scalar
+values) of a two-dimensional array of arrays (i.e., in which each
+first-level element is itself an array, not necessarily of the same
+length), you could use the following code:
+
+ for (i in array)
+ for (j in array[i])
+ print array[i][j]
+
+ The 'isarray()' function (*note Type Functions::) lets you test if an
+array element is itself an array:
+
+ for (i in array) {
+ if (isarray(array[i])) {
+ for (j in array[i]) {
+ print array[i][j]
+ }
+ }
+ else
+ print array[i]
+ }
+
+ If the structure of a jagged array of arrays is known in advance, you
+can often devise workarounds using control statements. For example, the
+following code prints the elements of our main array 'a':
+
+ for (i in a) {
+ for (j in a[i]) {
+ if (j == 3) {
+ for (k in a[i][j])
+ print a[i][j][k]
+ } else
+ print a[i][j]
+ }
+ }
+
+*Note Walking Arrays:: for a user-defined function that "walks" an
+arbitrarily dimensioned array of arrays.
+
+ Recall that a reference to an uninitialized array element yields a
+value of '""', the null string. This has one important implication when
+you intend to use a subarray as an argument to a function, as
+illustrated by the following example:
+
+ $ gawk 'BEGIN { split("a b c d", b[1]); print b[1][1] }'
+ error-> gawk: cmd. line:1: fatal: split: second argument is not an array
+
+ The way to work around this is to first force 'b[1]' to be an array
+by creating an arbitrary index:
+
+ $ gawk 'BEGIN { b[1][1] = ""; split("a b c d", b[1]); print b[1][1] }'
+ -| a
+
+
+File: gawk.info, Node: Arrays Summary, Prev: Arrays of Arrays, Up: Arrays
+
+8.7 Summary
+===========
+
+ * Standard 'awk' provides one-dimensional associative arrays (arrays
+ indexed by string values). All arrays are associative; numeric
+ indices are converted automatically to strings.
+
+ * Array elements are referenced as 'ARRAY[INDX]'. Referencing an
+ element creates it if it did not exist previously.
+
+ * The proper way to see if an array has an element with a given index
+ is to use the 'in' operator: 'INDX in ARRAY'.
+
+ * Use 'for (INDX in ARRAY) ...' to scan through all the individual
+ elements of an array. In the body of the loop, INDX takes on the
+ value of each element's index in turn.
+
+ * The order in which a 'for (INDX in ARRAY)' loop traverses an array
+ is undefined in POSIX 'awk' and varies among implementations.
+ 'gawk' lets you control the order by assigning special predefined
+ values to 'PROCINFO["sorted_in"]'.
+
+ * Use 'delete ARRAY[INDX]' to delete an individual element. To
+ delete all of the elements in an array, use 'delete ARRAY'. This
+ latter feature has been a common extension for many years and is
+ now standard, but may not be supported by all commercial versions
+ of 'awk'.
+
+ * Standard 'awk' simulates multidimensional arrays by separating
+ subscript values with commas. The values are concatenated into a
+ single string, separated by the value of 'SUBSEP'. The fact that
+ such a subscript was created in this way is not retained; thus,
+ changing 'SUBSEP' may have unexpected consequences. You can use
+ '(SUB1, SUB2, ...) in ARRAY' to see if such a multidimensional
+ subscript exists in ARRAY.
+
+ * 'gawk' provides true arrays of arrays. You use a separate set of
+ square brackets for each dimension in such an array:
+ 'data[row][col]', for example. Array elements may thus be either
+ scalar values (number or string) or other arrays.
+
+ * Use the 'isarray()' built-in function to determine if an array
+ element is itself a subarray.
+
+
+File: gawk.info, Node: Functions, Next: Library Functions, Prev: Arrays,
Up: Top
+
+9 Functions
+***********
+
+This major node describes 'awk''s built-in functions, which fall into
+three categories: numeric, string, and I/O. 'gawk' provides additional
+groups of functions to work with values that represent time, do bit
+manipulation, sort arrays, provide type information, and
+internationalize and localize programs.
+
+ Besides the built-in functions, 'awk' has provisions for writing new
+functions that the rest of a program can use. The second half of this
+major node describes these "user-defined" functions. Finally, we
+explore indirect function calls, a 'gawk'-specific extension that lets
+you determine at runtime what function is to be called.
+
+* Menu:
+
+* Built-in:: Summarizes the built-in functions.
+* User-defined:: Describes User-defined functions in detail.
+* Indirect Calls:: Choosing the function to call at runtime.
+* Functions Summary:: Summary of functions.
+
+
+File: gawk.info, Node: Built-in, Next: User-defined, Up: Functions
+
+9.1 Built-in Functions
+======================
+
+"Built-in" functions are always available for your 'awk' program to
+call. This minor node defines all the built-in functions in 'awk'; some
+of these are mentioned in other minor nodes but are summarized here for
+your convenience.
+
+* Menu:
+
+* Calling Built-in:: How to call built-in functions.
+* Boolean Functions:: A function that returns Boolean values.
+* Numeric Functions:: Functions that work with numbers, including
+ 'int()', 'sin()' and 'rand()'.
+* String Functions:: Functions for string manipulation, such as
+ 'split()', 'match()' and
+ 'sprintf()'.
+* I/O Functions:: Functions for files and shell commands.
+* Time Functions:: Functions for dealing with timestamps.
+* Bitwise Functions:: Functions for bitwise operations.
+* Type Functions:: Functions for type information.
+* I18N Functions:: Functions for string translation.
+
+
+File: gawk.info, Node: Calling Built-in, Next: Boolean Functions, Up:
Built-in
+
+9.1.1 Calling Built-in Functions
+--------------------------------
+
+To call one of 'awk''s built-in functions, write the name of the
+function followed by arguments in parentheses. For example, 'atan2(y +
+z, 1)' is a call to the function 'atan2()' and has two arguments.
+
+ Whitespace is ignored between the built-in function name and the
+opening parenthesis, but nonetheless it is good practice to avoid using
+whitespace there. User-defined functions do not permit whitespace in
+this way, and it is easier to avoid mistakes by following a simple
+convention that always works--no whitespace after a function name.
+
+ Each built-in function accepts a certain number of arguments. In
+some cases, arguments can be omitted. The defaults for omitted
+arguments vary from function to function and are described under the
+individual functions. In some 'awk' implementations, extra arguments
+given to built-in functions are ignored. However, in 'gawk', it is a
+fatal error to give extra arguments to a built-in function.
+
+ When a function is called, expressions that create the function's
+actual parameters are evaluated completely before the call is performed.
+For example, in the following code fragment:
+
+ i = 4
+ j = sqrt(i++)
+
+the variable 'i' is incremented to the value five before 'sqrt()' is
+called with a value of four for its actual parameter. The order of
+evaluation of the expressions used for the function's parameters is
+undefined. Thus, avoid writing programs that assume that parameters are
+evaluated from left to right or from right to left. For example:
+
+ i = 5
+ j = atan2(++i, i *= 2)
+
+ If the order of evaluation is left to right, then 'i' first becomes
+six, and then 12, and 'atan2()' is called with the two arguments six and
+12. But if the order of evaluation is right to left, 'i' first becomes
+10, then 11, and 'atan2()' is called with the two arguments 11 and 10.
+
+
+File: gawk.info, Node: Boolean Functions, Next: Numeric Functions, Prev:
Calling Built-in, Up: Built-in
+
+9.1.2 Generating Boolean Values
+-------------------------------
+
+This function is specific to 'gawk'. It is not available in
+compatibility mode (*note Options::):
+
+'mkbool(EXPRESSION)'
+ Return a Boolean-typed value based on the regular Boolean value of
+ EXPRESSION. Boolean "true" values have numeric value one. Boolean
+ "false" values have numeric zero. This is discussed in more detail
+ in *note Boolean Typed Values::.
+
+
+File: gawk.info, Node: Numeric Functions, Next: String Functions, Prev:
Boolean Functions, Up: Built-in
+
+9.1.3 Numeric Functions
+-----------------------
+
+The following list describes all of the built-in functions that work
+with numbers. Optional parameters are enclosed in square
+brackets ([ ]):
+
+'atan2(Y, X)'
+ Return the arctangent of 'Y / X' in radians. You can use 'pi =
+ atan2(0, -1)' to retrieve the value of pi.
+
+'cos(X)'
+ Return the cosine of X, with X in radians.
+
+'exp(X)'
+ Return the exponential of X ('e ^ X') or report an error if X is
+ out of range. The range of values X can have depends on your
+ machine's floating-point representation.
+
+'int(X)'
+ Return the nearest integer to X, located between X and zero and
+ truncated toward zero. For example, 'int(3)' is 3, 'int(3.9)' is
+ 3, 'int(-3.9)' is -3, and 'int(-3)' is -3 as well.
+
+'log(X)'
+ Return the natural logarithm of X, if X is positive; otherwise,
+ return NaN ("not a number") on IEEE 754 systems. Additionally,
+ 'gawk' prints a warning message when 'x' is negative.
+
+'rand()'
+ Return a random number. The values of 'rand()' are uniformly
+ distributed between zero and one. The value could be zero but is
+ never one.(1)
+
+ Often random integers are needed instead. Following is a
+ user-defined function that can be used to obtain a random
+ nonnegative integer less than N:
+
+ function randint(n)
+ {
+ return int(n * rand())
+ }
+
+ The multiplication produces a random number greater than or equal
+ to zero and less than 'n'. Using 'int()', this result is made into
+ an integer between zero and 'n' - 1, inclusive.
+
+ The following example uses a similar function to produce random
+ integers between one and N. This program prints a new random
+ number for each input record:
+
+ # Function to roll a simulated die.
+ function roll(n) { return 1 + int(rand() * n) }
+
+ # Roll 3 six-sided dice and
+ # print total number of points.
+ {
+ printf("%d points\n", roll(6) + roll(6) + roll(6))
+ }
+
+ CAUTION: In most 'awk' implementations, including 'gawk',
+ 'rand()' starts generating numbers from the same starting
+ number, or "seed", each time you run 'awk'.(2) Thus, a
+ program generates the same results each time you run it. The
+ numbers are random within one 'awk' run but predictable from
+ run to run. This is convenient for debugging, but if you want
+ a program to do different things each time it is used, you
+ must change the seed to a value that is different in each run.
+ To do this, use 'srand()'.
+
+'sin(X)'
+ Return the sine of X, with X in radians.
+
+'sqrt(X)'
+ Return the positive square root of X. 'gawk' prints a warning
+ message if X is negative. Thus, 'sqrt(4)' is 2.
+
+'srand('[X]')'
+ Set the starting point, or seed, for generating random numbers to
+ the value X.
+
+ Each seed value leads to a particular sequence of random
+ numbers.(3) Thus, if the seed is set to the same value a second
+ time, the same sequence of random numbers is produced again.
+
+ CAUTION: Different 'awk' implementations use different
+ random-number generators internally. Don't expect the same
+ 'awk' program to produce the same series of random numbers
+ when executed by different versions of 'awk'.
+
+ If the argument X is omitted, as in 'srand()', then the current
+ date and time of day are used for a seed. This is the way to get
+ random numbers that are truly unpredictable.
+
+ The return value of 'srand()' is the previous seed. This makes it
+ easy to keep track of the seeds in case you need to consistently
+ reproduce sequences of random numbers.
+
+ POSIX does not specify the initial seed; it differs among 'awk'
+ implementations.
+
+ ---------- Footnotes ----------
+
+ (1) The C version of 'rand()' on many Unix systems is known to
+produce fairly poor sequences of random numbers. However, nothing
+requires that an 'awk' implementation use the C 'rand()' to implement
+the 'awk' version of 'rand()'. In fact, for many years, 'gawk' used the
+BSD 'random()' function, which is considerably better than 'rand()', to
+produce random numbers. From version 4.1.4, courtesy of Nelson H.F.
+Beebe, 'gawk' uses the Bayes-Durham shuffle buffer algorithm which
+considerably extends the period of the random number generator, and
+eliminates short-range and long-range correlations that might exist in
+the original generator.
+
+ (2) 'mawk' uses a different seed each time.
+
+ (3) Computer-generated random numbers really are not truly random.
+They are technically known as "pseudorandom". This means that although
+the numbers in a sequence appear to be random, you can in fact generate
+the same sequence of random numbers over and over again.
+
+
+File: gawk.info, Node: String Functions, Next: I/O Functions, Prev: Numeric
Functions, Up: Built-in
+
+9.1.4 String-Manipulation Functions
+-----------------------------------
+
+The functions in this minor node look at or change the text of one or
+more strings.
+
+ 'gawk' understands locales (*note Locales::) and does all string
+processing in terms of _characters_, not _bytes_. This distinction is
+particularly important to understand for locales where one character may
+be represented by multiple bytes. Thus, for example, 'length()' returns
+the number of characters in a string, and not the number of bytes used
+to represent those characters. Similarly, 'index()' works with
+character indices, and not byte indices.
+
+ CAUTION: A number of functions deal with indices into strings. For
+ these functions, the first character of a string is at position
+ (index) one. This is different from C and the languages descended
+ from it, where the first character is at position zero. You need
+ to remember this when doing index calculations, particularly if you
+ are used to C.
+
+ In the following list, optional parameters are enclosed in square
+brackets ([ ]). Several functions perform string substitution; the full
+discussion is provided in the description of the 'sub()' function, which
+comes toward the end, because the list is presented alphabetically.
+
+ Those functions that are specific to 'gawk' are marked with a pound
+sign ('#'). They are not available in compatibility mode (*note
+Options::):
+
+* Menu:
+
+* Gory Details:: More than you want to know about '\' and
+ '&' with 'sub()', 'gsub()', and
+ 'gensub()'.
+
+'asort('SOURCE [',' DEST [',' HOW ] ]') #'
+'asorti('SOURCE [',' DEST [',' HOW ] ]') #'
+ These two functions are similar in behavior, so they are described
+ together.
+
+ NOTE: The following description ignores the third argument,
+ HOW, as it requires understanding features that we have not
+ discussed yet. Thus, the discussion here is a deliberate
+ simplification. (We do provide all the details later on; see
+ *note Array Sorting Functions:: for the full story.)
+
+ Both functions return the number of elements in the array SOURCE.
+ For 'asort()', 'gawk' sorts the values of SOURCE and replaces the
+ indices of the sorted values of SOURCE with sequential integers
+ starting with one. If the optional array DEST is specified, then
+ SOURCE is duplicated into DEST. DEST is then sorted, leaving the
+ indices of SOURCE unchanged.
+
+ When comparing strings, 'IGNORECASE' affects the sorting (*note
+ Array Sorting Functions::). If the SOURCE array contains subarrays
+ as values (*note Arrays of Arrays::), they will come last, after
+ all scalar values. Subarrays are _not_ recursively sorted.
+
+ For example, if the contents of 'a' are as follows:
+
+ a["last"] = "de"
+ a["first"] = "sac"
+ a["middle"] = "cul"
+
+ A call to 'asort()':
+
+ asort(a)
+
+ results in the following contents of 'a':
+
+ a[1] = "cul"
+ a[2] = "de"
+ a[3] = "sac"
+
+ The 'asorti()' function works similarly to 'asort()'; however, the
+ _indices_ are sorted, instead of the values. Thus, in the previous
+ example, starting with the same initial set of indices and values
+ in 'a', calling 'asorti(a)' would yield:
+
+ a[1] = "first"
+ a[2] = "last"
+ a[3] = "middle"
+
+ NOTE: You may not use either 'SYMTAB' or 'FUNCTAB' as the
+ second argument to these functions. Attempting to do so
+ produces a fatal error. You may use them as the first
+ argument, but only if providing a second array to use for the
+ actual sorting.
+
+ You are allowed to use the same array for both the SOURCE and DEST
+ arguments, but doing so only makes sense if you're also supplying
+ the third argument.
+
+'gensub(REGEXP, REPLACEMENT, HOW' [', TARGET']') #'
+ Search the target string TARGET for matches of the regular
+ expression REGEXP. If HOW is a string beginning with 'g' or 'G'
+ (short for "global"), then replace all matches of REGEXP with
+ REPLACEMENT. Otherwise, treat HOW as a number indicating which
+ match of REGEXP to replace. Treat numeric values less than one as
+ if they were one. If no TARGET is supplied, use '$0'. Return the
+ modified string as the result of the function. The original target
+ string is _not_ changed.
+
+ The returned value is _always_ a string, even if the original
+ TARGET was a number or a regexp value.
+
+ 'gensub()' is a general substitution function. Its purpose is to
+ provide more features than the standard 'sub()' and 'gsub()'
+ functions.
+
+ 'gensub()' provides an additional feature that is not available in
+ 'sub()' or 'gsub()': the ability to specify components of a regexp
+ in the replacement text. This is done by using parentheses in the
+ regexp to mark the components and then specifying '\N' in the
+ replacement text, where N is a digit from 1 to 9. For example:
+
+ $ gawk '
+ > BEGIN {
+ > a = "abc def"
+ > b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
+ > print b
+ > }'
+ -| def abc
+
+ As with 'sub()', you must type two backslashes in order to get one
+ into the string. In the replacement text, the sequence '\0'
+ represents the entire matched text, as does the character '&'.
+
+ The following example shows how you can use the third argument to
+ control which match of the regexp should be changed:
+
+ $ echo a b c a b c |
+ > gawk '{ print gensub(/a/, "AA", 2) }'
+ -| a b c AA b c
+
+ In this case, '$0' is the default target string. 'gensub()'
+ returns the new string as its result, which is passed directly to
+ 'print' for printing.
+
+ If the HOW argument is a string that does not begin with 'g' or
+ 'G', or if it is a number that is less than or equal to zero, only
+ one substitution is performed. If HOW is zero, 'gawk' issues a
+ warning message.
+
+ If REGEXP does not match TARGET, 'gensub()''s return value is the
+ original unchanged value of TARGET. Note that, as mentioned above,
+ the returned value is a string, even if TARGET was not.
+
+'gsub(REGEXP, REPLACEMENT' [', TARGET']')'
+ Search TARGET for _all_ of the longest, leftmost, _nonoverlapping_
+ matching substrings it can find and replace them with REPLACEMENT.
+ The 'g' in 'gsub()' stands for "global," which means replace
+ everywhere. For example:
+
+ { gsub(/Britain/, "United Kingdom"); print }
+
+ replaces all occurrences of the string 'Britain' with 'United
+ Kingdom' for all input records.
+
+ The 'gsub()' function returns the number of substitutions made. If
+ the variable to search and alter (TARGET) is omitted, then the
+ entire input record ('$0') is used. As in 'sub()', the characters
+ '&' and '\' are special, and the third argument must be assignable.
+
+'index(IN, FIND)'
+ Search the string IN for the first occurrence of the string FIND,
+ and return the position in characters where that occurrence begins
+ in the string IN. Consider the following example:
+
+ $ awk 'BEGIN { print index("peanut", "an") }'
+ -| 3
+
+ If FIND is not found, 'index()' returns zero.
+
+ With BWK 'awk' and 'gawk', it is a fatal error to use a regexp
+ constant for FIND. Other implementations allow it, simply treating
+ the regexp constant as an expression meaning '$0 ~ /regexp/'.
+ (d.c.)
+
+'length('[STRING]')'
+ Return the number of characters in STRING. If STRING is a number,
+ the length of the digit string representing that number is
+ returned. For example, 'length("abcde")' is five. By contrast,
+ 'length(15 * 35)' works out to three. In this example, 15 * 35 =
+ 525, and 525 is then converted to the string '"525"', which has
+ three characters.
+
+ If no argument is supplied, 'length()' returns the length of '$0'.
+
+ NOTE: In older versions of 'awk', the 'length()' function
+ could be called without any parentheses. Doing so is
+ considered poor practice, although the 2008 POSIX standard
+ explicitly allows it, to support historical practice. For
+ programs to be maximally portable, always supply the
+ parentheses.
+
+ If 'length()' is called with a variable that has not been used,
+ 'gawk' forces the variable to be a scalar. Other implementations
+ of 'awk' leave the variable without a type. (d.c.) Consider:
+
+ $ gawk 'BEGIN { print length(x) ; x[1] = 1 }'
+ -| 0
+ error-> gawk: fatal: attempt to use scalar `x' as array
+
+ $ nawk 'BEGIN { print length(x) ; x[1] = 1 }'
+ -| 0
+
+ If '--lint' has been specified on the command line, 'gawk' issues a
+ warning about this.
+
+ With 'gawk' and several other 'awk' implementations, when given an
+ array argument, the 'length()' function returns the number of
+ elements in the array. (c.e.) This is less useful than it might
+ seem at first, as the array is not guaranteed to be indexed from
+ one to the number of elements in it. If '--lint' is provided on
+ the command line (*note Options::), 'gawk' warns that passing an
+ array argument is not portable. If '--posix' is supplied, using an
+ array argument is a fatal error (*note Arrays::).
+
+'match(STRING, REGEXP' [', ARRAY']')'
+ Search STRING for the longest, leftmost substring matched by the
+ regular expression REGEXP and return the character position (index)
+ at which that substring begins (one, if it starts at the beginning
+ of STRING). If no match is found, return zero.
+
+ The REGEXP argument may be either a regexp constant ('/'...'/') or
+ a string constant ('"'...'"'). In the latter case, the string is
+ treated as a regexp to be matched. *Note Computed Regexps:: for a
+ discussion of the difference between the two forms, and the
+ implications for writing your program correctly.
+
+ The order of the first two arguments is the opposite of most other
+ string functions that work with regular expressions, such as
+ 'sub()' and 'gsub()'. It might help to remember that for
+ 'match()', the order is the same as for the '~' operator: 'STRING ~
+ REGEXP'.
+
+ The 'match()' function sets the predefined variable 'RSTART' to the
+ index. It also sets the predefined variable 'RLENGTH' to the
+ length in characters of the matched substring. If no match is
+ found, 'RSTART' is set to zero, and 'RLENGTH' to -1.
+
+ For example:
+
+ {
+ if ($1 == "FIND")
+ regex = $2
+ else {
+ where = match($0, regex)
+ if (where != 0)
+ print "Match of", regex, "found at", where, "in", $0
+ }
+ }
+
+ This program looks for lines that match the regular expression
+ stored in the variable 'regex'. This regular expression can be
+ changed. If the first word on a line is 'FIND', 'regex' is changed
+ to be the second word on that line. Therefore, if given:
+
+ FIND ru+n
+ My program runs
+ but not very quickly
+ FIND Melvin
+ JF+KM
+ This line is property of Reality Engineering Co.
+ Melvin was here.
+
+ 'awk' prints:
+
+ Match of ru+n found at 12 in My program runs
+ Match of Melvin found at 1 in Melvin was here.
+
+ If ARRAY is present, it is cleared, and then the zeroth element of
+ ARRAY is set to the entire portion of STRING matched by REGEXP. If
+ REGEXP contains parentheses, the integer-indexed elements of ARRAY
+ are set to contain the portion of STRING matching the corresponding
+ parenthesized subexpression. For example:
+
+ $ echo foooobazbarrrrr |
+ > gawk '{ match($0, /(fo+).+(bar*)/, arr)
+ > print arr[1], arr[2] }'
+ -| foooo barrrrr
+
+ In addition, multidimensional subscripts are available providing
+ the start index and length of each matched subexpression:
+
+ $ echo foooobazbarrrrr |
+ > gawk '{ match($0, /(fo+).+(bar*)/, arr)
+ > print arr[1], arr[2]
+ > print arr[1, "start"], arr[1, "length"]
+ > print arr[2, "start"], arr[2, "length"]
+ > }'
+ -| foooo barrrrr
+ -| 1 5
+ -| 9 7
+
+ There may not be subscripts for the start and index for every
+ parenthesized subexpression, because they may not all have matched
+ text; thus, they should be tested for with the 'in' operator (*note
+ Reference to Elements::).
+
+ The ARRAY argument to 'match()' is a 'gawk' extension. In
+ compatibility mode (*note Options::), using a third argument is a
+ fatal error.
+
+'patsplit(STRING, ARRAY' [', FIELDPAT' [', SEPS' ] ]') #'
+ Divide STRING into pieces (or "fields") defined by FIELDPAT and
+ store the pieces in ARRAY and the separator strings in the SEPS
+ array. The first piece is stored in 'ARRAY[1]', the second piece
+ in 'ARRAY[2]', and so forth. The third argument, FIELDPAT, is a
+ regexp describing the fields in STRING (just as 'FPAT' is a regexp
+ describing the fields in input records). It may be either a regexp
+ constant or a string. If FIELDPAT is omitted, the value of 'FPAT'
+ is used. 'patsplit()' returns the number of elements created.
+ 'SEPS[I]' is the possibly null separator string after 'ARRAY[I]'.
+ The possibly null leading separator will be in 'SEPS[0]'. So a
+ non-null STRING with N fields will have N+1 separators. A null
+ STRING has no fields or separators.
+
+ The 'patsplit()' function splits strings into pieces in a manner
+ similar to the way input lines are split into fields using 'FPAT'
+ (*note Splitting By Content::).
+
+ Before splitting the string, 'patsplit()' deletes any previously
+ existing elements in the arrays ARRAY and SEPS.
+
+'split(STRING, ARRAY' [', FIELDSEP' [', SEPS' ] ]')'
+ Divide STRING into pieces separated by FIELDSEP and store the
+ pieces in ARRAY and the separator strings in the SEPS array. The
+ first piece is stored in 'ARRAY[1]', the second piece in
+ 'ARRAY[2]', and so forth. The string value of the third argument,
+ FIELDSEP, is a regexp describing where to split STRING (much as
+ 'FS' can be a regexp describing where to split input records). If
+ FIELDSEP is omitted, the value of 'FS' is used. 'split()' returns
+ the number of elements created. SEPS is a 'gawk' extension, with
+ 'SEPS[I]' being the separator string between 'ARRAY[I]' and
+ 'ARRAY[I+1]'. If FIELDSEP is a single space, then any leading
+ whitespace goes into 'SEPS[0]' and any trailing whitespace goes
+ into 'SEPS[N]', where N is the return value of 'split()' (i.e., the
+ number of elements in ARRAY).
+
+ The 'split()' function splits strings into pieces in the same way
+ that input lines are split into fields. For example:
+
+ split("cul-de-sac", a, "-", seps)
+
+ splits the string '"cul-de-sac"' into three fields using '-' as the
+ separator. It sets the contents of the array 'a' as follows:
+
+ a[1] = "cul"
+ a[2] = "de"
+ a[3] = "sac"
+
+ and sets the contents of the array 'seps' as follows:
+
+ seps[1] = "-"
+ seps[2] = "-"
+
+ The value returned by this call to 'split()' is three.
+
+ As with input field-splitting, when the value of FIELDSEP is '" "',
+ leading and trailing whitespace is ignored in values assigned to
+ the elements of ARRAY but not in SEPS, and the elements are
+ separated by runs of whitespace. Also, as with input field
+ splitting, if FIELDSEP is the null string, each individual
+ character in the string is split into its own array element.
+ (c.e.) Additionally, if FIELDSEP is a single-character string,
+ that string acts as the separator, even if its value is a regular
+ expression metacharacter.
+
+ Note, however, that 'RS' has no effect on the way 'split()' works.
+ Even though 'RS = ""' causes the newline character to also be an
+ input field separator, this does not affect how 'split()' splits
+ strings.
+
+ Modern implementations of 'awk', including 'gawk', allow the third
+ argument to be a regexp constant ('/'...'/') as well as a string.
+ (d.c.) The POSIX standard allows this as well. *Note Computed
+ Regexps:: for a discussion of the difference between using a string
+ constant or a regexp constant, and the implications for writing
+ your program correctly.
+
+ Before splitting the string, 'split()' deletes any previously
+ existing elements in the arrays ARRAY and SEPS.
+
+ If STRING is null, the array has no elements. (So this is a
+ portable way to delete an entire array with one statement. *Note
+ Delete::.)
+
+ If STRING does not match FIELDSEP at all (but is not null), ARRAY
+ has one element only. The value of that element is the original
+ STRING.
+
+ In POSIX mode (*note Options::), the fourth argument is not
+ allowed.
+
+'sprintf(FORMAT, EXPRESSION1, ...)'
+ Return (without printing) the string that 'printf' would have
+ printed out with the same arguments (*note Printf::). For example:
+
+ pival = sprintf("pi = %.2f (approx.)", 22/7)
+
+ assigns the string 'pi = 3.14 (approx.)' to the variable 'pival'.
+
+'strtonum(STR) #'
+ Examine STR and return its numeric value. If STR begins with a
+ leading '0', 'strtonum()' assumes that STR is an octal number. If
+ STR begins with a leading '0x' or '0X', 'strtonum()' assumes that
+ STR is a hexadecimal number. For example:
+
+ $ echo 0x11 |
+ > gawk '{ printf "%d\n", strtonum($1) }'
+ -| 17
+
+ Using the 'strtonum()' function is _not_ the same as adding zero to
+ a string value; the automatic coercion of strings to numbers works
+ only for decimal data, not for octal or hexadecimal.(1)
+
+ Note also that 'strtonum()' uses the current locale's decimal point
+ for recognizing numbers (*note Locales::).
+
+'sub(REGEXP, REPLACEMENT' [', TARGET']')'
+ Search TARGET, which is treated as a string, for the leftmost,
+ longest substring matched by the regular expression REGEXP. Modify
+ the entire string by replacing the matched text with REPLACEMENT.
+ The modified string becomes the new value of TARGET. Return the
+ number of substitutions made (zero or one).
+
+ The REGEXP argument may be either a regexp constant ('/'...'/') or
+ a string constant ('"'...'"'). In the latter case, the string is
+ treated as a regexp to be matched. *Note Computed Regexps:: for a
+ discussion of the difference between the two forms, and the
+ implications for writing your program correctly.
+
+ This function is peculiar because TARGET is not simply used to
+ compute a value, and not just any expression will do--it must be a
+ variable, field, or array element so that 'sub()' can store a
+ modified value there. If this argument is omitted, then the
+ default is to use and alter '$0'.(2) For example:
+
+ str = "water, water, everywhere"
+ sub(/at/, "ith", str)
+
+ sets 'str' to 'wither, water, everywhere', by replacing the
+ leftmost longest occurrence of 'at' with 'ith'.
+
+ If the special character '&' appears in REPLACEMENT, it stands for
+ the precise substring that was matched by REGEXP. (If the regexp
+ can match more than one string, then this precise substring may
+ vary.) For example:
+
+ { sub(/candidate/, "& and his wife"); print }
+
+ changes the first occurrence of 'candidate' to 'candidate and his
+ wife' on each input line. Here is another example:
+
+ $ awk 'BEGIN {
+ > str = "daabaaa"
+ > sub(/a+/, "C&C", str)
+ > print str
+ > }'
+ -| dCaaCbaaa
+
+ This shows how '&' can represent a nonconstant string and also
+ illustrates the "leftmost, longest" rule in regexp matching (*note
+ Leftmost Longest::).
+
+ The effect of this special character ('&') can be turned off by
+ putting a backslash before it in the string. As usual, to insert
+ one backslash in the string, you must write two backslashes.
+ Therefore, write '\\&' in a string constant to include a literal
+ '&' in the replacement. For example, the following shows how to
+ replace the first '|' on each line with an '&':
+
+ { sub(/\|/, "\\&"); print }
+
+ As mentioned, the third argument to 'sub()' must be a variable,
+ field, or array element. Some versions of 'awk' allow the third
+ argument to be an expression that is not an lvalue. In such a
+ case, 'sub()' still searches for the pattern and returns zero or
+ one, but the result of the substitution (if any) is thrown away
+ because there is no place to put it. Such versions of 'awk' accept
+ expressions like the following:
+
+ sub(/USA/, "United States", "the USA and Canada")
+
+ For historical compatibility, 'gawk' accepts such erroneous code.
+ However, using any other nonchangeable object as the third
+ parameter causes a fatal error and your program will not run.
+
+ Finally, if the REGEXP is not a regexp constant, it is converted
+ into a string, and then the value of that string is treated as the
+ regexp to match.
+
+'substr(STRING, START' [', LENGTH' ]')'
+ Return a LENGTH-character-long substring of STRING, starting at
+ character number START. The first character of a string is
+ character number one.(3) For example, 'substr("washington", 5, 3)'
+ returns '"ing"'.
+
+ If LENGTH is not present, 'substr()' returns the whole suffix of
+ STRING that begins at character number START. For example,
+ 'substr("washington", 5)' returns '"ington"'. The whole suffix is
+ also returned if LENGTH is greater than the number of characters
+ remaining in the string, counting from character START.
+
+ If START is less than one, 'substr()' treats it as if it was one.
+ (POSIX doesn't specify what to do in this case: BWK 'awk' acts this
+ way, and therefore 'gawk' does too.) If START is greater than the
+ number of characters in the string, 'substr()' returns the null
+ string. Similarly, if LENGTH is present but less than or equal to
+ zero, the null string is returned.
+
+ The string returned by 'substr()' _cannot_ be assigned. Thus, it
+ is a mistake to attempt to change a portion of a string, as shown
+ in the following example:
+
+ string = "abcdef"
+ # try to get "abCDEf", won't work
+ substr(string, 3, 3) = "CDE"
+
+ It is also a mistake to use 'substr()' as the third argument of
+ 'sub()' or 'gsub()':
+
+ gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG
+
+ (Some commercial versions of 'awk' treat 'substr()' as assignable,
+ but doing so is not portable.)
+
+ If you need to replace bits and pieces of a string, combine
+ 'substr()' with string concatenation, in the following manner:
+
+ string = "abcdef"
+ ...
+ string = substr(string, 1, 2) "CDE" substr(string, 6)
+
+'tolower(STRING)'
+ Return a copy of STRING, with each uppercase character in the
+ string replaced with its corresponding lowercase character.
+ Nonalphabetic characters are left unchanged. For example,
+ 'tolower("MiXeD cAsE 123")' returns '"mixed case 123"'.
+
+'toupper(STRING)'
+ Return a copy of STRING, with each lowercase character in the
+ string replaced with its corresponding uppercase character.
+ Nonalphabetic characters are left unchanged. For example,
+ 'toupper("MiXeD cAsE 123")' returns '"MIXED CASE 123"'.
+
+ At first glance, the 'split()' and 'patsplit()' functions appear to
+be mirror images of each other. But there are differences:
+
+ * 'split()' treats its third argument like 'FS', with all the special
+ rules involved for 'FS'.
+
+ * Matching of null strings differs. This is discussed in *note FS
+ versus FPAT::.
+
+ Matching the Null String
+
+ In 'awk', the '*' operator can match the null string. This is
+particularly important for the 'sub()', 'gsub()', and 'gensub()'
+functions. For example:
+
+ $ echo abc | awk '{ gsub(/m*/, "X"); print }'
+ -| XaXbXcX
+
+Although this makes a certain amount of sense, it can be surprising.
+
+ ---------- Footnotes ----------
+
+ (1) Unless you use the '--non-decimal-data' option, which isn't
+recommended. *Note Nondecimal Data:: for more information.
+
+ (2) Note that this means that the record will first be regenerated
+using the value of 'OFS' if any fields have been changed, and that the
+fields will be updated after the substitution, even if the operation is
+a "no-op" such as 'sub(/^/, "")'.
+
+ (3) This is different from C and C++, in which the first character is
+number zero.
+
+
+File: gawk.info, Node: Gory Details, Up: String Functions
+
+9.1.4.1 More about '\' and '&' with 'sub()', 'gsub()', and 'gensub()'
+.....................................................................
+
+ CAUTION: This subsubsection has been reported to cause headaches.
+ You might want to skip it upon first reading.
+
+ When using 'sub()', 'gsub()', or 'gensub()', and trying to get
+literal backslashes and ampersands into the replacement text, you need
+to remember that there are several levels of "escape processing" going
+on.
+
+ First, there is the "lexical" level, which is when 'awk' reads your
+program and builds an internal copy of it to execute. Then there is the
+runtime level, which is when 'awk' actually scans the replacement string
+to determine what to generate.
+
+ At both levels, 'awk' looks for a defined set of characters that can
+come after a backslash. At the lexical level, it looks for the escape
+sequences listed in *note Escape Sequences::. Thus, for every '\' that
+'awk' processes at the runtime level, you must type two backslashes at
+the lexical level. When a character that is not valid for an escape
+sequence follows the '\', BWK 'awk' and 'gawk' both simply remove the
+initial '\' and put the next character into the string. Thus, for
+example, '"a\qb"' is treated as '"aqb"'.
+
+ At the runtime level, the various functions handle sequences of '\'
+and '&' differently. The situation is (sadly) somewhat complex.
+Historically, the 'sub()' and 'gsub()' functions treated the
+two-character sequence '\&' specially; this sequence was replaced in the
+generated text with a single '&'. Any other '\' within the REPLACEMENT
+string that did not precede an '&' was passed through unchanged. This
+is illustrated in *note Table 9.1: table-sub-escapes.
+
+
+ You type 'sub()' sees 'sub()' generates
+ ----- ------- ----------
+ '\&' '&' The matched text
+ '\\&' '\&' A literal '&'
+ '\\\&' '\&' A literal '&'
+ '\\\\&' '\\&' A literal '\&'
+ '\\\\\&' '\\&' A literal '\&'
+ '\\\\\\&' '\\\&' A literal '\\&'
+ '\\q' '\q' A literal '\q'
+
+Table 9.1: Historical escape sequence processing for 'sub()' and
+'gsub()'
+
+This table shows the lexical-level processing, where an odd number of
+backslashes becomes an even number at the runtime level, as well as the
+runtime processing done by 'sub()'. (For the sake of simplicity, the
+rest of the following tables only show the case of even numbers of
+backslashes entered at the lexical level.)
+
+ The problem with the historical approach is that there is no way to
+get a literal '\' followed by the matched text.
+
+ Several editions of the POSIX standard attempted to fix this problem
+but weren't successful. The details are irrelevant at this point in
+time.
+
+ At one point, the 'gawk' maintainer submitted proposed text for a
+revised standard that reverts to rules that correspond more closely to
+the original existing practice. The proposed rules have special cases
+that make it possible to produce a '\' preceding the matched text. This
+is shown in *note Table 9.2: table-sub-proposed.
+
+
+ You type 'sub()' sees 'sub()' generates
+ ----- ------- ----------
+ '\\\\\\&' '\\\&' A literal '\&'
+ '\\\\&' '\\&' A literal '\', followed by the
matched text
+ '\\&' '\&' A literal '&'
+ '\\q' '\q' A literal '\q'
+ '\\\\' '\\' '\\'
+
+Table 9.2: 'gawk' rules for 'sub()' and backslash
+
+ In a nutshell, at the runtime level, there are now three special
+sequences of characters ('\\\&', '\\&', and '\&') whereas historically
+there was only one. However, as in the historical case, any '\' that is
+not part of one of these three sequences is not special and appears in
+the output literally.
+
+ 'gawk' 3.0 and 3.1 follow these rules for 'sub()' and 'gsub()'. The
+POSIX standard took much longer to be revised than was expected. In
+addition, the 'gawk' maintainer's proposal was lost during the
+standardization process. The final rules are somewhat simpler. The
+results are similar except for one case.
+
+ The POSIX rules state that '\&' in the replacement string produces a
+literal '&', '\\' produces a literal '\', and '\' followed by anything
+else is not special; the '\' is placed straight into the output. These
+rules are presented in *note Table 9.3: table-posix-sub.
+
+
+ You type 'sub()' sees 'sub()' generates
+ ----- ------- ----------
+ '\\\\\\&' '\\\&' A literal '\&'
+ '\\\\&' '\\&' A literal '\', followed by the
matched text
+ '\\&' '\&' A literal '&'
+ '\\q' '\q' A literal '\q'
+ '\\\\' '\\' '\'
+
+Table 9.3: POSIX rules for 'sub()' and 'gsub()'
+
+ The only case where the difference is noticeable is the last one:
+'\\\\' is seen as '\\' and produces '\' instead of '\\'.
+
+ Starting with version 3.1.4, 'gawk' followed the POSIX rules when
+'--posix' was specified (*note Options::). Otherwise, it continued to
+follow the proposed rules, as that had been its behavior for many years.
+
+ When version 4.0.0 was released, the 'gawk' maintainer made the POSIX
+rules the default, breaking well over a decade's worth of backward
+compatibility.(1) Needless to say, this was a bad idea, and as of
+version 4.0.1, 'gawk' resumed its historical behavior, and only follows
+the POSIX rules when '--posix' is given.
+
+ The rules for 'gensub()' are considerably simpler. At the runtime
+level, whenever 'gawk' sees a '\', if the following character is a
+digit, then the text that matched the corresponding parenthesized
+subexpression is placed in the generated output. Otherwise, no matter
+what character follows the '\', it appears in the generated text and the
+'\' does not, as shown in *note Table 9.4: table-gensub-escapes.
+
+
+ You type 'gensub()' sees 'gensub()' generates
+ ----- --------- ------------
+ '&' '&' The matched text
+ '\\&' '\&' A literal '&'
+ '\\\\' '\\' A literal '\'
+ '\\\\&' '\\&' A literal '\', then the
matched text
+ '\\\\\\&' '\\\&' A literal '\&'
+ '\\q' '\q' A literal 'q'
+
+Table 9.4: Escape sequence processing for 'gensub()'
+
+ Because of the complexity of the lexical- and runtime-level
+processing and the special cases for 'sub()' and 'gsub()', we recommend
+the use of 'gawk' and 'gensub()' when you have to do substitutions.
+
+ ---------- Footnotes ----------
+
+ (1) This was rather naive of him, despite there being a note in this
+minor node indicating that the next major version would move to the
+POSIX rules.
+
+
+File: gawk.info, Node: I/O Functions, Next: Time Functions, Prev: String
Functions, Up: Built-in
+
+9.1.5 Input/Output Functions
+----------------------------
+
+The following functions relate to input/output (I/O). Optional
+parameters are enclosed in square brackets ([ ]):
+
+'close('FILENAME [',' HOW]')'
+ Close the file FILENAME for input or output. Alternatively, the
+ argument may be a shell command that was used for creating a
+ coprocess, or for redirecting to or from a pipe; then the coprocess
+ or pipe is closed. *Note Close Files And Pipes:: for more
+ information.
+
+ When closing a coprocess, it is occasionally useful to first close
+ one end of the two-way pipe and then to close the other. This is
+ done by providing a second argument to 'close()'. This second
+ argument (HOW) should be one of the two string values '"to"' or
+ '"from"', indicating which end of the pipe to close. Case in the
+ string does not matter. *Note Two-way I/O::, which discusses this
+ feature in more detail and gives an example.
+
+ Note that the second argument to 'close()' is a 'gawk' extension;
+ it is not available in compatibility mode (*note Options::).
+
+'fflush('[FILENAME]')'
+ Flush any buffered output associated with FILENAME, which is either
+ a file opened for writing or a shell command for redirecting output
+ to a pipe or coprocess.
+
+ Many utility programs "buffer" their output (i.e., they save
+ information to write to a disk file or the screen in memory until
+ there is enough for it to be worthwhile to send the data to the
+ output device). This is often more efficient than writing every
+ little bit of information as soon as it is ready. However,
+ sometimes it is necessary to force a program to "flush" its buffers
+ (i.e., write the information to its destination, even if a buffer
+ is not full). This is the purpose of the 'fflush()'
+ function--'gawk' also buffers its output, and the 'fflush()'
+ function forces 'gawk' to flush its buffers.
+
+ Brian Kernighan added 'fflush()' to his 'awk' in April 1992. For
+ two decades, it was a common extension. In December 2012, it was
+ accepted for inclusion into the POSIX standard. See the Austin
+ Group website (http://austingroupbugs.net/view.php?id=634).
+
+ POSIX standardizes 'fflush()' as follows: if there is no argument,
+ or if the argument is the null string ('""'), then 'awk' flushes
+ the buffers for _all_ open output files and pipes.
+
+ NOTE: Prior to version 4.0.2, 'gawk' would flush only the
+ standard output if there was no argument, and flush all output
+ files and pipes if the argument was the null string. This was
+ changed in order to be compatible with BWK 'awk', in the hope
+ that standardizing this feature in POSIX would then be easier
+ (which indeed proved to be the case).
+
+ With 'gawk', you can use 'fflush("/dev/stdout")' if you wish
+ to flush only the standard output.
+
+ 'fflush()' returns zero if the buffer is successfully flushed;
+ otherwise, it returns a nonzero value. ('gawk' returns -1.) In
+ the case where all buffers are flushed, the return value is zero
+ only if all buffers were flushed successfully. Otherwise, it is
+ -1, and 'gawk' warns about the problem FILENAME.
+
+ 'gawk' also issues a warning message if you attempt to flush a file
+ or pipe that was opened for reading (such as with 'getline'), or if
+ FILENAME is not an open file, pipe, or coprocess. In such a case,
+ 'fflush()' returns -1, as well.
+
+ Interactive Versus Noninteractive Buffering
+
+ As a side point, buffering issues can be even more confusing if your
+program is "interactive" (i.e., communicating with a user sitting at a
+keyboard).(1)
+
+ Interactive programs generally "line buffer" their output (i.e., they
+write out every line). Noninteractive programs wait until they have a
+full buffer, which may be many lines of output. Here is an example of
+the difference:
+
+ $ awk '{ print $1 + $2 }'
+ 1 1
+ -| 2
+ 2 3
+ -| 5
+ Ctrl-d
+
+Each line of output is printed immediately. Compare that behavior with
+this example:
+
+ $ awk '{ print $1 + $2 }' | cat
+ 1 1
+ 2 3
+ Ctrl-d
+ -| 2
+ -| 5
+
+Here, no output is printed until after the 'Ctrl-d' is typed, because it
+is all buffered and sent down the pipe to 'cat' in one shot.
+
+'system(COMMAND)'
+ Execute the operating system command COMMAND and then return to the
+ 'awk' program. Return COMMAND's exit status (see further on).
+
+ For example, if the following fragment of code is put in your 'awk'
+ program:
+
+ END {
+ system("date | mail -s 'awk run done' root")
+ }
+
+ the system administrator is sent mail when the 'awk' program
+ finishes processing input and begins its end-of-input processing.
+
+ Note that redirecting 'print' or 'printf' into a pipe is often
+ enough to accomplish your task. If you need to run many commands,
+ it is more efficient to simply print them down a pipeline to the
+ shell:
+
+ while (MORE STUFF TO DO)
+ print COMMAND | "/bin/sh"
+ close("/bin/sh")
+
+ However, if your 'awk' program is interactive, 'system()' is useful
+ for running large self-contained programs, such as a shell or an
+ editor. Some operating systems cannot implement the 'system()'
+ function. 'system()' causes a fatal error if it is not supported.
+
+ NOTE: When '--sandbox' is specified, the 'system()' function
+ is disabled (*note Options::).
+
+ On POSIX systems, a command's exit status is a 16-bit number. The
+ exit value passed to the C 'exit()' function is held in the
+ high-order eight bits. The low-order bits indicate if the process
+ was killed by a signal (bit 7) and if so, the guilty signal number
+ (bits 0-6).
+
+ Traditionally, 'awk''s 'system()' function has simply returned the
+ exit status value divided by 256. In the normal case this gives
+ the exit status but in the case of death-by-signal it yields a
+ fractional floating-point value.(2) POSIX states that 'awk''s
+ 'system()' should return the full 16-bit value.
+
+ 'gawk' steers a middle ground. The return values are summarized in
+ *note Table 9.5: table-system-return-values.
+
+
+ Situation Return value from 'system()'
+ --------------------------------------------------------------------------
+ '--traditional' C 'system()''s value divided by 256
+ '--posix' C 'system()''s value
+ Normal exit of command Command's exit status
+ Death by signal of command 256 + number of murderous signal
+ Death by signal of command 512 + number of murderous signal
+ with core dump
+ Some kind of error -1
+
+ Table 9.5: Return values from 'system()'
+
+ As of August, 2018, BWK 'awk' now follows 'gawk''s behavior for the
+return value of 'system()'.
+
+ Controlling Output Buffering with 'system()'
+
+ The 'fflush()' function provides explicit control over output
+buffering for individual files and pipes. However, its use is not
+portable to many older 'awk' implementations. An alternative method to
+flush output buffers is to call 'system()' with a null string as its
+argument:
+
+ system("") # flush output
+
+'gawk' treats this use of the 'system()' function as a special case and
+is smart enough not to run a shell (or other command interpreter) with
+the empty command. Therefore, with 'gawk', this idiom is not only
+useful, it is also efficient. Although this method should work with
+other 'awk' implementations, it does not necessarily avoid starting an
+unnecessary shell. (Other implementations may only flush the buffer
+associated with the standard output and not necessarily all buffered
+output.)
+
+ If you think about what a programmer expects, it makes sense that
+'system()' should flush any pending output. The following program:
+
+ BEGIN {
+ print "first print"
+ system("echo system echo")
+ print "second print"
+ }
+
+must print:
+
+ first print
+ system echo
+ second print
+
+and not:
+
+ system echo
+ first print
+ second print
+
+ If 'awk' did not flush its buffers before calling 'system()', you
+would see the latter (undesirable) output.
+
+ ---------- Footnotes ----------
+
+ (1) A program is interactive if the standard output is connected to a
+terminal device. On modern systems, this means your keyboard and
+screen.
+
+ (2) In private correspondence, Dr. Kernighan has indicated to me that
+the way this was done was probably a mistake.
+
+
+File: gawk.info, Node: Time Functions, Next: Bitwise Functions, Prev: I/O
Functions, Up: Built-in
+
+9.1.6 Time Functions
+--------------------
+
+'awk' programs are commonly used to process log files containing
+timestamp information, indicating when a particular log record was
+written. Many programs log their timestamps in the form returned by the
+'time()' system call, which is the number of seconds since a particular
+epoch. On POSIX-compliant systems, it is the number of seconds since
+1970-01-01 00:00:00 UTC, not counting leap seconds.(1) All known
+POSIX-compliant systems support timestamps from 0 through 2^31 - 1,
+which is sufficient to represent times through 2038-01-19 03:14:07 UTC.
+Many systems support a wider range of timestamps, including negative
+timestamps that represent times before the epoch.
+
+ In order to make it easier to process such log files and to produce
+useful reports, 'gawk' provides the following functions for working with
+timestamps. They are 'gawk' extensions; they are not specified in the
+POSIX standard.(2) However, recent versions of 'mawk' (*note Other
+Versions::) also support these functions. Optional parameters are
+enclosed in square brackets ([ ]):
+
+'mktime(DATESPEC' [', UTC-FLAG' ]')'
+ Turn DATESPEC into a timestamp in the same form as is returned by
+ 'systime()'. It is similar to the function of the same name in ISO
+ C. The argument, DATESPEC, is a string of the form
+ '"YYYY MM DD HH MM SS [DST]"'. The string consists of six or seven
+ numbers representing, respectively, the full year including
+ century, the month from 1 to 12, the day of the month from 1 to 31,
+ the hour of the day from 0 to 23, the minute from 0 to 59, the
+ second from 0 to 60,(3) and an optional daylight-savings flag.
+
+ The values of these numbers need not be within the ranges
+ specified; for example, an hour of -1 means 1 hour before midnight.
+ The origin-zero Gregorian calendar is assumed, with year 0
+ preceding year 1 and year -1 preceding year 0. If UTC-FLAG is
+ present and is either nonzero or non-null, the time is assumed to
+ be in the UTC time zone; otherwise, the time is assumed to be in
+ the local time zone. If the DST daylight-savings flag is positive,
+ the time is assumed to be daylight savings time; if zero, the time
+ is assumed to be standard time; and if negative (the default),
+ 'mktime()' attempts to determine whether daylight savings time is
+ in effect for the specified time.
+
+ If DATESPEC does not contain enough elements or if the resulting
+ time is out of range, 'mktime()' returns -1.
+
+'strftime('[FORMAT [',' TIMESTAMP [',' UTC-FLAG] ] ]')'
+ Format the time specified by TIMESTAMP based on the contents of the
+ FORMAT string and return the result. It is similar to the function
+ of the same name in ISO C. If UTC-FLAG is present and is either
+ nonzero or non-null, the value is formatted as UTC (Coordinated
+ Universal Time, formerly GMT or Greenwich Mean Time). Otherwise,
+ the value is formatted for the local time zone. The TIMESTAMP is
+ in the same format as the value returned by the 'systime()'
+ function. If no TIMESTAMP argument is supplied, 'gawk' uses the
+ current time of day as the timestamp. Without a FORMAT argument,
+ 'strftime()' uses the value of 'PROCINFO["strftime"]' as the format
+ string (*note Built-in Variables::). The default string value is
+ '"%a %b %e %H:%M:%S %Z %Y"'. This format string produces output
+ that is equivalent to that of the 'date' utility. You can assign a
+ new value to 'PROCINFO["strftime"]' to change the default format;
+ see the following list for the various format directives.
+
+'systime()'
+ Return the current time as the number of seconds since the system
+ epoch. On POSIX systems, this is the number of seconds since
+ 1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a
+ different number on other systems.
+
+ The 'systime()' function allows you to compare a timestamp from a log
+file with the current time of day. In particular, it is easy to
+determine how long ago a particular record was logged. It also allows
+you to produce log records using the "seconds since the epoch" format.
+
+ The 'mktime()' function allows you to convert a textual
+representation of a date and time into a timestamp. This makes it easy
+to do before/after comparisons of dates and times, particularly when
+dealing with date and time data coming from an external source, such as
+a log file.
+
+ The 'strftime()' function allows you to easily turn a timestamp into
+human-readable information. It is similar in nature to the 'sprintf()'
+function (*note String Functions::), in that it copies nonformat
+specification characters verbatim to the returned string, while
+substituting date and time values for format specifications in the
+FORMAT string.
+
+ 'strftime()' is guaranteed by the 1999 ISO C standard(4) to support
+the following date format specifications:
+
+'%a'
+ The locale's abbreviated weekday name.
+
+'%A'
+ The locale's full weekday name.
+
+'%b'
+ The locale's abbreviated month name.
+
+'%B'
+ The locale's full month name.
+
+'%c'
+ The locale's "appropriate" date and time representation. (This is
+ '%A %B %d %T %Y' in the '"C"' locale.)
+
+'%C'
+ The century part of the current year. This is the year divided by
+ 100 and truncated to the next lower integer.
+
+'%d'
+ The day of the month as a decimal number (01-31).
+
+'%D'
+ Equivalent to specifying '%m/%d/%y'.
+
+'%e'
+ The day of the month, padded with a space if it is only one digit.
+
+'%F'
+ Equivalent to specifying '%Y-%m-%d'. This is the ISO 8601 date
+ format.
+
+'%g'
+ The year modulo 100 of the ISO 8601 week number, as a decimal
+ number (00-99). For example, January 1, 2012, is in week 53 of
+ 2011. Thus, the year of its ISO 8601 week number is 2011, even
+ though its year is 2012. Similarly, December 31, 2012, is in week
+ 1 of 2013. Thus, the year of its ISO week number is 2013, even
+ though its year is 2012.
+
+'%G'
+ The full year of the ISO week number, as a decimal number.
+
+'%h'
+ Equivalent to '%b'.
+
+'%H'
+ The hour (24-hour clock) as a decimal number (00-23).
+
+'%I'
+ The hour (12-hour clock) as a decimal number (01-12).
+
+'%j'
+ The day of the year as a decimal number (001-366).
+
+'%m'
+ The month as a decimal number (01-12).
+
+'%M'
+ The minute as a decimal number (00-59).
+
+'%n'
+ A newline character (ASCII LF).
+
+'%p'
+ The locale's equivalent of the AM/PM designations associated with a
+ 12-hour clock.
+
+'%r'
+ The locale's 12-hour clock time. (This is '%I:%M:%S %p' in the
+ '"C"' locale.)
+
+'%R'
+ Equivalent to specifying '%H:%M'.
+
+'%S'
+ The second as a decimal number (00-60).
+
+'%t'
+ A TAB character.
+
+'%T'
+ Equivalent to specifying '%H:%M:%S'.
+
+'%u'
+ The weekday as a decimal number (1-7). Monday is day one.
+
+'%U'
+ The week number of the year (with the first Sunday as the first day
+ of week one) as a decimal number (00-53).
+
+'%V'
+ The week number of the year (with the first Monday as the first day
+ of week one) as a decimal number (01-53). The method for
+ determining the week number is as specified by ISO 8601. (To wit:
+ if the week containing January 1 has four or more days in the new
+ year, then it is week one; otherwise it is the last week [52 or 53]
+ of the previous year and the next week is week one.)
+
+'%w'
+ The weekday as a decimal number (0-6). Sunday is day zero.
+
+'%W'
+ The week number of the year (with the first Monday as the first day
+ of week one) as a decimal number (00-53).
+
+'%x'
+ The locale's "appropriate" date representation. (This is '%A %B %d
+ %Y' in the '"C"' locale.)
+
+'%X'
+ The locale's "appropriate" time representation. (This is '%T' in
+ the '"C"' locale.)
+
+'%y'
+ The year modulo 100 as a decimal number (00-99).
+
+'%Y'
+ The full year as a decimal number (e.g., 2015).
+
+'%z'
+ The time zone offset in a '+HHMM' format (e.g., the format
+ necessary to produce RFC 822/RFC 1036 date headers).
+
+'%Z'
+ The time zone name or abbreviation; no characters if no time zone
+ is determinable.
+
+'%Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH'
+'%OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
+ "Alternative representations" for the specifications that use only
+ the second letter ('%c', '%C', and so on).(5) (These facilitate
+ compliance with the POSIX 'date' utility.)
+
+'%%'
+ A literal '%'.
+
+ If a conversion specifier is not one of those just listed, the
+behavior is undefined.(6)
+
+ For systems that are not yet fully standards-compliant, 'gawk'
+supplies a copy of 'strftime()' from the GNU C Library. It supports all
+of the just-listed format specifications. If that version is used to
+compile 'gawk' (*note Installation::), then the following additional
+format specifications are available:
+
+'%k'
+ The hour (24-hour clock) as a decimal number (0-23). Single-digit
+ numbers are padded with a space.
+
+'%l'
+ The hour (12-hour clock) as a decimal number (1-12). Single-digit
+ numbers are padded with a space.
+
+'%s'
+ The time as a decimal timestamp in seconds since the epoch.
+
+ Additionally, the alternative representations are recognized but
+their normal representations are used.
+
+ The following example is an 'awk' implementation of the POSIX 'date'
+utility. Normally, the 'date' utility prints the current date and time
+of day in a well-known format. However, if you provide an argument to
+it that begins with a '+', 'date' copies nonformat specifier characters
+to the standard output and interprets the current time according to the
+format specifiers in the string. For example:
+
+ $ date '+Today is %A, %B %d, %Y.'
+ -| Today is Monday, September 22, 2014.
+
+ Here is the 'gawk' version of the 'date' utility. It has a shell
+"wrapper" to handle the '-u' option, which requires that 'date' run as
+if the time zone is set to UTC:
+
+ #! /bin/sh
+ #
+ # date --- approximate the POSIX 'date' command
+
+ case $1 in
+ -u) TZ=UTC0 # use UTC
+ export TZ
+ shift ;;
+ esac
+
+ gawk 'BEGIN {
+ format = PROCINFO["strftime"]
+ exitval = 0
+
+ if (ARGC > 2)
+ exitval = 1
+ else if (ARGC == 2) {
+ format = ARGV[1]
+ if (format ~ /^\+/)
+ format = substr(format, 2) # remove leading +
+ }
+ print strftime(format)
+ exit exitval
+ }' "$@"
+
+ ---------- Footnotes ----------
+
+ (1) *Note Glossary::, especially the entries "Epoch" and "UTC."
+
+ (2) The GNU 'date' utility can also do many of the things described
+here. Its use may be preferable for simple time-related operations in
+shell scripts.
+
+ (3) Occasionally there are minutes in a year with a leap second,
+which is why the seconds can go up to 60.
+
+ (4) Unfortunately, not every system's 'strftime()' necessarily
+supports all of the conversions listed here.
+
+ (5) If you don't understand any of this, don't worry about it; these
+facilities are meant to make it easier to "internationalize" programs.
+Other internationalization features are described in *note
+Internationalization::.
+
+ (6) This is because ISO C leaves the behavior of the C version of
+'strftime()' undefined and 'gawk' uses the system's version of
+'strftime()' if it's there. Typically, the conversion specifier either
+does not appear in the returned string or appears literally.
+
+
+File: gawk.info, Node: Bitwise Functions, Next: Type Functions, Prev: Time
Functions, Up: Built-in
+
+9.1.7 Bit-Manipulation Functions
+--------------------------------
+
+ I can explain it for you, but I can't understand it for you.
+ -- _Anonymous_
+
+ Many languages provide the ability to perform "bitwise" operations on
+two integer numbers. In other words, the operation is performed on each
+successive pair of bits in the operands. Three common operations are
+bitwise AND, OR, and XOR. The operations are described in *note Table
+9.6: table-bitwise-ops.
+
+
+ Bit operator
+ | AND | OR | XOR
+ |---+---+---+---+---+---
+Operands | 0 | 1 | 0 | 1 | 0 | 1
+----------+---+---+---+---+---+---
+ 0 | 0 0 | 0 1 | 0 1
+ 1 | 0 1 | 1 1 | 1 0
+
+Table 9.6: Bitwise operations
+
+ As you can see, the result of an AND operation is 1 only when _both_
+bits are 1. The result of an OR operation is 1 if _either_ bit is 1.
+The result of an XOR operation is 1 if either bit is 1, but not both.
+The next operation is the "complement"; the complement of 1 is 0 and the
+complement of 0 is 1. Thus, this operation "flips" all the bits of a
+given value.
+
+ Finally, two other common operations are to shift the bits left or
+right. For example, if you have a bit string '10111001' and you shift
+it right by three bits, you end up with '00010111'.(1) If you start
+over again with '10111001' and shift it left by three bits, you end up
+with '11001000'. The following list describes 'gawk''s built-in
+functions that implement the bitwise operations. Optional parameters
+are enclosed in square brackets ([ ]):
+
+'and('V1',' V2 [',' ...]')'
+ Return the bitwise AND of the arguments. There must be at least
+ two.
+
+'compl(VAL)'
+ Return the bitwise complement of VAL.
+
+'lshift(VAL, COUNT)'
+ Return the value of VAL, shifted left by COUNT bits.
+
+'or('V1',' V2 [',' ...]')'
+ Return the bitwise OR of the arguments. There must be at least
+ two.
+
+'rshift(VAL, COUNT)'
+ Return the value of VAL, shifted right by COUNT bits.
+
+'xor('V1',' V2 [',' ...]')'
+ Return the bitwise XOR of the arguments. There must be at least
+ two.
+
+ CAUTION: Beginning with 'gawk' version 4.2, negative operands are
+ not allowed for any of these functions. A negative operand
+ produces a fatal error. See the sidebar "Beware The Smoke and
+ Mirrors!" for more information as to why.
+
+ Here is a user-defined function (*note User-defined::) that
+illustrates the use of these functions:
+
+ # bits2str --- turn an integer into readable ones and zeros
+
+ function bits2str(bits, data, mask)
+ {
+ if (bits == 0)
+ return "0"
+
+ mask = 1
+ for (; bits != 0; bits = rshift(bits, 1))
+ data = (and(bits, mask) ? "1" : "0") data
+
+ while ((length(data) % 8) != 0)
+ data = "0" data
+
+ return data
+ }
+
+ BEGIN {
+ printf "123 = %s\n", bits2str(123)
+ printf "0123 = %s\n", bits2str(0123)
+ printf "0x99 = %s\n", bits2str(0x99)
+ comp = compl(0x99)
+ printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp)
+ shift = lshift(0x99, 2)
+ printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
+ shift = rshift(0x99, 2)
+ printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
+ }
+
+This program produces the following output when run:
+
+ $ gawk -f testbits.awk
+ -| 123 = 01111011
+ -| 0123 = 01010011
+ -| 0x99 = 10011001
+ -| compl(0x99) = 0x3fffffffffff66 =
+ -| 00111111111111111111111111111111111111111111111101100110
+ -| lshift(0x99, 2) = 0x264 = 0000001001100100
+ -| rshift(0x99, 2) = 0x26 = 00100110
+
+ The 'bits2str()' function turns a binary number into a string.
+Initializing 'mask' to one creates a binary value where the rightmost
+bit is set to one. Using this mask, the function repeatedly checks the
+rightmost bit. ANDing the mask with the value indicates whether the
+rightmost bit is one or not. If so, a '"1"' is concatenated onto the
+front of the string. Otherwise, a '"0"' is added. The value is then
+shifted right by one bit and the loop continues until there are no more
+one bits.
+
+ If the initial value is zero, it returns a simple '"0"'. Otherwise,
+at the end, it pads the value with zeros to represent multiples of 8-bit
+quantities. This is typical in modern computers.
+
+ The main code in the 'BEGIN' rule shows the difference between the
+decimal and octal values for the same numbers (*note
+Nondecimal-numbers::), and then demonstrates the results of the
+'compl()', 'lshift()', and 'rshift()' functions.
+
+ Beware The Smoke and Mirrors!
+
+ It other languages, bitwise operations are performed on integer
+values, not floating-point values. As a general statement, such
+operations work best when performed on unsigned integers.
+
+ 'gawk' attempts to treat the arguments to the bitwise functions as
+unsigned integers. For this reason, negative arguments produce a fatal
+error.
+
+ In normal operation, for all of these functions, first the
+double-precision floating-point value is converted to the widest C
+unsigned integer type, then the bitwise operation is performed. If the
+result cannot be represented exactly as a C 'double', leading nonzero
+bits are removed one by one until it can be represented exactly. The
+result is then converted back into a C 'double'.(2)
+
+ However, when using arbitrary precision arithmetic with the '-M'
+option (*note Arbitrary Precision Arithmetic::), the results may differ.
+This is particularly noticeable with the 'compl()' function:
+
+ $ gawk 'BEGIN { print compl(42) }'
+ -| 9007199254740949
+ $ gawk -M 'BEGIN { print compl(42) }'
+ -| -43
+
+ What's going on becomes clear when printing the results in
+hexadecimal:
+
+ $ gawk 'BEGIN { printf "%#x\n", compl(42) }'
+ -| 0x1fffffffffffd5
+ $ gawk -M 'BEGIN { printf "%#x\n", compl(42) }'
+ -| 0xffffffffffffffd5
+
+ When using the '-M' option, under the hood, 'gawk' uses GNU MP
+arbitrary precision integers which have at least 64 bits of precision.
+When not using '-M', 'gawk' stores integral values in regular
+double-precision floating point, which only maintain 53 bits of
+precision. Furthermore, the GNU MP library treats (or at least seems to
+treat) the leading bit as a sign bit; thus the result with '-M' in this
+case is a negative number.
+
+ In short, using 'gawk' for any but the simplest kind of bitwise
+operations is probably a bad idea; caveat emptor!
+
+ ---------- Footnotes ----------
+
+ (1) This example shows that zeros come in on the left side. For
+'gawk', this is always true, but in some languages, it's possible to
+have the left side fill with ones.
+
+ (2) If you don't understand this paragraph, the upshot is that 'gawk'
+can only store a particular range of integer values; numbers outside
+that range are reduced to fit within the range.
+
+
+File: gawk.info, Node: Type Functions, Next: I18N Functions, Prev: Bitwise
Functions, Up: Built-in
+
+9.1.8 Getting Type Information
+------------------------------
+
+'gawk' provides two functions that let you distinguish the type of a
+variable. This is necessary for writing code that traverses every
+element of an array of arrays (*note Arrays of Arrays::), and in other
+contexts.
+
+'isarray(X)'
+ Return a true value if X is an array. Otherwise, return false.
+
+'typeof(X)'
+ Return one of the following strings, depending upon the type of X:
+
+ '"array"'
+ X is an array.
+
+ '"regexp"'
+ X is a strongly typed regexp (*note Strong Regexp
+ Constants::).
+
+ '"number"'
+ X is a number.
+
+ '"number|bool"'
+ X is a Boolean typed value (*note Boolean Typed Values::).
+
+ '"string"'
+ X is a string.
+
+ '"strnum"'
+ X is a number that started life as user input, such as a field
+ or the result of calling 'split()'. (I.e., X has the strnum
+ attribute; *note Variable Typing::.)
+
+ '"unassigned"'
+ X is a scalar variable that has not been assigned a value yet.
+ For example:
+
+ BEGIN {
+ # creates a[1] but it has no assigned value
+ a[1]
+ print typeof(a[1]) # unassigned
+ }
+
+ '"untyped"'
+ X has not yet been used yet at all; it can become a scalar or
+ an array. The typing could even conceivably differ from run
+ to run of the same program! For example:
+
+ BEGIN {
+ print "initially, typeof(v) = ", typeof(v)
+
+ if ("FOO" in ENVIRON)
+ make_scalar(v)
+ else
+ make_array(v)
+
+ print "typeof(v) =", typeof(v)
+ }
+
+ function make_scalar(p, l) { l = p }
+
+ function make_array(p) { p[1] = 1 }
+
+ 'isarray()' is meant for use in two circumstances. The first is when
+traversing a multidimensional array: you can test if an element is
+itself an array or not. The second is inside the body of a user-defined
+function (not discussed yet; *note User-defined::), to test if a
+parameter is an array or not.
+
+ NOTE: While you can use 'isarray()' at the global level to test
+ variables, doing so makes no sense. Because _you_ are the one
+ writing the program, _you_ are supposed to know if your variables
+ are arrays or not.
+
+ The 'typeof()' function is general; it allows you to determine if a
+variable or function parameter is a scalar (number, string, or strongly
+typed regexp) or an array.
+
+ Normally, passing a variable that has never been used to a built-in
+function causes it to become a scalar variable (unassigned). However,
+'isarray()' and 'typeof()' are different; they do not change their
+arguments from untyped to unassigned.
+
+ By "variable" we mean one denoted by a simple identifier. Array
+elements that come into existence simply by referencing them are
+different, they are automatically forced to be scalars. Consider:
+
+ $ gawk 'BEGIN { print typeof(x) }'
+ -| untyped
+ $ gawk 'BEGIN { print typeof(x["foo"]) }'
+ -| unassigned
+
+'x["foo"]' comes into existence before it is passed to 'typeof()';
+'typeof()' cannot tell that it didn't exist prior to being called.
+(d.c.)
+
+
+File: gawk.info, Node: I18N Functions, Prev: Type Functions, Up: Built-in
+
+9.1.9 String-Translation Functions
+----------------------------------
+
+'gawk' provides facilities for internationalizing 'awk' programs. These
+include the functions described in the following list. The descriptions
+here are purposely brief. *Note Internationalization::, for the full
+story. Optional parameters are enclosed in square brackets ([ ]):
+
+'bindtextdomain(DIRECTORY' [',' DOMAIN]')'
+ Set the directory in which 'gawk' will look for message translation
+ files, in case they will not or cannot be placed in the "standard"
+ locations (e.g., during testing). It returns the directory in
+ which DOMAIN is "bound."
+
+ The default DOMAIN is the value of 'TEXTDOMAIN'. If DIRECTORY is
+ the null string ('""'), then 'bindtextdomain()' returns the current
+ binding for the given DOMAIN.
+
+'dcgettext(STRING' [',' DOMAIN [',' CATEGORY] ]')'
+ Return the translation of STRING in text domain DOMAIN for locale
+ category CATEGORY. The default value for DOMAIN is the current
+ value of 'TEXTDOMAIN'. The default value for CATEGORY is
+ '"LC_MESSAGES"'.
+
+'dcngettext(STRING1, STRING2, NUMBER' [',' DOMAIN [',' CATEGORY] ]')'
+ Return the plural form used for NUMBER of the translation of
+ STRING1 and STRING2 in text domain DOMAIN for locale category
+ CATEGORY. STRING1 is the English singular variant of a message,
+ and STRING2 is the English plural variant of the same message. The
+ default value for DOMAIN is the current value of 'TEXTDOMAIN'. The
+ default value for CATEGORY is '"LC_MESSAGES"'.
+
+
+File: gawk.info, Node: User-defined, Next: Indirect Calls, Prev: Built-in,
Up: Functions
+
+9.2 User-Defined Functions
+==========================
+
+Complicated 'awk' programs can often be simplified by defining your own
+functions. User-defined functions can be called just like built-in ones
+(*note Function Calls::), but it is up to you to define them (i.e., to
+tell 'awk' what they should do).
+
+* Menu:
+
+* Definition Syntax:: How to write definitions and what they mean.
+* Function Example:: An example function definition and what it
+ does.
+* Function Calling:: Calling user-defined functions.
+* Return Statement:: Specifying the value a function returns.
+* Dynamic Typing:: How variable types can change at runtime.
+
+
+File: gawk.info, Node: Definition Syntax, Next: Function Example, Up:
User-defined
+
+9.2.1 Function Definition Syntax
+--------------------------------
+
+ It's entirely fair to say that the awk syntax for local variable
+ definitions is appallingly awful.
+ -- _Brian Kernighan_
+
+ Definitions of functions can appear anywhere between the rules of an
+'awk' program. Thus, the general form of an 'awk' program is extended
+to include sequences of rules _and_ user-defined function definitions.
+There is no need to put the definition of a function before all uses of
+the function. This is because 'awk' reads the entire program before
+starting to execute any of it.
+
+ The definition of a function named NAME looks like this:
+
+ 'function' NAME'('[PARAMETER-LIST]')'
+ '{'
+ BODY-OF-FUNCTION
+ '}'
+
+Here, NAME is the name of the function to define. A valid function name
+is like a valid variable name: a sequence of letters, digits, and
+underscores that doesn't start with a digit. Here too, only the 52
+upper- and lowercase English letters may be used in a function name.
+Within a single 'awk' program, any particular name can only be used as a
+variable, array, or function.
+
+ PARAMETER-LIST is an optional list of the function's arguments and
+local variable names, separated by commas. When the function is called,
+the argument names are used to hold the argument values given in the
+call.
+
+ A function cannot have two parameters with the same name, nor may it
+have a parameter with the same name as the function itself.
+
+ CAUTION: According to the POSIX standard, function parameters
+ cannot have the same name as one of the special predefined
+ variables (*note Built-in Variables::), nor may a function
+ parameter have the same name as another function.
+
+ Not all versions of 'awk' enforce these restrictions. (d.c.)
+ 'gawk' always enforces the first restriction. With '--posix'
+ (*note Options::), it also enforces the second restriction.
+
+ Local variables act like the empty string if referenced where a
+string value is required, and like zero if referenced where a numeric
+value is required. This is the same as the behavior of regular
+variables that have never been assigned a value. (There is more to
+understand about local variables; *note Dynamic Typing::.)
+
+ The BODY-OF-FUNCTION consists of 'awk' statements. It is the most
+important part of the definition, because it says what the function
+should actually _do_. The argument names exist to give the body a way
+to talk about the arguments; local variables exist to give the body
+places to keep temporary values.
+
+ Argument names are not distinguished syntactically from local
+variable names. Instead, the number of arguments supplied when the
+function is called determines how many argument variables there are.
+Thus, if three argument values are given, the first three names in
+PARAMETER-LIST are arguments and the rest are local variables.
+
+ It follows that if the number of arguments is not the same in all
+calls to the function, some of the names in PARAMETER-LIST may be
+arguments on some occasions and local variables on others. Another way
+to think of this is that omitted arguments default to the null string.
+
+ Usually when you write a function, you know how many names you intend
+to use for arguments and how many you intend to use as local variables.
+It is conventional to place some extra space between the arguments and
+the local variables, in order to document how your function is supposed
+to be used.
+
+ During execution of the function body, the arguments and local
+variable values hide, or "shadow", any variables of the same names used
+in the rest of the program. The shadowed variables are not accessible
+in the function definition, because there is no way to name them while
+their names have been taken away for the arguments and local variables.
+All other variables used in the 'awk' program can be referenced or set
+normally in the function's body.
+
+ The arguments and local variables last only as long as the function
+body is executing. Once the body finishes, you can once again access
+the variables that were shadowed while the function was running.
+
+ The function body can contain expressions that call functions. They
+can even call this function, either directly or by way of another
+function. When this happens, we say the function is "recursive". The
+act of a function calling itself is called "recursion".
+
+ All the built-in functions return a value to their caller.
+User-defined functions can do so also, using the 'return' statement,
+which is described in detail in *note Return Statement::. Many of the
+subsequent examples in this minor node use the 'return' statement.
+
+ In many 'awk' implementations, including 'gawk', the keyword
+'function' may be abbreviated 'func'. (c.e.) However, POSIX only
+specifies the use of the keyword 'function'. This actually has some
+practical implications. If 'gawk' is in POSIX-compatibility mode (*note
+Options::), then the following statement does _not_ define a function:
+
+ func foo() { a = sqrt($1) ; print a }
+
+Instead, it defines a rule that, for each record, concatenates the value
+of the variable 'func' with the return value of the function 'foo'. If
+the resulting string is non-null, the action is executed. This is
+probably not what is desired. ('awk' accepts this input as
+syntactically valid, because functions may be used before they are
+defined in 'awk' programs.(1))
+
+ To ensure that your 'awk' programs are portable, always use the
+keyword 'function' when defining a function.
+
+ ---------- Footnotes ----------
+
+ (1) This program won't actually run, because 'foo()' is undefined.
+
+
+File: gawk.info, Node: Function Example, Next: Function Calling, Prev:
Definition Syntax, Up: User-defined
+
+9.2.2 Function Definition Examples
+----------------------------------
+
+Here is an example of a user-defined function, called 'myprint()', that
+takes a number and prints it in a specific format:
+
+ function myprint(num)
+ {
+ printf "%6.3g\n", num
+ }
+
+To illustrate, here is an 'awk' rule that uses our 'myprint()' function:
+
+ $3 > 0 { myprint($3) }
+
+This program prints, in our special format, all the third fields that
+contain a positive number in our input. Therefore, when given the
+following input:
+
+ 1.2 3.4 5.6 7.8
+ 9.10 11.12 -13.14 15.16
+ 17.18 19.20 21.22 23.24
+
+this program, using our function to format the results, prints:
+
+ 5.6
+ 21.2
+
+ This function deletes all the elements in an array (recall that the
+extra whitespace signifies the start of the local variable list):
+
+ function delarray(a, i)
+ {
+ for (i in a)
+ delete a[i]
+ }
+
+ When working with arrays, it is often necessary to delete all the
+elements in an array and start over with a new list of elements (*note
+Delete::). Instead of having to repeat this loop everywhere that you
+need to clear out an array, your program can just call 'delarray()'.
+(This guarantees portability. The use of 'delete ARRAY' to delete the
+contents of an entire array is a relatively recent(1) addition to the
+POSIX standard.)
+
+ The following is an example of a recursive function. It takes a
+string as an input parameter and returns the string in reverse order.
+Recursive functions must always have a test that stops the recursion.
+In this case, the recursion terminates when the input string is already
+empty:
+
+ function rev(str)
+ {
+ if (str == "")
+ return ""
+
+ return (rev(substr(str, 2)) substr(str, 1, 1))
+ }
+
+ If this function is in a file named 'rev.awk', it can be tested this
+way:
+
+ $ echo "Don't Panic!" |
+ > gawk -e '{ print rev($0) }' -f rev.awk
+ -| !cinaP t'noD
+
+ The C 'ctime()' function takes a timestamp and returns it as a
+string, formatted in a well-known fashion. The following example uses
+the built-in 'strftime()' function (*note Time Functions::) to create an
+'awk' version of 'ctime()':
+
+ # ctime.awk
+ #
+ # awk version of C ctime(3) function
+
+ function ctime(ts, format)
+ {
+ format = "%a %b %e %H:%M:%S %Z %Y"
+
+ if (ts == 0)
+ ts = systime() # use current time as default
+ return strftime(format, ts)
+ }
+
+ You might think that 'ctime()' could use 'PROCINFO["strftime"]' for
+its format string. That would be a mistake, because 'ctime()' is
+supposed to return the time formatted in a standard fashion, and
+user-level code could have changed 'PROCINFO["strftime"]'.
+
+ ---------- Footnotes ----------
+
+ (1) Late in 2012.
+
+
+File: gawk.info, Node: Function Calling, Next: Return Statement, Prev:
Function Example, Up: User-defined
+
+9.2.3 Calling User-Defined Functions
+------------------------------------
+
+"Calling a function" means causing the function to run and do its job.
+A function call is an expression and its value is the value returned by
+the function.
+
+* Menu:
+
+* Calling A Function:: Don't use spaces.
+* Variable Scope:: Controlling variable scope.
+* Pass By Value/Reference:: Passing parameters.
+* Function Caveats:: Other points to know about functions.
+
+
+File: gawk.info, Node: Calling A Function, Next: Variable Scope, Up:
Function Calling
+
+9.2.3.1 Writing a Function Call
+...............................
+
+A function call consists of the function name followed by the arguments
+in parentheses. 'awk' expressions are what you write in the call for
+the arguments. Each time the call is executed, these expressions are
+evaluated, and the values become the actual arguments. For example,
+here is a call to 'foo()' with three arguments (the first being a string
+concatenation):
+
+ foo(x y, "lose", 4 * z)
+
+ CAUTION: Whitespace characters (spaces and TABs) are not allowed
+ between the function name and the opening parenthesis of the
+ argument list. If you write whitespace by mistake, 'awk' might
+ think that you mean to concatenate a variable with an expression in
+ parentheses. However, it notices that you used a function name and
+ not a variable name, and reports an error.
+
+
+File: gawk.info, Node: Variable Scope, Next: Pass By Value/Reference, Prev:
Calling A Function, Up: Function Calling
+
+9.2.3.2 Controlling Variable Scope
+..................................
+
+Unlike in many languages, there is no way to make a variable local to a
+'{' ... '}' block in 'awk', but you can make a variable local to a
+function. It is good practice to do so whenever a variable is needed
+only in that function.
+
+ To make a variable local to a function, simply declare the variable
+as an argument after the actual function arguments (*note Definition
+Syntax::). Look at the following example, where variable 'i' is a
+global variable used by both functions 'foo()' and 'bar()':
+
+ function bar()
+ {
+ for (i = 0; i < 3; i++)
+ print "bar's i=" i
+ }
+
+ function foo(j)
+ {
+ i = j + 1
+ print "foo's i=" i
+ bar()
+ print "foo's i=" i
+ }
+
+ BEGIN {
+ i = 10
+ print "top's i=" i
+ foo(0)
+ print "top's i=" i
+ }
+
+ Running this script produces the following, because the 'i' in
+functions 'foo()' and 'bar()' and at the top level refer to the same
+variable instance:
+
+ top's i=10
+ foo's i=1
+ bar's i=0
+ bar's i=1
+ bar's i=2
+ foo's i=3
+ top's i=3
+
+ If you want 'i' to be local to both 'foo()' and 'bar()', do as
+follows (the extra space before 'i' is a coding convention to indicate
+that 'i' is a local variable, not an argument):
+
+ function bar( i)
+ {
+ for (i = 0; i < 3; i++)
+ print "bar's i=" i
+ }
+
+ function foo(j, i)
+ {
+ i = j + 1
+ print "foo's i=" i
+ bar()
+ print "foo's i=" i
+ }
+
+ BEGIN {
+ i = 10
+ print "top's i=" i
+ foo(0)
+ print "top's i=" i
+ }
+
+ Running the corrected script produces the following:
+
+ top's i=10
+ foo's i=1
+ bar's i=0
+ bar's i=1
+ bar's i=2
+ foo's i=1
+ top's i=10
+
+ Besides scalar values (strings and numbers), you may also have local
+arrays. By using a parameter name as an array, 'awk' treats it as an
+array, and it is local to the function. In addition, recursive calls
+create new arrays. Consider this example:
+
+ function some_func(p1, a)
+ {
+ if (p1++ > 3)
+ return
+
+ a[p1] = p1
+
+ some_func(p1)
+
+ printf("At level %d, index %d %s found in a\n",
+ p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not")
+ printf("At level %d, index %d %s found in a\n",
+ p1, p1, p1 in a ? "is" : "is not")
+ print ""
+ }
+
+ BEGIN {
+ some_func(1)
+ }
+
+ When run, this program produces the following output:
+
+ At level 4, index 3 is not found in a
+ At level 4, index 4 is found in a
+
+ At level 3, index 2 is not found in a
+ At level 3, index 3 is found in a
+
+ At level 2, index 1 is not found in a
+ At level 2, index 2 is found in a
+
+
+File: gawk.info, Node: Pass By Value/Reference, Next: Function Caveats,
Prev: Variable Scope, Up: Function Calling
+
+9.2.3.3 Passing Function Arguments by Value Or by Reference
+...........................................................
+
+In 'awk', when you declare a function, there is no way to declare
+explicitly whether the arguments are passed "by value" or "by
+reference".
+
+ Instead, the passing convention is determined at runtime when the
+function is called, according to the following rule: if the argument is
+an array variable, then it is passed by reference. Otherwise, the
+argument is passed by value.
+
+ Passing an argument by value means that when a function is called, it
+is given a _copy_ of the value of this argument. The caller may use a
+variable as the expression for the argument, but the called function
+does not know this--it only knows what value the argument had. For
+example, if you write the following code:
+
+ foo = "bar"
+ z = myfunc(foo)
+
+then you should not think of the argument to 'myfunc()' as being "the
+variable 'foo'." Instead, think of the argument as the string value
+'"bar"'. If the function 'myfunc()' alters the values of its local
+variables, this has no effect on any other variables. Thus, if
+'myfunc()' does this:
+
+ function myfunc(str)
+ {
+ print str
+ str = "zzz"
+ print str
+ }
+
+to change its first argument variable 'str', it does _not_ change the
+value of 'foo' in the caller. The role of 'foo' in calling 'myfunc()'
+ended when its value ('"bar"') was computed. If 'str' also exists
+outside of 'myfunc()', the function body cannot alter this outer value,
+because it is shadowed during the execution of 'myfunc()' and cannot be
+seen or changed from there.
+
+ However, when arrays are the parameters to functions, they are _not_
+copied. Instead, the array itself is made available for direct
+manipulation by the function. This is usually termed "call by
+reference". Changes made to an array parameter inside the body of a
+function _are_ visible outside that function.
+
+ NOTE: Changing an array parameter inside a function can be very
+ dangerous if you do not watch what you are doing. For example:
+
+ function changeit(array, ind, nvalue)
+ {
+ array[ind] = nvalue
+ }
+
+ BEGIN {
+ a[1] = 1; a[2] = 2; a[3] = 3
+ changeit(a, 2, "two")
+ printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
+ a[1], a[2], a[3]
+ }
+
+ prints 'a[1] = 1, a[2] = two, a[3] = 3', because 'changeit()'
+ stores '"two"' in the second element of 'a'.
+
+
+File: gawk.info, Node: Function Caveats, Prev: Pass By Value/Reference, Up:
Function Calling
+
+9.2.3.4 Other Points About Calling Functions
+............................................
+
+Some 'awk' implementations allow you to call a function that has not
+been defined. They only report a problem at runtime, when the program
+actually tries to call the function. For example:
+
+ BEGIN {
+ if (0)
+ foo()
+ else
+ bar()
+ }
+ function bar() { ... }
+ # note that `foo' is not defined
+
+Because the 'if' statement will never be true, it is not really a
+problem that 'foo()' has not been defined. Usually, though, it is a
+problem if a program calls an undefined function.
+
+ If '--lint' is specified (*note Options::), 'gawk' reports calls to
+undefined functions.
+
+ Some 'awk' implementations generate a runtime error if you use either
+the 'next' statement or the 'nextfile' statement (*note Next
+Statement::, and *note Nextfile Statement::) inside a user-defined
+function. 'gawk' does not have this limitation.
+
+ You can call a function and pass it more parameters than it was
+declared with, like so:
+
+ function foo(p1, p2)
+ {
+ ...
+ }
+
+ BEGIN {
+ foo(1, 2, 3, 4)
+ }
+
+ Doing so is bad practice, however. The called function cannot do
+anything with the additional values being passed to it, so 'awk'
+evaluates the expressions but then just throws them away.
+
+ More importantly, such a call is confusing for whoever will next read
+your program.(1) Function parameters generally are input items that
+influence the computation performed by the function. Calling a function
+with more parameters than it accepts gives the false impression that
+those values are important to the function, when in fact they are not.
+
+ Because this is such a bad practice, 'gawk' _unconditionally_ issues
+a warning whenever it executes such a function call. (If you don't like
+the warning, fix your code! It's incorrect, after all.)
+
+ ---------- Footnotes ----------
+
+ (1) Said person might even be you, sometime in the future, at which
+point you will wonder, "what was I thinking?!?"
+
+
+File: gawk.info, Node: Return Statement, Next: Dynamic Typing, Prev:
Function Calling, Up: User-defined
+
+9.2.4 The 'return' Statement
+----------------------------
+
+As seen in several earlier examples, the body of a user-defined function
+can contain a 'return' statement. This statement returns control to the
+calling part of the 'awk' program. It can also be used to return a
+value for use in the rest of the 'awk' program. It looks like this:
+
+ 'return' [EXPRESSION]
+
+ The EXPRESSION part is optional. Due most likely to an oversight,
+POSIX does not define what the return value is if you omit the
+EXPRESSION. Technically speaking, this makes the returned value
+undefined, and therefore, unpredictable. In practice, though, all
+versions of 'awk' simply return the null string, which acts like zero if
+used in a numeric context.
+
+ A 'return' statement without an EXPRESSION is assumed at the end of
+every function definition. So, if control reaches the end of the
+function body, then technically the function returns an unpredictable
+value. In practice, it returns the empty string. 'awk' does _not_ warn
+you if you use the return value of such a function.
+
+ Sometimes, you want to write a function for what it does, not for
+what it returns. Such a function corresponds to a 'void' function in C,
+C++, or Java, or to a 'procedure' in Ada. Thus, it may be appropriate
+to not return any value; simply bear in mind that you should not be
+using the return value of such a function.
+
+ The following is an example of a user-defined function that returns a
+value for the largest number among the elements of an array:
+
+ function maxelt(vec, i, ret)
+ {
+ for (i in vec) {
+ if (ret == "" || vec[i] > ret)
+ ret = vec[i]
+ }
+ return ret
+ }
+
+You call 'maxelt()' with one argument, which is an array name. The
+local variables 'i' and 'ret' are not intended to be arguments; there is
+nothing to stop you from passing more than one argument to 'maxelt()'
+but the results would be strange. The extra space before 'i' in the
+function parameter list indicates that 'i' and 'ret' are local
+variables. You should follow this convention when defining functions.
+
+ The following program uses the 'maxelt()' function. It loads an
+array, calls 'maxelt()', and then reports the maximum number in that
+array:
+
+ function maxelt(vec, i, ret)
+ {
+ for (i in vec) {
+ if (ret == "" || vec[i] > ret)
+ ret = vec[i]
+ }
+ return ret
+ }
+
+ # Load all fields of each record into nums.
+ {
+ for(i = 1; i <= NF; i++)
+ nums[NR, i] = $i
+ }
+
+ END {
+ print maxelt(nums)
+ }
+
+ Given the following input:
+
+ 1 5 23 8 16
+ 44 3 5 2 8 26
+ 256 291 1396 2962 100
+ -6 467 998 1101
+ 99385 11 0 225
+
+the program reports (predictably) that 99,385 is the largest value in
+the array.
+
+
+File: gawk.info, Node: Dynamic Typing, Prev: Return Statement, Up:
User-defined
+
+9.2.5 Functions and Their Effects on Variable Typing
+----------------------------------------------------
+
+'awk' is a very fluid language. It is possible that 'awk' can't tell if
+an identifier represents a scalar variable or an array until runtime.
+Here is an annotated sample program:
+
+ function foo(a)
+ {
+ a[1] = 1 # parameter is an array
+ }
+
+ BEGIN {
+ b = 1
+ foo(b) # invalid: fatal type mismatch
+
+ foo(x) # x uninitialized, becomes an array dynamically
+ x = 1 # now not allowed, runtime error
+ }
+
+ In this example, the first call to 'foo()' generates a fatal error,
+so 'awk' will not report the second error. If you comment out that
+call, though, then 'awk' does report the second error.
+
+ Usually, such things aren't a big issue, but it's worth being aware
+of them.
+
+
+File: gawk.info, Node: Indirect Calls, Next: Functions Summary, Prev:
User-defined, Up: Functions
+
+9.3 Indirect Function Calls
+===========================
+
+This minor node describes an advanced, 'gawk'-specific extension.
+
+ Often, you may wish to defer the choice of function to call until
+runtime. For example, you may have different kinds of records, each of
+which should be processed differently.
+
+ Normally, you would have to use a series of 'if'-'else' statements to
+decide which function to call. By using "indirect" function calls, you
+can specify the name of the function to call as a string variable, and
+then call the function. Let's look at an example.
+
+ Suppose you have a file with your test scores for the classes you are
+taking, and you wish to get the sum and the average of your test scores.
+The first field is the class name. The following fields are the
+functions to call to process the data, up to a "marker" field 'data:'.
+Following the marker, to the end of the record, are the various numeric
+test scores.
+
+ Here is the initial file:
+
+ Biology_101 sum average data: 87.0 92.4 78.5 94.9
+ Chemistry_305 sum average data: 75.2 98.3 94.7 88.2
+ English_401 sum average data: 100.0 95.6 87.1 93.4
+
+ To process the data, you might write initially:
+
+ {
+ class = $1
+ for (i = 2; $i != "data:"; i++) {
+ if ($i == "sum")
+ sum() # processes the whole record
+ else if ($i == "average")
+ average()
+ ... # and so on
+ }
+ }
+
+This style of programming works, but can be awkward. With "indirect"
+function calls, you tell 'gawk' to use the _value_ of a variable as the
+_name_ of the function to call.
+
+ The syntax is similar to that of a regular function call: an
+identifier immediately followed by an opening parenthesis, any
+arguments, and then a closing parenthesis, with the addition of a
+leading '@' character:
+
+ the_function = "sum"
+ result = @the_function() # calls the sum() function
+
+ Here is a full program that processes the previously shown data,
+using indirect function calls:
+
+ # indirectcall.awk --- Demonstrate indirect function calls
+
+ # average --- return the average of the values in fields $first - $last
+
+ function average(first, last, sum, i)
+ {
+ sum = 0;
+ for (i = first; i <= last; i++)
+ sum += $i
+
+ return sum / (last - first + 1)
+ }
+
+ # sum --- return the sum of the values in fields $first - $last
+
+ function sum(first, last, ret, i)
+ {
+ ret = 0;
+ for (i = first; i <= last; i++)
+ ret += $i
+
+ return ret
+ }
+
+ These two functions expect to work on fields; thus, the parameters
+'first' and 'last' indicate where in the fields to start and end.
+Otherwise, they perform the expected computations and are not unusual:
+
+ # For each record, print the class name and the requested statistics
+ {
+ class_name = $1
+ gsub(/_/, " ", class_name) # Replace _ with spaces
+
+ # find start
+ for (i = 1; i <= NF; i++) {
+ if ($i == "data:") {
+ start = i + 1
+ break
+ }
+ }
+
+ printf("%s:\n", class_name)
+ for (i = 2; $i != "data:"; i++) {
+ the_function = $i
+ printf("\t%s: <%s>\n", $i, @the_function(start, NF) "")
+ }
+ print ""
+ }
+
+ This is the main processing for each record. It prints the class
+name (with underscores replaced with spaces). It then finds the start
+of the actual data, saving it in 'start'. The last part of the code
+loops through each function name (from '$2' up to the marker, 'data:'),
+calling the function named by the field. The indirect function call
+itself occurs as a parameter in the call to 'printf'. (The 'printf'
+format string uses '%s' as the format specifier so that we can use
+functions that return strings, as well as numbers. Note that the result
+from the indirect call is concatenated with the empty string, in order
+to force it to be a string value.)
+
+ Here is the result of running the program:
+
+ $ gawk -f indirectcall.awk class_data1
+ -| Biology 101:
+ -| sum: <352.8>
+ -| average: <88.2>
+ -|
+ -| Chemistry 305:
+ -| sum: <356.4>
+ -| average: <89.1>
+ -|
+ -| English 401:
+ -| sum: <376.1>
+ -| average: <94.025>
+
+ The ability to use indirect function calls is more powerful than you
+may think at first. The C and C++ languages provide "function
+pointers," which are a mechanism for calling a function chosen at
+runtime. One of the most well-known uses of this ability is the C
+'qsort()' function, which sorts an array using the famous "quicksort"
+algorithm (see the Wikipedia article
+(https://en.wikipedia.org/wiki/Quicksort) for more information). To use
+this function, you supply a pointer to a comparison function. This
+mechanism allows you to sort arbitrary data in an arbitrary fashion.
+
+ We can do something similar using 'gawk', like this:
+
+ # quicksort.awk --- Quicksort algorithm, with user-supplied
+ # comparison function
+
+ # quicksort --- C.A.R. Hoare's quicksort algorithm. See Wikipedia
+ # or almost any algorithms or computer science text.
+
+ function quicksort(data, left, right, less_than, i, last)
+ {
+ if (left >= right) # do nothing if array contains fewer
+ return # than two elements
+
+ quicksort_swap(data, left, int((left + right) / 2))
+ last = left
+ for (i = left + 1; i <= right; i++)
+ if (@less_than(data[i], data[left]))
+ quicksort_swap(data, ++last, i)
+ quicksort_swap(data, left, last)
+ quicksort(data, left, last - 1, less_than)
+ quicksort(data, last + 1, right, less_than)
+ }
+
+ # quicksort_swap --- helper function for quicksort, should really be
inline
+
+ function quicksort_swap(data, i, j, temp)
+ {
+ temp = data[i]
+ data[i] = data[j]
+ data[j] = temp
+ }
+
+ The 'quicksort()' function receives the 'data' array, the starting
+and ending indices to sort ('left' and 'right'), and the name of a
+function that performs a "less than" comparison. It then implements the
+quicksort algorithm.
+
+ To make use of the sorting function, we return to our previous
+example. The first thing to do is write some comparison functions:
+
+ # num_lt --- do a numeric less than comparison
+
+ function num_lt(left, right)
+ {
+ return ((left + 0) < (right + 0))
+ }
+
+ # num_ge --- do a numeric greater than or equal to comparison
+
+ function num_ge(left, right)
+ {
+ return ((left + 0) >= (right + 0))
+ }
+
+ The 'num_ge()' function is needed to perform a descending sort; when
+used to perform a "less than" test, it actually does the opposite
+(greater than or equal to), which yields data sorted in descending
+order.
+
+ Next comes a sorting function. It is parameterized with the starting
+and ending field numbers and the comparison function. It builds an
+array with the data and calls 'quicksort()' appropriately, and then
+formats the results as a single string:
+
+ # do_sort --- sort the data according to `compare'
+ # and return it as a string
+
+ function do_sort(first, last, compare, data, i, retval)
+ {
+ delete data
+ for (i = 1; first <= last; first++) {
+ data[i] = $first
+ i++
+ }
+
+ quicksort(data, 1, i-1, compare)
+
+ retval = data[1]
+ for (i = 2; i in data; i++)
+ retval = retval " " data[i]
+
+ return retval
+ }
+
+ Finally, the two sorting functions call 'do_sort()', passing in the
+names of the two comparison functions:
+
+ # sort --- sort the data in ascending order and return it as a string
+
+ function sort(first, last)
+ {
+ return do_sort(first, last, "num_lt")
+ }
+
+ # rsort --- sort the data in descending order and return it as a string
+
+ function rsort(first, last)
+ {
+ return do_sort(first, last, "num_ge")
+ }
+
+ Here is an extended version of the data file:
+
+ Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9
+ Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2
+ English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4
+
+ Finally, here are the results when the enhanced program is run:
+
+ $ gawk -f quicksort.awk -f indirectcall.awk class_data2
+ -| Biology 101:
+ -| sum: <352.8>
+ -| average: <88.2>
+ -| sort: <78.5 87.0 92.4 94.9>
+ -| rsort: <94.9 92.4 87.0 78.5>
+ -|
+ -| Chemistry 305:
+ -| sum: <356.4>
+ -| average: <89.1>
+ -| sort: <75.2 88.2 94.7 98.3>
+ -| rsort: <98.3 94.7 88.2 75.2>
+ -|
+ -| English 401:
+ -| sum: <376.1>
+ -| average: <94.025>
+ -| sort: <87.1 93.4 95.6 100.0>
+ -| rsort: <100.0 95.6 93.4 87.1>
+
+ Another example where indirect functions calls are useful can be
+found in processing arrays. This is described in *note Walking
+Arrays::.
+
+ Remember that you must supply a leading '@' in front of an indirect
+function call.
+
+ Starting with version 4.1.2 of 'gawk', indirect function calls may
+also be used with built-in functions and with extension functions (*note
+Dynamic Extensions::). There are some limitations when calling built-in
+functions indirectly, as follows.
+
+ * You cannot pass a regular expression constant to a built-in
+ function through an indirect function call.(1) This applies to the
+ 'sub()', 'gsub()', 'gensub()', 'match()', 'split()' and
+ 'patsplit()' functions.
+
+ * If calling 'sub()' or 'gsub()', you may only pass two arguments,
+ since those functions are unusual in that they update their third
+ argument. This means that '$0' will be updated.
+
+ 'gawk' does its best to make indirect function calls efficient. For
+example, in the following case:
+
+ for (i = 1; i <= n; i++)
+ @the_function()
+
+'gawk' looks up the actual function to call only once.
+
+ ---------- Footnotes ----------
+
+ (1) This may change in a future version; recheck the documentation
+that comes with your version of 'gawk' to see if it has.
+
+
+File: gawk.info, Node: Functions Summary, Prev: Indirect Calls, Up:
Functions
+
+9.4 Summary
+===========
+
+ * 'awk' provides built-in functions and lets you define your own
+ functions.
+
+ * POSIX 'awk' provides three kinds of built-in functions: numeric,
+ string, and I/O. 'gawk' provides functions that sort arrays, work
+ with values representing time, do bit manipulation, determine
+ variable type (array versus scalar), and internationalize and
+ localize programs. 'gawk' also provides several extensions to some
+ of standard functions, typically in the form of additional
+ arguments.
+
+ * Functions accept zero or more arguments and return a value. The
+ expressions that provide the argument values are completely
+ evaluated before the function is called. Order of evaluation is
+ not defined. The return value can be ignored.
+
+ * The handling of backslash in 'sub()' and 'gsub()' is not simple.
+ It is more straightforward in 'gawk''s 'gensub()' function, but
+ that function still requires care in its use.
+
+ * User-defined functions provide important capabilities but come with
+ some syntactic inelegancies. In a function call, there cannot be
+ any space between the function name and the opening left
+ parenthesis of the argument list. Also, there is no provision for
+ local variables, so the convention is to add extra parameters, and
+ to separate them visually from the real parameters by extra
+ whitespace.
+
+ * User-defined functions may call other user-defined (and built-in)
+ functions and may call themselves recursively. Function parameters
+ "hide" any global variables of the same names. You cannot use the
+ name of a reserved variable (such as 'ARGC') as the name of a
+ parameter in user-defined functions.
+
+ * Scalar values are passed to user-defined functions by value. Array
+ parameters are passed by reference; any changes made by the
+ function to array parameters are thus visible after the function
+ has returned.
+
+ * Use the 'return' statement to return from a user-defined function.
+ An optional expression becomes the function's return value. Only
+ scalar values may be returned by a function.
+
+ * If a variable that has never been used is passed to a user-defined
+ function, how that function treats the variable can set its nature:
+ either scalar or array.
+
+ * 'gawk' provides indirect function calls using a special syntax. By
+ setting a variable to the name of a function, you can determine at
+ runtime what function will be called at that point in the program.
+ This is equivalent to function pointers in C and C++.
+
+
+File: gawk.info, Node: Library Functions, Next: Sample Programs, Prev:
Functions, Up: Top
+
+10 A Library of 'awk' Functions
+*******************************
+
+*note User-defined:: describes how to write your own 'awk' functions.
+Writing functions is important, because it allows you to encapsulate
+algorithms and program tasks in a single place. It simplifies
+programming, making program development more manageable and making
+programs more readable.
+
+ In their seminal 1976 book, 'Software Tools',(1) Brian Kernighan and
+P.J. Plauger wrote:
+
+ Good Programming is not learned from generalities, but by seeing
+ how significant programs can be made clean, easy to read, easy to
+ maintain and modify, human-engineered, efficient and reliable, by
+ the application of common sense and good programming practices.
+ Careful study and imitation of good programs leads to better
+ writing.
+
+ In fact, they felt this idea was so important that they placed this
+statement on the cover of their book. Because we believe strongly that
+their statement is correct, this major node and *note Sample Programs::,
+provide a good-sized body of code for you to read and, we hope, to learn
+from.
+
+ This major node presents a library of useful 'awk' functions. Many
+of the sample programs presented later in this Info file use these
+functions. The functions are presented here in a progression from
+simple to complex.
+
+ *note Extract Program:: presents a program that you can use to
+extract the source code for these example library functions and programs
+from the Texinfo source for this Info file. (This has already been done
+as part of the 'gawk' distribution.)
+
+ If you have written one or more useful, general-purpose 'awk'
+functions and would like to contribute them to the 'awk' user community,
+see *note How To Contribute::, for more information.
+
+ The programs in this major node and in *note Sample Programs::,
+freely use 'gawk'-specific features. Rewriting these programs for
+different implementations of 'awk' is pretty straightforward:
+
+ * Diagnostic error messages are sent to '/dev/stderr'. Use '| "cat
+ 1>&2"' instead of '> "/dev/stderr"' if your system does not have a
+ '/dev/stderr', or if you cannot use 'gawk'.
+
+ * A number of programs use 'nextfile' (*note Nextfile Statement::) to
+ skip any remaining input in the input file.
+
+ * Finally, some of the programs choose to ignore upper- and lowercase
+ distinctions in their input. They do so by assigning one to
+ 'IGNORECASE'. You can achieve almost the same effect(2) by adding
+ the following rule to the beginning of the program:
+
+ # ignore case
+ { $0 = tolower($0) }
+
+ Also, verify that all regexp and string constants used in
+ comparisons use only lowercase letters.
+
+* Menu:
+
+* Library Names:: How to best name private global variables in
+ library functions.
+* General Functions:: Functions that are of general use.
+* Data File Management:: Functions for managing command-line data
+ files.
+* Getopt Function:: A function for processing command-line
+ arguments.
+* Passwd Functions:: Functions for getting user information.
+* Group Functions:: Functions for getting group information.
+* Walking Arrays:: A function to walk arrays of arrays.
+* Library Functions Summary:: Summary of library functions.
+* Library Exercises:: Exercises.
+
+ ---------- Footnotes ----------
+
+ (1) Sadly, over 35 years later, many of the lessons taught by this
+book have yet to be learned by a vast number of practicing programmers.
+
+ (2) The effects are not identical. Output of the transformed record
+will be in all lowercase, while 'IGNORECASE' preserves the original
+contents of the input record.
+
+
+File: gawk.info, Node: Library Names, Next: General Functions, Up: Library
Functions
+
+10.1 Naming Library Function Global Variables
+=============================================
+
+Due to the way the 'awk' language evolved, variables are either "global"
+(usable by the entire program) or "local" (usable just by a specific
+function). There is no intermediate state analogous to 'static'
+variables in C.
+
+ Library functions often need to have global variables that they can
+use to preserve state information between calls to the function--for
+example, 'getopt()''s variable '_opti' (*note Getopt Function::). Such
+variables are called "private", as the only functions that need to use
+them are the ones in the library.
+
+ When writing a library function, you should try to choose names for
+your private variables that will not conflict with any variables used by
+either another library function or a user's main program. For example,
+a name like 'i' or 'j' is not a good choice, because user programs often
+use variable names like these for their own purposes.
+
+ The example programs shown in this major node all start the names of
+their private variables with an underscore ('_'). Users generally don't
+use leading underscores in their variable names, so this convention
+immediately decreases the chances that the variable names will be
+accidentally shared with the user's program.
+
+ In addition, several of the library functions use a prefix that helps
+indicate what function or set of functions use the variables--for
+example, '_pw_byname()' in the user database routines (*note Passwd
+Functions::). This convention is recommended, as it even further
+decreases the chance of inadvertent conflict among variable names. Note
+that this convention is used equally well for variable names and for
+private function names.(1)
+
+ As a final note on variable naming, if a function makes global
+variables available for use by a main program, it is a good convention
+to start those variables' names with a capital letter--for example,
+'getopt()''s 'Opterr' and 'Optind' variables (*note Getopt Function::).
+The leading capital letter indicates that it is global, while the fact
+that the variable name is not all capital letters indicates that the
+variable is not one of 'awk''s predefined variables, such as 'FS'.
+
+ It is also important that _all_ variables in library functions that
+do not need to save state are, in fact, declared local.(2) If this is
+not done, the variables could accidentally be used in the user's
+program, leading to bugs that are very difficult to track down:
+
+ function lib_func(x, y, l1, l2)
+ {
+ ...
+ # some_var should be local but by oversight is not
+ USE VARIABLE some_var
+ ...
+ }
+
+ A different convention, common in the Tcl community, is to use a
+single associative array to hold the values needed by the library
+function(s), or "package." This significantly decreases the number of
+actual global names in use. For example, the functions described in
+*note Passwd Functions:: might have used array elements
+'PW_data["inited"]', 'PW_data["total"]', 'PW_data["count"]', and
+'PW_data["awklib"]', instead of '_pw_inited', '_pw_awklib', '_pw_total',
+and '_pw_count'.
+
+ The conventions presented in this minor node are exactly that:
+conventions. You are not required to write your programs this way--we
+merely recommend that you do so.
+
+ Beginning with version 5.0, 'gawk' provides a powerful mechanism for
+solving the problems described in this section: "namespaces".
+Namespaces and their use are described in detail in *note Namespaces::.
+
+ ---------- Footnotes ----------
+
+ (1) Although all the library routines could have been rewritten to
+use this convention, this was not done, in order to show how our own
+'awk' programming style has evolved and to provide some basis for this
+discussion.
+
+ (2) 'gawk''s '--dump-variables' command-line option is useful for
+verifying this.
+
+
+File: gawk.info, Node: General Functions, Next: Data File Management, Prev:
Library Names, Up: Library Functions
+
+10.2 General Programming
+========================
+
+This minor node presents a number of functions that are of general
+programming use.
+
+* Menu:
+
+* Strtonum Function:: A replacement for the built-in
+ 'strtonum()' function.
+* Assert Function:: A function for assertions in 'awk'
+ programs.
+* Round Function:: A function for rounding if 'sprintf()'
+ does not do it correctly.
+* Cliff Random Function:: The Cliff Random Number Generator.
+* Ordinal Functions:: Functions for using characters as numbers and
+ vice versa.
+* Join Function:: A function to join an array into a string.
+* Getlocaltime Function:: A function to get formatted times.
+* Readfile Function:: A function to read an entire file at once.
+* Shell Quoting:: A function to quote strings for the shell.
+* Isnumeric Function:: A function to test whether a value is numeric.
+
+
+File: gawk.info, Node: Strtonum Function, Next: Assert Function, Up:
General Functions
+
+10.2.1 Converting Strings to Numbers
+------------------------------------
+
+The 'strtonum()' function (*note String Functions::) is a 'gawk'
+extension. The following function provides an implementation for other
+versions of 'awk':
+
+ # mystrtonum --- convert string to number
+
+ function mystrtonum(str, ret, n, i, k, c)
+ {
+ if (str ~ /^0[0-7]*$/) {
+ # octal
+ n = length(str)
+ ret = 0
+ for (i = 1; i <= n; i++) {
+ c = substr(str, i, 1)
+ # index() returns 0 if c not in string,
+ # includes c == "0"
+ k = index("1234567", c)
+
+ ret = ret * 8 + k
+ }
+ } else if (str ~ /^0[xX][[:xdigit:]]+$/) {
+ # hexadecimal
+ str = substr(str, 3) # lop off leading 0x
+ n = length(str)
+ ret = 0
+ for (i = 1; i <= n; i++) {
+ c = substr(str, i, 1)
+ c = tolower(c)
+ # index() returns 0 if c not in string,
+ # includes c == "0"
+ k = index("123456789abcdef", c)
+
+ ret = ret * 16 + k
+ }
+ } else if (str ~ \
+
/^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {
+ # decimal number, possibly floating point
+ ret = str + 0
+ } else
+ ret = "NOT-A-NUMBER"
+
+ return ret
+ }
+
+ # BEGIN { # gawk test harness
+ # a[1] = "25"
+ # a[2] = ".31"
+ # a[3] = "0123"
+ # a[4] = "0xdeadBEEF"
+ # a[5] = "123.45"
+ # a[6] = "1.e3"
+ # a[7] = "1.32"
+ # a[8] = "1.32E2"
+ #
+ # for (i = 1; i in a; i++)
+ # print a[i], strtonum(a[i]), mystrtonum(a[i])
+ # }
+
+ The function first looks for C-style octal numbers (base 8). If the
+input string matches a regular expression describing octal numbers, then
+'mystrtonum()' loops through each character in the string. It sets 'k'
+to the index in '"1234567"' of the current octal digit. The return
+value will either be the same number as the digit, or zero if the
+character is not there, which will be true for a '0'. This is safe,
+because the regexp test in the 'if' ensures that only octal values are
+converted.
+
+ Similar logic applies to the code that checks for and converts a
+hexadecimal value, which starts with '0x' or '0X'. The use of
+'tolower()' simplifies the computation for finding the correct numeric
+value for each hexadecimal digit.
+
+ Finally, if the string matches the (rather complicated) regexp for a
+regular decimal integer or floating-point number, the computation 'ret =
+str + 0' lets 'awk' convert the value to a number.
+
+ A commented-out test program is included, so that the function can be
+tested with 'gawk' and the results compared to the built-in 'strtonum()'
+function.
+
+
+File: gawk.info, Node: Assert Function, Next: Round Function, Prev:
Strtonum Function, Up: General Functions
+
+10.2.2 Assertions
+-----------------
+
+When writing large programs, it is often useful to know that a condition
+or set of conditions is true. Before proceeding with a particular
+computation, you make a statement about what you believe to be the case.
+Such a statement is known as an "assertion". The C language provides an
+'<assert.h>' header file and corresponding 'assert()' macro that a
+programmer can use to make assertions. If an assertion fails, the
+'assert()' macro arranges to print a diagnostic message describing the
+condition that should have been true but was not, and then it kills the
+program. In C, using 'assert()' looks this:
+
+ #include <assert.h>
+
+ int myfunc(int a, double b)
+ {
+ assert(a <= 5 && b >= 17.1);
+ ...
+ }
+
+ If the assertion fails, the program prints a message similar to this:
+
+ prog.c:5: assertion failed: a <= 5 && b >= 17.1
+
+ The C language makes it possible to turn the condition into a string
+for use in printing the diagnostic message. This is not possible in
+'awk', so this 'assert()' function also requires a string version of the
+condition that is being tested. Following is the function:
+
+ # assert --- assert that a condition is true. Otherwise, exit.
+
+ function assert(condition, string)
+ {
+ if (! condition) {
+ printf("%s:%d: assertion failed: %s\n",
+ FILENAME, FNR, string) > "/dev/stderr"
+ _assert_exit = 1
+ exit 1
+ }
+ }
+
+ END {
+ if (_assert_exit)
+ exit 1
+ }
+
+ The 'assert()' function tests the 'condition' parameter. If it is
+false, it prints a message to standard error, using the 'string'
+parameter to describe the failed condition. It then sets the variable
+'_assert_exit' to one and executes the 'exit' statement. The 'exit'
+statement jumps to the 'END' rule. If the 'END' rule finds
+'_assert_exit' to be true, it exits immediately.
+
+ The purpose of the test in the 'END' rule is to keep any other 'END'
+rules from running. When an assertion fails, the program should exit
+immediately. If no assertions fail, then '_assert_exit' is still false
+when the 'END' rule is run normally, and the rest of the program's 'END'
+rules execute. For all of this to work correctly, 'assert.awk' must be
+the first source file read by 'awk'. The function can be used in a
+program in the following way:
+
+ function myfunc(a, b)
+ {
+ assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1")
+ ...
+ }
+
+If the assertion fails, you see a message similar to the following:
+
+ mydata:1357: assertion failed: a <= 5 && b >= 17.1
+
+ There is a small problem with this version of 'assert()'. An 'END'
+rule is automatically added to the program calling 'assert()'.
+Normally, if a program consists of just a 'BEGIN' rule, the input files
+and/or standard input are not read. However, now that the program has
+an 'END' rule, 'awk' attempts to read the input data files or standard
+input (*note Using BEGIN/END::), most likely causing the program to hang
+as it waits for input.
+
+ There is a simple workaround to this: make sure that such a 'BEGIN'
+rule always ends with an 'exit' statement.
+
+
+File: gawk.info, Node: Round Function, Next: Cliff Random Function, Prev:
Assert Function, Up: General Functions
+
+10.2.3 Rounding Numbers
+-----------------------
+
+The way 'printf' and 'sprintf()' (*note Printf::) perform rounding often
+depends upon the system's C 'sprintf()' subroutine. On many machines,
+'sprintf()' rounding is "unbiased", which means it doesn't always round
+a trailing .5 up, contrary to naive expectations. In unbiased rounding,
+.5 rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5
+rounds to 4. This means that if you are using a format that does
+rounding (e.g., '"%.0f"'), you should check what your system does. The
+following function does traditional rounding; it might be useful if your
+'awk''s 'printf' does unbiased rounding:
+
+ # round.awk --- do normal rounding
+
+ function round(x, ival, aval, fraction)
+ {
+ ival = int(x) # integer part, int() truncates
+
+ # see if fractional part
+ if (ival == x) # no fraction
+ return ival # ensure no decimals
+
+ if (x < 0) {
+ aval = -x # absolute value
+ ival = int(aval)
+ fraction = aval - ival
+ if (fraction >= .5)
+ return int(x) - 1 # -2.5 --> -3
+ else
+ return int(x) # -2.3 --> -2
+ } else {
+ fraction = x - ival
+ if (fraction >= .5)
+ return ival + 1
+ else
+ return ival
+ }
+ }
+ # test harness
+ # { print $0, round($0) }
+
+
+File: gawk.info, Node: Cliff Random Function, Next: Ordinal Functions,
Prev: Round Function, Up: General Functions
+
+10.2.4 The Cliff Random Number Generator
+----------------------------------------
+
+The Cliff random number generator
+(http://mathworld.wolfram.com/CliffRandomNumberGenerator.html) is a very
+simple random number generator that "passes the noise sphere test for
+randomness by showing no structure." It is easily programmed, in less
+than 10 lines of 'awk' code:
+
+ # cliff_rand.awk --- generate Cliff random numbers
+
+ BEGIN { _cliff_seed = 0.1 }
+
+ function cliff_rand()
+ {
+ _cliff_seed = (100 * log(_cliff_seed)) % 1
+ if (_cliff_seed < 0)
+ _cliff_seed = - _cliff_seed
+ return _cliff_seed
+ }
+
+ This algorithm requires an initial "seed" of 0.1. Each new value
+uses the current seed as input for the calculation. If the built-in
+'rand()' function (*note Numeric Functions::) isn't random enough, you
+might try using this function instead.
+
+
+File: gawk.info, Node: Ordinal Functions, Next: Join Function, Prev: Cliff
Random Function, Up: General Functions
+
+10.2.5 Translating Between Characters and Numbers
+-------------------------------------------------
+
+One commercial implementation of 'awk' supplies a built-in function,
+'ord()', which takes a character and returns the numeric value for that
+character in the machine's character set. If the string passed to
+'ord()' has more than one character, only the first one is used.
+
+ The inverse of this function is 'chr()' (from the function of the
+same name in Pascal), which takes a number and returns the corresponding
+character. Both functions are written very nicely in 'awk'; there is no
+real reason to build them into the 'awk' interpreter:
+
+ # ord.awk --- do ord and chr
+
+ # Global identifiers:
+ # _ord_: numerical values indexed by characters
+ # _ord_init: function to initialize _ord_
+
+ BEGIN { _ord_init() }
+
+ function _ord_init( low, high, i, t)
+ {
+ low = sprintf("%c", 7) # BEL is ascii 7
+ if (low == "\a") { # regular ascii
+ low = 0
+ high = 127
+ } else if (sprintf("%c", 128 + 7) == "\a") {
+ # ascii, mark parity
+ low = 128
+ high = 255
+ } else { # ebcdic(!)
+ low = 0
+ high = 255
+ }
+
+ for (i = low; i <= high; i++) {
+ t = sprintf("%c", i)
+ _ord_[t] = i
+ }
+ }
+
+ Some explanation of the numbers used by '_ord_init()' is worthwhile.
+The most prominent character set in use today is ASCII.(1) Although an
+8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only
+defines characters that use the values from 0 to 127.(2) In the now
+distant past, at least one minicomputer manufacturer used ASCII, but
+with mark parity, meaning that the leftmost bit in the byte is always 1.
+This means that on those systems, characters have numeric values from
+128 to 255. Finally, large mainframe systems use the EBCDIC character
+set, which uses all 256 values. There are other character sets in use
+on some older systems, but they are not really worth worrying about:
+
+ function ord(str, c)
+ {
+ # only first character is of interest
+ c = substr(str, 1, 1)
+ return _ord_[c]
+ }
+
+ function chr(c)
+ {
+ # force c to be numeric by adding 0
+ return sprintf("%c", c + 0)
+ }
+
+ #### test code ####
+ # BEGIN {
+ # for (;;) {
+ # printf("enter a character: ")
+ # if (getline var <= 0)
+ # break
+ # printf("ord(%s) = %d\n", var, ord(var))
+ # }
+ # }
+
+ An obvious improvement to these functions is to move the code for the
+'_ord_init' function into the body of the 'BEGIN' rule. It was written
+this way initially for ease of development. There is a "test program"
+in a 'BEGIN' rule, to test the function. It is commented out for
+production use.
+
+ ---------- Footnotes ----------
+
+ (1) This is changing; many systems use Unicode, a very large
+character set that includes ASCII as a subset. On systems with full
+Unicode support, a character can occupy up to 32 bits, making simple
+tests such as used here prohibitively expensive.
+
+ (2) ASCII has been extended in many countries to use the values from
+128 to 255 for country-specific characters. If your system uses these
+extensions, you can simplify '_ord_init()' to loop from 0 to 255.
+
+
+File: gawk.info, Node: Join Function, Next: Getlocaltime Function, Prev:
Ordinal Functions, Up: General Functions
+
+10.2.6 Merging an Array into a String
+-------------------------------------
+
+When doing string processing, it is often useful to be able to join all
+the strings in an array into one long string. The following function,
+'join()', accomplishes this task. It is used later in several of the
+application programs (*note Sample Programs::).
+
+ Good function design is important; this function needs to be general,
+but it should also have a reasonable default behavior. It is called
+with an array as well as the beginning and ending indices of the
+elements in the array to be merged. This assumes that the array indices
+are numeric--a reasonable assumption, as the array was likely created
+with 'split()' (*note String Functions::):
+
+ # join.awk --- join an array into a string
+
+ function join(array, start, end, sep, result, i)
+ {
+ if (sep == "")
+ sep = " "
+ else if (sep == SUBSEP) # magic value
+ sep = ""
+ result = array[start]
+ for (i = start + 1; i <= end; i++)
+ result = result sep array[i]
+ return result
+ }
+
+ An optional additional argument is the separator to use when joining
+the strings back together. If the caller supplies a nonempty value,
+'join()' uses it; if it is not supplied, it has a null value. In this
+case, 'join()' uses a single space as a default separator for the
+strings. If the value is equal to 'SUBSEP', then 'join()' joins the
+strings with no separator between them. 'SUBSEP' serves as a "magic"
+value to indicate that there should be no separation between the
+component strings.(1)
+
+ ---------- Footnotes ----------
+
+ (1) It would be nice if 'awk' had an assignment operator for
+concatenation. The lack of an explicit operator for concatenation makes
+string operations more difficult than they really need to be.
+
+
+File: gawk.info, Node: Getlocaltime Function, Next: Readfile Function,
Prev: Join Function, Up: General Functions
+
+10.2.7 Managing the Time of Day
+-------------------------------
+
+The 'systime()' and 'strftime()' functions described in *note Time
+Functions:: provide the minimum functionality necessary for dealing with
+the time of day in human-readable form. Although 'strftime()' is
+extensive, the control formats are not necessarily easy to remember or
+intuitively obvious when reading a program.
+
+ The following function, 'getlocaltime()', populates a user-supplied
+array with preformatted time information. It returns a string with the
+current time formatted in the same way as the 'date' utility:
+
+ # getlocaltime.awk --- get the time of day in a usable format
+
+ # Returns a string in the format of output of date(1)
+ # Populates the array argument time with individual values:
+ # time["second"] -- seconds (0 - 59)
+ # time["minute"] -- minutes (0 - 59)
+ # time["hour"] -- hours (0 - 23)
+ # time["althour"] -- hours (0 - 12)
+ # time["monthday"] -- day of month (1 - 31)
+ # time["month"] -- month of year (1 - 12)
+ # time["monthname"] -- name of the month
+ # time["shortmonth"] -- short name of the month
+ # time["year"] -- year modulo 100 (0 - 99)
+ # time["fullyear"] -- full year
+ # time["weekday"] -- day of week (Sunday = 0)
+ # time["altweekday"] -- day of week (Monday = 0)
+ # time["dayname"] -- name of weekday
+ # time["shortdayname"] -- short name of weekday
+ # time["yearday"] -- day of year (0 - 365)
+ # time["timezone"] -- abbreviation of timezone name
+ # time["ampm"] -- AM or PM designation
+ # time["weeknum"] -- week number, Sunday first day
+ # time["altweeknum"] -- week number, Monday first day
+
+ function getlocaltime(time, ret, now, i)
+ {
+ # get time once, avoids unnecessary system calls
+ now = systime()
+
+ # return date(1)-style output
+ ret = strftime("%a %b %e %H:%M:%S %Z %Y", now)
+
+ # clear out target array
+ delete time
+
+ # fill in values, force numeric values to be
+ # numeric by adding 0
+ time["second"] = strftime("%S", now) + 0
+ time["minute"] = strftime("%M", now) + 0
+ time["hour"] = strftime("%H", now) + 0
+ time["althour"] = strftime("%I", now) + 0
+ time["monthday"] = strftime("%d", now) + 0
+ time["month"] = strftime("%m", now) + 0
+ time["monthname"] = strftime("%B", now)
+ time["shortmonth"] = strftime("%b", now)
+ time["year"] = strftime("%y", now) + 0
+ time["fullyear"] = strftime("%Y", now) + 0
+ time["weekday"] = strftime("%w", now) + 0
+ time["altweekday"] = strftime("%u", now) + 0
+ time["dayname"] = strftime("%A", now)
+ time["shortdayname"] = strftime("%a", now)
+ time["yearday"] = strftime("%j", now) + 0
+ time["timezone"] = strftime("%Z", now)
+ time["ampm"] = strftime("%p", now)
+ time["weeknum"] = strftime("%U", now) + 0
+ time["altweeknum"] = strftime("%W", now) + 0
+
+ return ret
+ }
+
+ The string indices are easier to use and read than the various
+formats required by 'strftime()'. The 'alarm' program presented in
+*note Alarm Program:: uses this function. A more general design for the
+'getlocaltime()' function would have allowed the user to supply an
+optional timestamp value to use instead of the current time.
+
+
+File: gawk.info, Node: Readfile Function, Next: Shell Quoting, Prev:
Getlocaltime Function, Up: General Functions
+
+10.2.8 Reading a Whole File at Once
+-----------------------------------
+
+Often, it is convenient to have the entire contents of a file available
+in memory as a single string. A straightforward but naive way to do
+that might be as follows:
+
+ function readfile1(file, tmp, contents)
+ {
+ if ((getline tmp < file) < 0)
+ return
+
+ contents = tmp RT
+ while ((getline tmp < file) > 0)
+ contents = contents tmp RT
+
+ close(file)
+ return contents
+ }
+
+ This function reads from 'file' one record at a time, building up the
+full contents of the file in the local variable 'contents'. It works,
+but is not necessarily efficient.
+
+ The following function, based on a suggestion by Denis Shirokov,
+reads the entire contents of the named file in one shot:
+
+ # readfile.awk --- read an entire file at once
+
+ function readfile(file, tmp, save_rs)
+ {
+ save_rs = RS
+ RS = "^$"
+ getline tmp < file
+ close(file)
+ RS = save_rs
+
+ return tmp
+ }
+
+ It works by setting 'RS' to '^$', a regular expression that will
+never match if the file has contents. 'gawk' reads data from the file
+into 'tmp', attempting to match 'RS'. The match fails after each read,
+but fails quickly, such that 'gawk' fills 'tmp' with the entire contents
+of the file. (*Note Records:: for information on 'RT' and 'RS'.)
+
+ In the case that 'file' is empty, the return value is the null
+string. Thus, calling code may use something like:
+
+ contents = readfile("/some/path")
+ if (length(contents) == 0)
+ # file was empty ...
+
+ This tests the result to see if it is empty or not. An equivalent
+test would be 'contents == ""'.
+
+ *Note Extension Sample Readfile:: for an extension function that also
+reads an entire file into memory.
+
+
+File: gawk.info, Node: Shell Quoting, Next: Isnumeric Function, Prev:
Readfile Function, Up: General Functions
+
+10.2.9 Quoting Strings to Pass to the Shell
+-------------------------------------------
+
+Michael Brennan offers the following programming pattern, which he uses
+frequently:
+
+ #! /bin/sh
+
+ awkp='
+ ...
+ '
+
+ INPUT_PROGRAM | awk "$awkp" | /bin/sh
+
+ For example, a program of his named 'flac-edit' has this form:
+
+ $ flac-edit -song="Whoope! That's Great" file.flac
+
+ It generates the following output, which is to be piped to the shell
+('/bin/sh'):
+
+ chmod +w file.flac
+ metaflac --remove-tag=TITLE file.flac
+ LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great'
file.flac
+ chmod -w file.flac
+
+ Note the need for shell quoting. The function 'shell_quote()' does
+it. 'SINGLE' is the one-character string '"'"' and 'QSINGLE' is the
+three-character string '"\"'\""':
+
+ # shell_quote --- quote an argument for passing to the shell
+
+ function shell_quote(s, # parameter
+ SINGLE, QSINGLE, i, X, n, ret) # locals
+ {
+ if (s == "")
+ return "\"\""
+
+ SINGLE = "\x27" # single quote
+ QSINGLE = "\"\x27\""
+ n = split(s, X, SINGLE)
+
+ ret = SINGLE X[1] SINGLE
+ for (i = 2; i <= n; i++)
+ ret = ret QSINGLE SINGLE X[i] SINGLE
+
+ return ret
+ }
+
+
+File: gawk.info, Node: Isnumeric Function, Prev: Shell Quoting, Up: General
Functions
+
+10.2.10 Checking Whether A Value Is Numeric
+-------------------------------------------
+
+A frequent programming question is how to ascertain whether a value is
+numeric. This can be solved by using this example function
+'isnumeric()', which employs the trick of converting a string value to
+user input by using the 'split()' function:
+
+ # isnumeric --- check whether a value is numeric
+
+ function isnumeric(x, f)
+ {
+ switch (typeof(x)) {
+ case "strnum":
+ case "number":
+ return 1
+ case "string":
+ return (split(x, f, " ") == 1) && (typeof(f[1]) == "strnum")
+ default:
+ return 0
+ }
+ }
+
+ Please note that leading or trailing white space is disregarded in
+deciding whether a value is numeric or not, so if it matters to you, you
+may want to add an additional check for that.
+
+ Traditionally, it has been recommended to check for numeric values
+using the test 'x+0 == x'. This function is superior in two ways: it
+will not report that unassigned variables contain numeric values; and it
+recognizes string values with numeric contents where 'CONVFMT' does not
+yield the original string. On the other hand, it uses the 'typeof()'
+function (*note Type Functions::), which is specific to 'gawk'.
+
+
+File: gawk.info, Node: Data File Management, Next: Getopt Function, Prev:
General Functions, Up: Library Functions
+
+10.3 Data file Management
+=========================
+
+This minor node presents functions that are useful for managing
+command-line data files.
+
+* Menu:
+
+* Filetrans Function:: A function for handling data file transitions.
+* Rewind Function:: A function for rereading the current file.
+* File Checking:: Checking that data files are readable.
+* Empty Files:: Checking for zero-length files.
+* Ignoring Assigns:: Treating assignments as file names.
+
+
+File: gawk.info, Node: Filetrans Function, Next: Rewind Function, Up: Data
File Management
+
+10.3.1 Noting Data file Boundaries
+----------------------------------
+
+The 'BEGIN' and 'END' rules are each executed exactly once, at the
+beginning and end of your 'awk' program, respectively (*note
+BEGIN/END::). We (the 'gawk' authors) once had a user who mistakenly
+thought that the 'BEGIN' rules were executed at the beginning of each
+data file and the 'END' rules were executed at the end of each data
+file.
+
+ When informed that this was not the case, the user requested that we
+add new special patterns to 'gawk', named 'BEGIN_FILE' and 'END_FILE',
+that would have the desired behavior. He even supplied us the code to
+do so.
+
+ Adding these special patterns to 'gawk' wasn't necessary; the job can
+be done cleanly in 'awk' itself, as illustrated by the following library
+program. It arranges to call two user-supplied functions, 'beginfile()'
+and 'endfile()', at the beginning and end of each data file. Besides
+solving the problem in only nine(!) lines of code, it does so
+_portably_; this works with any implementation of 'awk':
+
+ # transfile.awk
+ #
+ # Give the user a hook for filename transitions
+ #
+ # The user must supply functions beginfile() and endfile()
+ # that each take the name of the file being started or
+ # finished, respectively.
+
+ FILENAME != _oldfilename {
+ if (_oldfilename != "")
+ endfile(_oldfilename)
+ _oldfilename = FILENAME
+ beginfile(FILENAME)
+ }
+
+ END { endfile(FILENAME) }
+
+ This file must be loaded before the user's "main" program, so that
+the rule it supplies is executed first.
+
+ This rule relies on 'awk''s 'FILENAME' variable, which automatically
+changes for each new data file. The current file name is saved in a
+private variable, '_oldfilename'. If 'FILENAME' does not equal
+'_oldfilename', then a new data file is being processed and it is
+necessary to call 'endfile()' for the old file. Because 'endfile()'
+should only be called if a file has been processed, the program first
+checks to make sure that '_oldfilename' is not the null string. The
+program then assigns the current file name to '_oldfilename' and calls
+'beginfile()' for the file. Because, like all 'awk' variables,
+'_oldfilename' is initialized to the null string, this rule executes
+correctly even for the first data file.
+
+ The program also supplies an 'END' rule to do the final processing
+for the last file. Because this 'END' rule comes before any 'END' rules
+supplied in the "main" program, 'endfile()' is called first. Once
+again, the value of multiple 'BEGIN' and 'END' rules should be clear.
+
+ If the same data file occurs twice in a row on the command line, then
+'endfile()' and 'beginfile()' are not executed at the end of the first
+pass and at the beginning of the second pass. The following version
+solves the problem:
+
+ # ftrans.awk --- handle datafile transitions
+ #
+ # user supplies beginfile() and endfile() functions
+
+ FNR == 1 {
+ if (_filename_ != "")
+ endfile(_filename_)
+ _filename_ = FILENAME
+ beginfile(FILENAME)
+ }
+
+ END { endfile(_filename_) }
+
+ *note Wc Program:: shows how this library function can be used and
+how it simplifies writing the main program.
+
+ So Why Does 'gawk' Have 'BEGINFILE' and 'ENDFILE'?
+
+ You are probably wondering, if 'beginfile()' and 'endfile()'
+functions can do the job, why does 'gawk' have 'BEGINFILE' and 'ENDFILE'
+patterns?
+
+ Good question. Normally, if 'awk' cannot open a file, this causes an
+immediate fatal error. In this case, there is no way for a user-defined
+function to deal with the problem, as the mechanism for calling it
+relies on the file being open and at the first record. Thus, the main
+reason for 'BEGINFILE' is to give you a "hook" to catch files that
+cannot be processed. 'ENDFILE' exists for symmetry, and because it
+provides an easy way to do per-file cleanup processing. For more
+information, refer to *note BEGINFILE/ENDFILE::.
+
+
+File: gawk.info, Node: Rewind Function, Next: File Checking, Prev:
Filetrans Function, Up: Data File Management
+
+10.3.2 Rereading the Current File
+---------------------------------
+
+Another request for a new built-in function was for a function that
+would make it possible to reread the current file. The requesting user
+didn't want to have to use 'getline' (*note Getline::) inside a loop.
+
+ However, as long as you are not in the 'END' rule, it is quite easy
+to arrange to immediately close the current input file and then start
+over with it from the top. For lack of a better name, we'll call the
+function 'rewind()':
+
+ # rewind.awk --- rewind the current file and start over
+
+ function rewind( i)
+ {
+ # shift remaining arguments up
+ for (i = ARGC; i > ARGIND; i--)
+ ARGV[i] = ARGV[i-1]
+
+ # make sure gawk knows to keep going
+ ARGC++
+
+ # make current file next to get done
+ ARGV[ARGIND+1] = FILENAME
+
+ # do it
+ nextfile
+ }
+
+ The 'rewind()' function relies on the 'ARGIND' variable (*note
+Auto-set::), which is specific to 'gawk'. It also relies on the
+'nextfile' keyword (*note Nextfile Statement::). Because of this, you
+should not call it from an 'ENDFILE' rule. (This isn't necessary
+anyway, because 'gawk' goes to the next file as soon as an 'ENDFILE'
+rule finishes!)
+
+ You need to be careful calling 'rewind()'. You can end up causing
+infinite recursion if you don't pay attention. Here is an example use:
+
+ $ cat data
+ -| a
+ -| b
+ -| c
+ -| d
+ -| e
+
+ $ cat test.awk
+ -| FNR == 3 && ! rewound {
+ -| rewound = 1
+ -| rewind()
+ -| }
+ -|
+ -| { print FILENAME, FNR, $0 }
+
+ $ gawk -f rewind.awk -f test.awk data
+ -| data 1 a
+ -| data 2 b
+ -| data 1 a
+ -| data 2 b
+ -| data 3 c
+ -| data 4 d
+ -| data 5 e
+
+
+File: gawk.info, Node: File Checking, Next: Empty Files, Prev: Rewind
Function, Up: Data File Management
+
+10.3.3 Checking for Readable Data files
+---------------------------------------
+
+Normally, if you give 'awk' a data file that isn't readable, it stops
+with a fatal error. There are times when you might want to just ignore
+such files and keep going.(1) You can do this by prepending the
+following program to your 'awk' program:
+
+ # readable.awk --- library file to skip over unreadable files
+
+ BEGIN {
+ for (i = 1; i < ARGC; i++) {
+ if (ARGV[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/ \
+ || ARGV[i] == "-" || ARGV[i] == "/dev/stdin")
+ continue # assignment or standard input
+ else if ((getline junk < ARGV[i]) < 0) # unreadable
+ delete ARGV[i]
+ else
+ close(ARGV[i])
+ }
+ }
+
+ This works, because the 'getline' won't be fatal. Removing the
+element from 'ARGV' with 'delete' skips the file (because it's no longer
+in the list). See also *note ARGC and ARGV::.
+
+ Because 'awk' variable names only allow the English letters, the
+regular expression check purposely does not use character classes such
+as '[:alpha:]' and '[:alnum:]' (*note Bracket Expressions::).
+
+ ---------- Footnotes ----------
+
+ (1) The 'BEGINFILE' special pattern (*note BEGINFILE/ENDFILE::)
+provides an alternative mechanism for dealing with files that can't be
+opened. However, the code here provides a portable solution.
+
+
+File: gawk.info, Node: Empty Files, Next: Ignoring Assigns, Prev: File
Checking, Up: Data File Management
+
+10.3.4 Checking for Zero-Length Files
+-------------------------------------
+
+All known 'awk' implementations silently skip over zero-length files.
+This is a by-product of 'awk''s implicit
+read-a-record-and-match-against-the-rules loop: when 'awk' tries to read
+a record from an empty file, it immediately receives an end-of-file
+indication, closes the file, and proceeds on to the next command-line
+data file, _without_ executing any user-level 'awk' program code.
+
+ Using 'gawk''s 'ARGIND' variable (*note Built-in Variables::), it is
+possible to detect when an empty data file has been skipped. Similar to
+the library file presented in *note Filetrans Function::, the following
+library file calls a function named 'zerofile()' that the user must
+provide. The arguments passed are the file name and the position in
+'ARGV' where it was found:
+
+ # zerofile.awk --- library file to process empty input files
+
+ BEGIN { Argind = 0 }
+
+ ARGIND > Argind + 1 {
+ for (Argind++; Argind < ARGIND; Argind++)
+ zerofile(ARGV[Argind], Argind)
+ }
+
+ ARGIND != Argind { Argind = ARGIND }
+
+ END {
+ if (ARGIND > Argind)
+ for (Argind++; Argind <= ARGIND; Argind++)
+ zerofile(ARGV[Argind], Argind)
+ }
+
+ The user-level variable 'Argind' allows the 'awk' program to track
+its progress through 'ARGV'. Whenever the program detects that 'ARGIND'
+is greater than 'Argind + 1', it means that one or more empty files were
+skipped. The action then calls 'zerofile()' for each such file,
+incrementing 'Argind' along the way.
+
+ The 'Argind != ARGIND' rule simply keeps 'Argind' up to date in the
+normal case.
+
+ Finally, the 'END' rule catches the case of any empty files at the
+end of the command-line arguments. Note that the test in the condition
+of the 'for' loop uses the '<=' operator, not '<'.
+
+
+File: gawk.info, Node: Ignoring Assigns, Prev: Empty Files, Up: Data File
Management
+
+10.3.5 Treating Assignments as File names
+-----------------------------------------
+
+Occasionally, you might not want 'awk' to process command-line variable
+assignments (*note Assignment Options::). In particular, if you have a
+file name that contains an '=' character, 'awk' treats the file name as
+an assignment and does not process it.
+
+ Some users have suggested an additional command-line option for
+'gawk' to disable command-line assignments. However, some simple
+programming with a library file does the trick:
+
+ # noassign.awk --- library file to avoid the need for a
+ # special option that disables command-line assignments
+
+ function disable_assigns(argc, argv, i)
+ {
+ for (i = 1; i < argc; i++)
+ if (argv[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/)
+ argv[i] = ("./" argv[i])
+ }
+
+ BEGIN {
+ if (No_command_assign)
+ disable_assigns(ARGC, ARGV)
+ }
+
+ You then run your program this way:
+
+ awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk *
+
+ The function works by looping through the arguments. It prepends
+'./' to any argument that matches the form of a variable assignment,
+turning that argument into a file name.
+
+ The use of 'No_command_assign' allows you to disable command-line
+assignments at invocation time, by giving the variable a true value.
+When not set, it is initially zero (i.e., false), so the command-line
+arguments are left alone.
+
+
+File: gawk.info, Node: Getopt Function, Next: Passwd Functions, Prev: Data
File Management, Up: Library Functions
+
+10.4 Processing Command-Line Options
+====================================
+
+Most utilities on POSIX-compatible systems take options on the command
+line that can be used to change the way a program behaves. 'awk' is an
+example of such a program (*note Options::). Often, options take
+"arguments" (i.e., data that the program needs to correctly obey the
+command-line option). For example, 'awk''s '-F' option requires a
+string to use as the field separator. The first occurrence on the
+command line of either '--' or a string that does not begin with '-'
+ends the options.
+
+ Modern Unix systems provide a C function named 'getopt()' for
+processing command-line arguments. The programmer provides a string
+describing the one-letter options. If an option requires an argument,
+it is followed in the string with a colon. 'getopt()' is also passed
+the count and values of the command-line arguments and is called in a
+loop. 'getopt()' processes the command-line arguments for option
+letters. Each time around the loop, it returns a single character
+representing the next option letter that it finds, or '?' if it finds an
+invalid option. When it returns -1, there are no options left on the
+command line.
+
+ When using 'getopt()', options that do not take arguments can be
+grouped together. Furthermore, options that take arguments require that
+the argument be present. The argument can immediately follow the option
+letter, or it can be a separate command-line argument.
+
+ Given a hypothetical program that takes three command-line options,
+'-a', '-b', and '-c', where '-b' requires an argument, all of the
+following are valid ways of invoking the program:
+
+ prog -a -b foo -c data1 data2 data3
+ prog -ac -bfoo -- data1 data2 data3
+ prog -acbfoo data1 data2 data3
+
+ Notice that when the argument is grouped with its option, the rest of
+the argument is considered to be the option's argument. In this
+example, '-acbfoo' indicates that all of the '-a', '-b', and '-c'
+options were supplied, and that 'foo' is the argument to the '-b'
+option.
+
+ 'getopt()' provides four external variables that the programmer can
+use:
+
+'optind'
+ The index in the argument value array ('argv') where the first
+ nonoption command-line argument can be found.
+
+'optarg'
+ The string value of the argument to an option.
+
+'opterr'
+ Usually 'getopt()' prints an error message when it finds an invalid
+ option. Setting 'opterr' to zero disables this feature. (An
+ application might want to print its own error message.)
+
+'optopt'
+ The letter representing the command-line option.
+
+ The following C fragment shows how 'getopt()' might process
+command-line arguments for 'awk':
+
+ int
+ main(int argc, char *argv[])
+ {
+ ...
+ /* print our own message */
+ opterr = 0;
+ while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {
+ switch (c) {
+ case 'f': /* file */
+ ...
+ break;
+ case 'F': /* field separator */
+ ...
+ break;
+ case 'v': /* variable assignment */
+ ...
+ break;
+ case 'W': /* extension */
+ ...
+ break;
+ case '?':
+ default:
+ usage();
+ break;
+ }
+ }
+ ...
+ }
+
+ The GNU project's version of the original Unix utilities popularized
+the use of long command line options. For example, '--help' in addition
+to '-h'. Arguments to long options are either provided as separate
+command line arguments ('--source 'PROGRAM-TEXT'') or separated from the
+option with an '=' sign ('--source='PROGRAM-TEXT'').
+
+ As a side point, 'gawk' actually uses the GNU 'getopt_long()'
+function to process both normal and GNU-style long options (*note
+Options::).
+
+ The abstraction provided by 'getopt()' is very useful and is quite
+handy in 'awk' programs as well. Following is an 'awk' version of
+'getopt()' that accepts both short and long options. (Support for long
+options was supplied by Greg Minshall. We thank him.)
+
+ This function highlights one of the greatest weaknesses in 'awk',
+which is that it is very poor at manipulating single characters. The
+function needs repeated calls to 'substr()' in order to access
+individual characters (*note String Functions::).(1)
+
+ The discussion that follows walks through the code a bit at a time:
+
+ # getopt.awk --- Do C library getopt(3) function in awk
+ # Also supports long options.
+
+ # External variables:
+ # Optind -- index in ARGV of first nonoption argument
+ # Optarg -- string value of argument to current option
+ # Opterr -- if nonzero, print our own diagnostic
+ # Optopt -- current option letter
+
+ # Returns:
+ # -1 at end of options
+ # "?" for unrecognized option
+ # <s> a string representing the current option
+
+ # Private Data:
+ # _opti -- index in multiflag option, e.g., -abc
+
+ The function starts out with comments presenting a list of the global
+variables it uses, what the return values are, what they mean, and any
+global variables that are "private" to this library function. Such
+documentation is essential for any program, and particularly for library
+functions.
+
+ The 'getopt()' function first checks that it was indeed called with a
+string of options (the 'options' parameter). If both 'options' and
+'longoptions' have a zero length, 'getopt()' immediately returns -1:
+
+ function getopt(argc, argv, options, longopts, thisopt, i, j)
+ {
+ if (length(options) == 0 && length(longopts) == 0)
+ return -1 # no options given
+
+ if (argv[Optind] == "--") { # all done
+ Optind++
+ _opti = 0
+ return -1
+ } else if (argv[Optind] !~ /^-[^:[:space:]]/) {
+ _opti = 0
+ return -1
+ }
+
+ The next thing to check for is the end of the options. A '--' ends
+the command-line options, as does any command-line argument that does
+not begin with a '-' (unless it is an argument to a preceding option).
+'Optind' steps through the array of command-line arguments; it retains
+its value across calls to 'getopt()', because it is a global variable.
+
+ The regular expression '/^-[^:[:space:]/' checks for a '-' followed
+by anything that is not whitespace and not a colon. If the current
+command-line argument does not match this pattern, it is not an option,
+and it ends option processing. Now, we check to see if we are
+processing a short (single letter) option, or a long option (indicated
+by two dashes, e.g., '--filename'). If it is a short option, we
+continue on:
+
+ if (argv[Optind] !~ /^--/) { # if this is a short option
+ if (_opti == 0)
+ _opti = 2
+ thisopt = substr(argv[Optind], _opti, 1)
+ Optopt = thisopt
+ i = index(options, thisopt)
+ if (i == 0) {
+ if (Opterr)
+ printf("%c -- invalid option\n", thisopt) > "/dev/stderr"
+ if (_opti >= length(argv[Optind])) {
+ Optind++
+ _opti = 0
+ } else
+ _opti++
+ return "?"
+ }
+
+ The '_opti' variable tracks the position in the current command-line
+argument ('argv[Optind]'). If multiple options are grouped together
+with one '-' (e.g., '-abx'), it is necessary to return them to the user
+one at a time.
+
+ If '_opti' is equal to zero, it is set to two, which is the index in
+the string of the next character to look at (we skip the '-', which is
+at position one). The variable 'thisopt' holds the character, obtained
+with 'substr()'. It is saved in 'Optopt' for the main program to use.
+
+ If 'thisopt' is not in the 'options' string, then it is an invalid
+option. If 'Opterr' is nonzero, 'getopt()' prints an error message on
+the standard error that is similar to the message from the C version of
+'getopt()'.
+
+ Because the option is invalid, it is necessary to skip it and move on
+to the next option character. If '_opti' is greater than or equal to
+the length of the current command-line argument, it is necessary to move
+on to the next argument, so 'Optind' is incremented and '_opti' is reset
+to zero. Otherwise, 'Optind' is left alone and '_opti' is merely
+incremented.
+
+ In any case, because the option is invalid, 'getopt()' returns '"?"'.
+The main program can examine 'Optopt' if it needs to know what the
+invalid option letter actually is. Continuing on:
+
+ if (substr(options, i + 1, 1) == ":") {
+ # get option argument
+ if (length(substr(argv[Optind], _opti + 1)) > 0)
+ Optarg = substr(argv[Optind], _opti + 1)
+ else
+ Optarg = argv[++Optind]
+ _opti = 0
+ } else
+ Optarg = ""
+
+ If the option requires an argument, the option letter is followed by
+a colon in the 'options' string. If there are remaining characters in
+the current command-line argument ('argv[Optind]'), then the rest of
+that string is assigned to 'Optarg'. Otherwise, the next command-line
+argument is used ('-xFOO' versus '-x FOO'). In either case, '_opti' is
+reset to zero, because there are no more characters left to examine in
+the current command-line argument. Continuing:
+
+ if (_opti == 0 || _opti >= length(argv[Optind])) {
+ Optind++
+ _opti = 0
+ } else
+ _opti++
+ return thisopt
+
+ Finally, for a short option, if '_opti' is either zero or greater
+than the length of the current command-line argument, it means this
+element in 'argv' is through being processed, so 'Optind' is incremented
+to point to the next element in 'argv'. If neither condition is true,
+then only '_opti' is incremented, so that the next option letter can be
+processed on the next call to 'getopt()'.
+
+ On the other hand, if the earlier test found that this was a long
+option, we take a different branch:
+
+ } else {
+ j = index(argv[Optind], "=")
+ if (j > 0)
+ thisopt = substr(argv[Optind], 3, j - 3)
+ else
+ thisopt = substr(argv[Optind], 3)
+ Optopt = thisopt
+
+ First, we search this option for a possible embedded equal sign, as
+the specification of long options allows an argument to an option
+'--someopt' to be specified as '--someopt=answer' as well as
+'--someopt answer'.
+
+ i = match(longopts, "(^|,)" thisopt "($|[,:])")
+ if (i == 0) {
+ if (Opterr)
+ printf("%s -- invalid option\n", thisopt) > "/dev/stderr"
+ Optind++
+ return "?"
+ }
+
+ Next, we try to find the current option in 'longopts'. The regular
+expression given to 'match()', '"(^|,)" thisopt "($|[,:])"', matches
+this option at the beginning of 'longopts', or at the beginning of a
+subsequent long option (the previous long option would have been
+terminated by a comma), and, in any case, either at the end of the
+'longopts' string ('$'), or followed by a comma (separating this option
+from a subsequent option) or a colon (indicating this long option takes
+an argument ('[,:]').
+
+ Using this regular expression, we check to see if the current option
+might possibly be in 'longopts' (if 'longopts' is not specified, this
+test will also fail). In case of an error, we possibly print an error
+message and then return '"?"'. Continuing on:
+
+ if (substr(longopts, i+1+length(thisopt), 1) == ":") {
+ if (j > 0)
+ Optarg = substr(argv[Optind], j + 1)
+ else
+ Optarg = argv[++Optind]
+ } else
+ Optarg = ""
+
+ We now check to see if this option takes an argument and, if so, we
+set 'Optarg' to the value of that argument (either a value after an
+equal sign specified on the command line, immediately adjoining the long
+option string, or as the next argument on the command line).
+
+ Optind++
+ return thisopt
+ }
+ }
+
+ We increase 'Optind' (which we already increased once if a required
+argument was separated from its option by an equal sign), and return the
+long option (minus its leading dashes).
+
+ The 'BEGIN' rule initializes both 'Opterr' and 'Optind' to one.
+'Opterr' is set to one, because the default behavior is for 'getopt()'
+to print a diagnostic message upon seeing an invalid option. 'Optind'
+is set to one, because there's no reason to look at the program name,
+which is in 'ARGV[0]':
+
+ BEGIN {
+ Opterr = 1 # default is to diagnose
+ Optind = 1 # skip ARGV[0]
+
+ # test program
+ if (_getopt_test) {
+ _myshortopts = "ab:cd"
+ _mylongopts = "longa,longb:,otherc,otherd"
+
+ while ((_go_c = getopt(ARGC, ARGV, _myshortopts, _mylongopts)) !=
-1)
+ printf("c = <%s>, Optarg = <%s>\n", _go_c, Optarg)
+ printf("non-option arguments:\n")
+ for (; Optind < ARGC; Optind++)
+ printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind])
+ }
+ }
+
+ The rest of the 'BEGIN' rule is a simple test program. Here are the
+results of some sample runs of the test program:
+
+ $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
+ -| c = <a>, Optarg = <>
+ -| c = <c>, Optarg = <>
+ -| c = <b>, Optarg = <ARG>
+ -| non-option arguments:
+ -| ARGV[3] = <bax>
+ -| ARGV[4] = <-x>
+
+ $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
+ -| c = <a>, Optarg = <>
+ error-> x -- invalid option
+ -| c = <?>, Optarg = <>
+ -| non-option arguments:
+ -| ARGV[4] = <xyz>
+ -| ARGV[5] = <abc>
+
+ $ awk -f getopt.awk -v _getopt_test=1 -- -a \
+ > --longa -b xx --longb=foo=bar --otherd --otherc arg1 arg2
+ -| c = <a>, Optarg = <>
+ -| c = <longa>, Optarg = <>
+ -| c = <b>, Optarg = <xx>
+ -| c = <longb>, Optarg = <foo=bar>
+ -| c = <otherd>, Optarg = <>
+ -| c = <otherc>, Optarg = <>
+ -| non-option arguments:
+ -| ARGV[8] = <arg1>
+ -| ARGV[9] = <arg2>
+
+ In all the runs, the first '--' terminates the arguments to 'awk', so
+that it does not try to interpret the '-a', etc., as its own options.
+
+ NOTE: After 'getopt()' is through, user-level code must clear out
+ all the elements of 'ARGV' from 1 to 'Optind', so that 'awk' does
+ not try to process the command-line options as file names.
+
+ Using '#!' with the '-E' option may help avoid conflicts between your
+program's options and 'gawk''s options, as '-E' causes 'gawk' to abandon
+processing of further options (*note Executable Scripts:: and *note
+Options::).
+
+ Several of the sample programs presented in *note Sample Programs::,
+use 'getopt()' to process their arguments.
+
+ ---------- Footnotes ----------
+
+ (1) This function was written before 'gawk' acquired the ability to
+split strings into single characters using '""' as the separator. We
+have left it alone, as using 'substr()' is more portable.
+
+
+File: gawk.info, Node: Passwd Functions, Next: Group Functions, Prev:
Getopt Function, Up: Library Functions
+
+10.5 Reading the User Database
+==============================
+
+The 'PROCINFO' array (*note Built-in Variables::) provides access to the
+current user's real and effective user and group ID numbers, and, if
+available, the user's supplementary group set. However, because these
+are numbers, they do not provide very useful information to the average
+user. There needs to be some way to find the user information
+associated with the user and group ID numbers. This minor node presents
+a suite of functions for retrieving information from the user database.
+*Note Group Functions:: for a similar suite that retrieves information
+from the group database.
+
+ The POSIX standard does not define the file where user information is
+kept. Instead, it provides the '<pwd.h>' header file and several C
+language subroutines for obtaining user information. The primary
+function is 'getpwent()', for "get password entry." The "password"
+comes from the original user database file, '/etc/passwd', which stores
+user information along with the encrypted passwords (hence the name).
+
+ Although an 'awk' program could simply read '/etc/passwd' directly,
+this file may not contain complete information about the system's set of
+users.(1) To be sure you are able to produce a readable and complete
+version of the user database, it is necessary to write a small C program
+that calls 'getpwent()'. 'getpwent()' is defined as returning a pointer
+to a 'struct passwd'. Each time it is called, it returns the next entry
+in the database. When there are no more entries, it returns 'NULL', the
+null pointer. When this happens, the C program should call 'endpwent()'
+to close the database. Following is 'pwcat', a C program that "cats"
+the password database:
+
+ /*
+ * pwcat.c
+ *
+ * Generate a printable version of the password database.
+ */
+ #include <stdio.h>
+ #include <pwd.h>
+
+ int
+ main(int argc, char **argv)
+ {
+ struct passwd *p;
+
+ while ((p = getpwent()) != NULL)
+ printf("%s:%s:%ld:%ld:%s:%s:%s\n",
+ p->pw_name, p->pw_passwd, (long) p->pw_uid,
+ (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
+
+ endpwent();
+ return 0;
+ }
+
+ If you don't understand C, don't worry about it. The output from
+'pwcat' is the user database, in the traditional '/etc/passwd' format of
+colon-separated fields. The fields are:
+
+Login name
+ The user's login name.
+
+Encrypted password
+ The user's encrypted password. This may not be available on some
+ systems.
+
+User-ID
+ The user's numeric user ID number. (On some systems, it's a C
+ 'long', and not an 'int'. Thus, we cast it to 'long' for all
+ cases.)
+
+Group-ID
+ The user's numeric group ID number. (Similar comments about 'long'
+ versus 'int' apply here.)
+
+Full name
+ The user's full name, and perhaps other information associated with
+ the user.
+
+Home directory
+ The user's login (or "home") directory (familiar to shell
+ programmers as '$HOME').
+
+Login shell
+ The program that is run when the user logs in. This is usually a
+ shell, such as Bash.
+
+ A few lines representative of 'pwcat''s output are as follows:
+
+ $ pwcat
+ -| root:x:0:1:Operator:/:/bin/sh
+ -| nobody:*:65534:65534::/:
+ -| daemon:*:1:1::/:
+ -| sys:*:2:2::/:/bin/csh
+ -| bin:*:3:3::/bin:
+ -| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
+ -| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
+ -| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
+ ...
+
+ With that introduction, following is a group of functions for getting
+user information. There are several functions here, corresponding to
+the C functions of the same names:
+
+ # passwd.awk --- access password file information
+
+ BEGIN {
+ # tailor this to suit your system
+ _pw_awklib = "/usr/local/libexec/awk/"
+ }
+
+ function _pw_init( oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)
+ {
+ if (_pw_inited)
+ return
+
+ oldfs = FS
+ oldrs = RS
+ olddol0 = $0
+ using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
+ using_fpat = (PROCINFO["FS"] == "FPAT")
+ FS = ":"
+ RS = "\n"
+
+ pwcat = _pw_awklib "pwcat"
+ while ((pwcat | getline) > 0) {
+ _pw_byname[$1] = $0
+ _pw_byuid[$3] = $0
+ _pw_bycount[++_pw_total] = $0
+ }
+ close(pwcat)
+ _pw_count = 0
+ _pw_inited = 1
+ FS = oldfs
+ if (using_fw)
+ FIELDWIDTHS = FIELDWIDTHS
+ else if (using_fpat)
+ FPAT = FPAT
+ RS = oldrs
+ $0 = olddol0
+ }
+
+ The 'BEGIN' rule sets a private variable to the directory where
+'pwcat' is stored. Because it is used to help out an 'awk' library
+routine, we have chosen to put it in '/usr/local/libexec/awk'; however,
+you might want it to be in a different directory on your system.
+
+ The function '_pw_init()' fills three copies of the user information
+into three associative arrays. The arrays are indexed by username
+('_pw_byname'), by user ID number ('_pw_byuid'), and by order of
+occurrence ('_pw_bycount'). The variable '_pw_inited' is used for
+efficiency, as '_pw_init()' needs to be called only once.
+
+ Because this function uses 'getline' to read information from
+'pwcat', it first saves the values of 'FS', 'RS', and '$0'. It notes in
+the variable 'using_fw' whether field splitting with 'FIELDWIDTHS' is in
+effect or not. Doing so is necessary, as these functions could be
+called from anywhere within a user's program, and the user may have his
+or her own way of splitting records and fields. This makes it possible
+to restore the correct field-splitting mechanism later. The test can
+only be true for 'gawk'. It is false if using 'FS' or 'FPAT', or on
+some other 'awk' implementation.
+
+ The code that checks for using 'FPAT', using 'using_fpat' and
+'PROCINFO["FS"]', is similar.
+
+ The main part of the function uses a loop to read database lines,
+split the lines into fields, and then store the lines into each array as
+necessary. When the loop is done, '_pw_init()' cleans up by closing the
+pipeline, setting '_pw_inited' to one, and restoring 'FS' (and
+'FIELDWIDTHS' or 'FPAT' if necessary), 'RS', and '$0'. The use of
+'_pw_count' is explained shortly.
+
+ The 'getpwnam()' function takes a username as a string argument. If
+that user is in the database, it returns the appropriate line.
+Otherwise, it relies on the array reference to a nonexistent element to
+create the element with the null string as its value:
+
+ function getpwnam(name)
+ {
+ _pw_init()
+ return _pw_byname[name]
+ }
+
+ Similarly, the 'getpwuid()' function takes a user ID number argument.
+If that user number is in the database, it returns the appropriate line.
+Otherwise, it returns the null string:
+
+ function getpwuid(uid)
+ {
+ _pw_init()
+ return _pw_byuid[uid]
+ }
+
+ The 'getpwent()' function simply steps through the database, one
+entry at a time. It uses '_pw_count' to track its current position in
+the '_pw_bycount' array:
+
+ function getpwent()
+ {
+ _pw_init()
+ if (_pw_count < _pw_total)
+ return _pw_bycount[++_pw_count]
+ return ""
+ }
+
+ The 'endpwent()' function resets '_pw_count' to zero, so that
+subsequent calls to 'getpwent()' start over again:
+
+ function endpwent()
+ {
+ _pw_count = 0
+ }
+
+ A conscious design decision in this suite is that each subroutine
+calls '_pw_init()' to initialize the database arrays. The overhead of
+running a separate process to generate the user database, and the I/O to
+scan it, are only incurred if the user's main program actually calls one
+of these functions. If this library file is loaded along with a user's
+program, but none of the routines are ever called, then there is no
+extra runtime overhead. (The alternative is move the body of
+'_pw_init()' into a 'BEGIN' rule, which always runs 'pwcat'. This
+simplifies the code but runs an extra process that may never be needed.)
+
+ In turn, calling '_pw_init()' is not too expensive, because the
+'_pw_inited' variable keeps the program from reading the data more than
+once. If you are worried about squeezing every last cycle out of your
+'awk' program, the check of '_pw_inited' could be moved out of
+'_pw_init()' and duplicated in all the other functions. In practice,
+this is not necessary, as most 'awk' programs are I/O-bound, and such a
+change would clutter up the code.
+
+ The 'id' program in *note Id Program:: uses these functions.
+
+ ---------- Footnotes ----------
+
+ (1) It is often the case that password information is stored in a
+network database.
+
+
+File: gawk.info, Node: Group Functions, Next: Walking Arrays, Prev: Passwd
Functions, Up: Library Functions
+
+10.6 Reading the Group Database
+===============================
+
+Much of the discussion presented in *note Passwd Functions:: applies to
+the group database as well. Although there has traditionally been a
+well-known file ('/etc/group') in a well-known format, the POSIX
+standard only provides a set of C library routines ('<grp.h>' and
+'getgrent()') for accessing the information. Even though this file may
+exist, it may not have complete information. Therefore, as with the
+user database, it is necessary to have a small C program that generates
+the group database as its output. 'grcat', a C program that "cats" the
+group database, is as follows:
+
+ /*
+ * grcat.c
+ *
+ * Generate a printable version of the group database.
+ */
+ #include <stdio.h>
+ #include <grp.h>
+
+ int
+ main(int argc, char **argv)
+ {
+ struct group *g;
+ int i;
+
+ while ((g = getgrent()) != NULL) {
+ printf("%s:%s:%ld:", g->gr_name, g->gr_passwd,
+ (long) g->gr_gid);
+ for (i = 0; g->gr_mem[i] != NULL; i++) {
+ printf("%s", g->gr_mem[i]);
+ if (g->gr_mem[i+1] != NULL)
+ putchar(',');
+ }
+ putchar('\n');
+ }
+ endgrent();
+ return 0;
+ }
+
+ Each line in the group database represents one group. The fields are
+separated with colons and represent the following information:
+
+Group Name
+ The group's name.
+
+Group Password
+ The group's encrypted password. In practice, this field is never
+ used; it is usually empty or set to '*'.
+
+Group ID Number
+ The group's numeric group ID number; the association of name to
+ number must be unique within the file. (On some systems it's a C
+ 'long', and not an 'int'. Thus, we cast it to 'long' for all
+ cases.)
+
+Group Member List
+ A comma-separated list of usernames. These users are members of
+ the group. Modern Unix systems allow users to be members of
+ several groups simultaneously. If your system does, then there are
+ elements '"group1"' through '"groupN"' in 'PROCINFO' for those
+ group ID numbers. (Note that 'PROCINFO' is a 'gawk' extension;
+ *note Built-in Variables::.)
+
+ Here is what running 'grcat' might produce:
+
+ $ grcat
+ -| wheel:*:0:arnold
+ -| nogroup:*:65534:
+ -| daemon:*:1:
+ -| kmem:*:2:
+ -| staff:*:10:arnold,miriam,andy
+ -| other:*:20:
+ ...
+
+ Here are the functions for obtaining information from the group
+database. There are several, modeled after the C library functions of
+the same names:
+
+ # group.awk --- functions for dealing with the group file
+
+ BEGIN {
+ # Change to suit your system
+ _gr_awklib = "/usr/local/libexec/awk/"
+ }
+
+ function _gr_init( oldfs, oldrs, olddol0, grcat,
+ using_fw, using_fpat, n, a, i)
+ {
+ if (_gr_inited)
+ return
+
+ oldfs = FS
+ oldrs = RS
+ olddol0 = $0
+ using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
+ using_fpat = (PROCINFO["FS"] == "FPAT")
+ FS = ":"
+ RS = "\n"
+
+ grcat = _gr_awklib "grcat"
+ while ((grcat | getline) > 0) {
+ if ($1 in _gr_byname)
+ _gr_byname[$1] = _gr_byname[$1] "," $4
+ else
+ _gr_byname[$1] = $0
+ if ($3 in _gr_bygid)
+ _gr_bygid[$3] = _gr_bygid[$3] "," $4
+ else
+ _gr_bygid[$3] = $0
+
+ n = split($4, a, "[ \t]*,[ \t]*")
+ for (i = 1; i <= n; i++)
+ if (a[i] in _gr_groupsbyuser)
+ _gr_groupsbyuser[a[i]] = _gr_groupsbyuser[a[i]] " " $1
+ else
+ _gr_groupsbyuser[a[i]] = $1
+
+ _gr_bycount[++_gr_count] = $0
+ }
+ close(grcat)
+ _gr_count = 0
+ _gr_inited++
+ FS = oldfs
+ if (using_fw)
+ FIELDWIDTHS = FIELDWIDTHS
+ else if (using_fpat)
+ FPAT = FPAT
+ RS = oldrs
+ $0 = olddol0
+ }
+
+ The 'BEGIN' rule sets a private variable to the directory where
+'grcat' is stored. Because it is used to help out an 'awk' library
+routine, we have chosen to put it in '/usr/local/libexec/awk'. You
+might want it to be in a different directory on your system.
+
+ These routines follow the same general outline as the user database
+routines (*note Passwd Functions::). The '_gr_inited' variable is used
+to ensure that the database is scanned no more than once. The
+'_gr_init()' function first saves 'FS', 'RS', and '$0', and then sets
+'FS' and 'RS' to the correct values for scanning the group information.
+It also takes care to note whether 'FIELDWIDTHS' or 'FPAT' is being
+used, and to restore the appropriate field-splitting mechanism.
+
+ The group information is stored in several associative arrays. The
+arrays are indexed by group name ('_gr_byname'), by group ID number
+('_gr_bygid'), and by position in the database ('_gr_bycount'). There
+is an additional array indexed by username ('_gr_groupsbyuser'), which
+is a space-separated list of groups to which each user belongs.
+
+ Unlike in the user database, it is possible to have multiple records
+in the database for the same group. This is common when a group has a
+large number of members. A pair of such entries might look like the
+following:
+
+ tvpeople:*:101:johnny,jay,arsenio
+ tvpeople:*:101:david,conan,tom,joan
+
+ For this reason, '_gr_init()' looks to see if a group name or group
+ID number is already seen. If so, the usernames are simply concatenated
+onto the previous list of users.(1)
+
+ Finally, '_gr_init()' closes the pipeline to 'grcat', restores 'FS'
+(and 'FIELDWIDTHS' or 'FPAT', if necessary), 'RS', and '$0', initializes
+'_gr_count' to zero (it is used later), and makes '_gr_inited' nonzero.
+
+ The 'getgrnam()' function takes a group name as its argument, and if
+that group exists, it is returned. Otherwise, it relies on the array
+reference to a nonexistent element to create the element with the null
+string as its value:
+
+ function getgrnam(group)
+ {
+ _gr_init()
+ return _gr_byname[group]
+ }
+
+ The 'getgrgid()' function is similar; it takes a numeric group ID and
+looks up the information associated with that group ID:
+
+ function getgrgid(gid)
+ {
+ _gr_init()
+ return _gr_bygid[gid]
+ }
+
+ The 'getgruser()' function does not have a C counterpart. It takes a
+username and returns the list of groups that have the user as a member:
+
+ function getgruser(user)
+ {
+ _gr_init()
+ return _gr_groupsbyuser[user]
+ }
+
+ The 'getgrent()' function steps through the database one entry at a
+time. It uses '_gr_count' to track its position in the list:
+
+ function getgrent()
+ {
+ _gr_init()
+ if (++_gr_count in _gr_bycount)
+ return _gr_bycount[_gr_count]
+ return ""
+ }
+
+ The 'endgrent()' function resets '_gr_count' to zero so that
+'getgrent()' can start over again:
+
+ function endgrent()
+ {
+ _gr_count = 0
+ }
+
+ As with the user database routines, each function calls '_gr_init()'
+to initialize the arrays. Doing so only incurs the extra overhead of
+running 'grcat' if these functions are used (as opposed to moving the
+body of '_gr_init()' into a 'BEGIN' rule).
+
+ Most of the work is in scanning the database and building the various
+associative arrays. The functions that the user calls are themselves
+very simple, relying on 'awk''s associative arrays to do work.
+
+ The 'id' program in *note Id Program:: uses these functions.
+
+ ---------- Footnotes ----------
+
+ (1) There is a subtle problem with the code just presented. Suppose
+that the first time there were no names. This code adds the names with
+a leading comma. It also doesn't check that there is a '$4'.
+
+
+File: gawk.info, Node: Walking Arrays, Next: Library Functions Summary,
Prev: Group Functions, Up: Library Functions
+
+10.7 Traversing Arrays of Arrays
+================================
+
+*note Arrays of Arrays:: described how 'gawk' provides arrays of arrays.
+In particular, any element of an array may be either a scalar or another
+array. The 'isarray()' function (*note Type Functions::) lets you
+distinguish an array from a scalar. The following function,
+'walk_array()', recursively traverses an array, printing the element
+indices and values. You call it with the array and a string
+representing the name of the array:
+
+ function walk_array(arr, name, i)
+ {
+ for (i in arr) {
+ if (isarray(arr[i]))
+ walk_array(arr[i], (name "[" i "]"))
+ else
+ printf("%s[%s] = %s\n", name, i, arr[i])
+ }
+ }
+
+It works by looping over each element of the array. If any given
+element is itself an array, the function calls itself recursively,
+passing the subarray and a new string representing the current index.
+Otherwise, the function simply prints the element's name, index, and
+value. Here is a main program to demonstrate:
+
+ BEGIN {
+ a[1] = 1
+ a[2][1] = 21
+ a[2][2] = 22
+ a[3] = 3
+ a[4][1][1] = 411
+ a[4][2] = 42
+
+ walk_array(a, "a")
+ }
+
+ When run, the program produces the following output:
+
+ $ gawk -f walk_array.awk
+ -| a[1] = 1
+ -| a[2][1] = 21
+ -| a[2][2] = 22
+ -| a[3] = 3
+ -| a[4][1][1] = 411
+ -| a[4][2] = 42
+
+ The function just presented simply prints the name and value of each
+scalar array element. However, it is easy to generalize it, by passing
+in the name of a function to call when walking an array. The modified
+function looks like this:
+
+ function process_array(arr, name, process, do_arrays, i, new_name)
+ {
+ for (i in arr) {
+ new_name = (name "[" i "]")
+ if (isarray(arr[i])) {
+ if (do_arrays)
+ @process(new_name, arr[i])
+ process_array(arr[i], new_name, process, do_arrays)
+ } else
+ @process(new_name, arr[i])
+ }
+ }
+
+ The arguments are as follows:
+
+'arr'
+ The array.
+
+'name'
+ The name of the array (a string).
+
+'process'
+ The name of the function to call.
+
+'do_arrays'
+ If this is true, the function can handle elements that are
+ subarrays.
+
+ If subarrays are to be processed, that is done before walking them
+further.
+
+ When run with the following scaffolding, the function produces the
+same results as does the earlier version of 'walk_array()':
+
+ BEGIN {
+ a[1] = 1
+ a[2][1] = 21
+ a[2][2] = 22
+ a[3] = 3
+ a[4][1][1] = 411
+ a[4][2] = 42
+
+ process_array(a, "a", "do_print", 0)
+ }
+
+ function do_print(name, element)
+ {
+ printf "%s = %s\n", name, element
+ }
+
+
+File: gawk.info, Node: Library Functions Summary, Next: Library Exercises,
Prev: Walking Arrays, Up: Library Functions
+
+10.8 Summary
+============
+
+ * Reading programs is an excellent way to learn Good Programming.
+ The functions and programs provided in this major node and the next
+ are intended to serve that purpose.
+
+ * When writing general-purpose library functions, put some thought
+ into how to name any global variables so that they won't conflict
+ with variables from a user's program.
+
+ * The functions presented here fit into the following categories:
+
+ General problems
+ Number-to-string conversion, testing assertions, rounding,
+ random number generation, converting characters to numbers,
+ joining strings, getting easily usable time-of-day
+ information, and reading a whole file in one shot
+
+ Managing data files
+ Noting data file boundaries, rereading the current file,
+ checking for readable files, checking for zero-length files,
+ and treating assignments as file names
+
+ Processing command-line options
+ An 'awk' version of the standard C 'getopt()' function
+
+ Reading the user and group databases
+ Two sets of routines that parallel the C library versions
+
+ Traversing arrays of arrays
+ Two functions that traverse an array of arrays to any depth
+
+
+File: gawk.info, Node: Library Exercises, Prev: Library Functions Summary,
Up: Library Functions
+
+10.9 Exercises
+==============
+
+ 1. In *note Empty Files::, we presented the 'zerofile.awk' program,
+ which made use of 'gawk''s 'ARGIND' variable. Can this problem be
+ solved without relying on 'ARGIND'? If so, how?
+
+ 2. As a related challenge, revise that code to handle the case where
+ an intervening value in 'ARGV' is a variable assignment.
+
+
+File: gawk.info, Node: Sample Programs, Next: Advanced Features, Prev:
Library Functions, Up: Top
+
+11 Practical 'awk' Programs
+***************************
+
+*note Library Functions::, presents the idea that reading programs in a
+language contributes to learning that language. This major node
+continues that theme, presenting a potpourri of 'awk' programs for your
+reading enjoyment.
+
+ Many of these programs use library functions presented in *note
+Library Functions::.
+
+* Menu:
+
+* Running Examples:: How to run these examples.
+* Clones:: Clones of common utilities.
+* Miscellaneous Programs:: Some interesting 'awk' programs.
+* Programs Summary:: Summary of programs.
+* Programs Exercises:: Exercises.
+
+
+File: gawk.info, Node: Running Examples, Next: Clones, Up: Sample Programs
+
+11.1 Running the Example Programs
+=================================
+
+To run a given program, you would typically do something like this:
+
+ awk -f PROGRAM -- OPTIONS FILES
+
+Here, PROGRAM is the name of the 'awk' program (such as 'cut.awk'),
+OPTIONS are any command-line options for the program that start with a
+'-', and FILES are the actual data files.
+
+ If your system supports the '#!' executable interpreter mechanism
+(*note Executable Scripts::), you can instead run your program directly:
+
+ cut.awk -c1-8 myfiles > results
+
+ If your 'awk' is not 'gawk', you may instead need to use this:
+
+ cut.awk -- -c1-8 myfiles > results
+
+
+File: gawk.info, Node: Clones, Next: Miscellaneous Programs, Prev: Running
Examples, Up: Sample Programs
+
+11.2 Reinventing Wheels for Fun and Profit
+==========================================
+
+This minor node presents a number of POSIX utilities implemented in
+'awk'. Reinventing these programs in 'awk' is often enjoyable, because
+the algorithms can be very clearly expressed, and the code is usually
+very concise and simple. This is true because 'awk' does so much for
+you.
+
+ It should be noted that these programs are not necessarily intended
+to replace the installed versions on your system. Nor may all of these
+programs be fully compliant with the most recent POSIX standard. This
+is not a problem; their purpose is to illustrate 'awk' language
+programming for "real-world" tasks.
+
+ The programs are presented in alphabetical order.
+
+* Menu:
+
+* Cut Program:: The 'cut' utility.
+* Egrep Program:: The 'egrep' utility.
+* Id Program:: The 'id' utility.
+* Split Program:: The 'split' utility.
+* Tee Program:: The 'tee' utility.
+* Uniq Program:: The 'uniq' utility.
+* Wc Program:: The 'wc' utility.
+
+
+File: gawk.info, Node: Cut Program, Next: Egrep Program, Up: Clones
+
+11.2.1 Cutting Out Fields and Columns
+-------------------------------------
+
+The 'cut' utility selects, or "cuts," characters or fields from its
+standard input and sends them to its standard output. Fields are
+separated by TABs by default, but you may supply a command-line option
+to change the field "delimiter" (i.e., the field-separator character).
+'cut''s definition of fields is less general than 'awk''s.
+
+ A common use of 'cut' might be to pull out just the login names of
+logged-on users from the output of 'who'. For example, the following
+pipeline generates a sorted, unique list of the logged-on users:
+
+ who | cut -c1-8 | sort | uniq
+
+ The options for 'cut' are:
+
+'-c LIST'
+ Use LIST as the list of characters to cut out. Items within the
+ list may be separated by commas, and ranges of characters can be
+ separated with dashes. The list '1-8,15,22-35' specifies
+ characters 1 through 8, 15, and 22 through 35.
+
+'-d DELIM'
+ Use DELIM as the field-separator character instead of the TAB
+ character.
+
+'-f LIST'
+ Use LIST as the list of fields to cut out.
+
+'-s'
+ Suppress printing of lines that do not contain the field delimiter.
+
+ The 'awk' implementation of 'cut' uses the 'getopt()' library
+function (*note Getopt Function::) and the 'join()' library function
+(*note Join Function::).
+
+ The current POSIX version of 'cut' has options to cut fields based on
+both bytes and characters. This version does not attempt to implement
+those options, as 'awk' works exclusively in terms of characters.
+
+ The program begins with a comment describing the options, the library
+functions needed, and a 'usage()' function that prints out a usage
+message and exits. 'usage()' is called if invalid arguments are
+supplied:
+
+ # cut.awk --- implement cut in awk
+
+ # Options:
+ # -c list Cut characters
+ # -f list Cut fields
+ # -d c Field delimiter character
+ #
+ # -s Suppress lines without the delimiter
+ #
+ # Requires getopt() and join() library functions
+
+ function usage()
+ {
+ print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr"
+ print(" cut [-c list] [files...]") > "/dev/stderr"
+ exit 1
+ }
+
+ Next comes a 'BEGIN' rule that parses the command-line options. It
+sets 'FS' to a single TAB character, because that is 'cut''s default
+field separator. The rule then sets the output field separator to be
+the same as the input field separator. A loop using 'getopt()' steps
+through the command-line options. Exactly one of the variables
+'by_fields' or 'by_chars' is set to true, to indicate that processing
+should be done by fields or by characters, respectively. When cutting
+by characters, the output field separator is set to the null string:
+
+ BEGIN {
+ FS = "\t" # default
+ OFS = FS
+ while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
+ if (c == "f") {
+ by_fields = 1
+ fieldlist = Optarg
+ } else if (c == "c") {
+ by_chars = 1
+ fieldlist = Optarg
+ OFS = ""
+ } else if (c == "d") {
+ if (length(Optarg) > 1) {
+ printf("cut: using first character of %s" \
+ " for delimiter\n", Optarg) > "/dev/stderr"
+ Optarg = substr(Optarg, 1, 1)
+ }
+ fs = FS = Optarg
+ OFS = FS
+ if (FS == " ") # defeat awk semantics
+ FS = "[ ]"
+ } else if (c == "s")
+ suppress = 1
+ else
+ usage()
+ }
+
+ # Clear out options
+ for (i = 1; i < Optind; i++)
+ ARGV[i] = ""
+
+ The code must take special care when the field delimiter is a space.
+Using a single space ('" "') for the value of 'FS' is incorrect--'awk'
+would separate fields with runs of spaces, TABs, and/or newlines, and we
+want them to be separated with individual spaces. To this end, we save
+the original space character in the variable 'fs' for later use; after
+setting 'FS' to '"[ ]"' we can't use it directly to see if the field
+delimiter character is in the string.
+
+ Also remember that after 'getopt()' is through (as described in *note
+Getopt Function::), we have to clear out all the elements of 'ARGV' from
+1 to 'Optind', so that 'awk' does not try to process the command-line
+options as file names.
+
+ After dealing with the command-line options, the program verifies
+that the options make sense. Only one or the other of '-c' and '-f'
+should be used, and both require a field list. Then the program calls
+either 'set_fieldlist()' or 'set_charlist()' to pull apart the list of
+fields or characters:
+
+ if (by_fields && by_chars)
+ usage()
+
+ if (by_fields == 0 && by_chars == 0)
+ by_fields = 1 # default
+
+ if (fieldlist == "") {
+ print "cut: needs list for -c or -f" > "/dev/stderr"
+ exit 1
+ }
+
+ if (by_fields)
+ set_fieldlist()
+ else
+ set_charlist()
+ }
+
+ 'set_fieldlist()' splits the field list apart at the commas into an
+array. Then, for each element of the array, it looks to see if the
+element is actually a range, and if so, splits it apart. The function
+checks the range to make sure that the first number is smaller than the
+second. Each number in the list is added to the 'flist' array, which
+simply lists the fields that will be printed. Normal field splitting is
+used. The program lets 'awk' handle the job of doing the field
+splitting:
+
+ function set_fieldlist( n, m, i, j, k, f, g)
+ {
+ n = split(fieldlist, f, ",")
+ j = 1 # index in flist
+ for (i = 1; i <= n; i++) {
+ if (index(f[i], "-") != 0) { # a range
+ m = split(f[i], g, "-")
+ if (m != 2 || g[1] >= g[2]) {
+ printf("cut: bad field list: %s\n",
+ f[i]) > "/dev/stderr"
+ exit 1
+ }
+ for (k = g[1]; k <= g[2]; k++)
+ flist[j++] = k
+ } else
+ flist[j++] = f[i]
+ }
+ nfields = j - 1
+ }
+
+ The 'set_charlist()' function is more complicated than
+'set_fieldlist()'. The idea here is to use 'gawk''s 'FIELDWIDTHS'
+variable (*note Constant Size::), which describes constant-width input.
+When using a character list, that is exactly what we have.
+
+ Setting up 'FIELDWIDTHS' is more complicated than simply listing the
+fields that need to be printed. We have to keep track of the fields to
+print and also the intervening characters that have to be skipped. For
+example, suppose you wanted characters 1 through 8, 15, and 22 through
+35. You would use '-c 1-8,15,22-35'. The necessary value for
+'FIELDWIDTHS' is '"8 6 1 6 14"'. This yields five fields, and the
+fields to print are '$1', '$3', and '$5'. The intermediate fields are
+"filler", which is stuff in between the desired data. 'flist' lists the
+fields to print, and 't' tracks the complete field list, including
+filler fields:
+
+ function set_charlist( field, i, j, f, g, n, m, t,
+ filler, last, len)
+ {
+ field = 1 # count total fields
+ n = split(fieldlist, f, ",")
+ j = 1 # index in flist
+ for (i = 1; i <= n; i++) {
+ if (index(f[i], "-") != 0) { # range
+ m = split(f[i], g, "-")
+ if (m != 2 || g[1] >= g[2]) {
+ printf("cut: bad character list: %s\n",
+ f[i]) > "/dev/stderr"
+ exit 1
+ }
+ len = g[2] - g[1] + 1
+ if (g[1] > 1) # compute length of filler
+ filler = g[1] - last - 1
+ else
+ filler = 0
+ if (filler)
+ t[field++] = filler
+ t[field++] = len # length of field
+ last = g[2]
+ flist[j++] = field - 1
+ } else {
+ if (f[i] > 1)
+ filler = f[i] - last - 1
+ else
+ filler = 0
+ if (filler)
+ t[field++] = filler
+ t[field++] = 1
+ last = f[i]
+ flist[j++] = field - 1
+ }
+ }
+ FIELDWIDTHS = join(t, 1, field - 1)
+ nfields = j - 1
+ }
+
+ Next is the rule that processes the data. If the '-s' option is
+given, then 'suppress' is true. The first 'if' statement makes sure
+that the input record does have the field separator. If 'cut' is
+processing fields, 'suppress' is true, and the field separator character
+is not in the record, then the record is skipped.
+
+ If the record is valid, then 'gawk' has split the data into fields,
+either using the character in 'FS' or using fixed-length fields and
+'FIELDWIDTHS'. The loop goes through the list of fields that should be
+printed. The corresponding field is printed if it contains data. If
+the next field also has data, then the separator character is written
+out between the fields:
+
+ {
+ if (by_fields && suppress && index($0, fs) == 0)
+ next
+
+ for (i = 1; i <= nfields; i++) {
+ if ($flist[i] != "") {
+ printf "%s", $flist[i]
+ if (i < nfields && $flist[i+1] != "")
+ printf "%s", OFS
+ }
+ }
+ print ""
+ }
+
+ This version of 'cut' relies on 'gawk''s 'FIELDWIDTHS' variable to do
+the character-based cutting. It is possible in other 'awk'
+implementations to use 'substr()' (*note String Functions::), but it is
+also extremely painful. The 'FIELDWIDTHS' variable supplies an elegant
+solution to the problem of picking the input line apart by characters.
+
+
+File: gawk.info, Node: Egrep Program, Next: Id Program, Prev: Cut Program,
Up: Clones
+
+11.2.2 Searching for Regular Expressions in Files
+-------------------------------------------------
+
+The 'grep' family of programs searches files for patterns. These
+programs have an unusual history. Initially there was 'grep' (Global
+Regular Expression Print), which used what are now called Basic Regular
+Expressions (BREs). Later there was 'egrep' (Extended 'grep') which
+used what are now called Extended Regular Expressions (EREs). (These
+are almost identical to those available in 'awk'; *note Regexp::).
+There was also 'fgrep' (Fast 'grep'), which searched for matches of one
+more fixed strings.
+
+ POSIX chose to combine these three programs into one, simply named
+'grep'. On a POSIX system, 'grep''s default behavior is to search using
+BREs. You use '-E' to specify the use of EREs, and '-F' to specify
+searching for fixed strings.
+
+ In practice, systems continue to come with separate 'egrep' and
+'fgrep' utilities, for backwards compatibility. This minor node
+provides an 'awk' implementation of 'egrep', which supports all of the
+POSIX-mandated options. You invoke it as follows:
+
+ 'egrep' [OPTIONS] ''PATTERN'' FILES ...
+
+ The PATTERN is a regular expression. In typical usage, the regular
+expression is quoted to prevent the shell from expanding any of the
+special characters as file name wildcards. Normally, 'egrep' prints the
+lines that matched. If multiple file names are provided on the command
+line, each output line is preceded by the name of the file and a colon.
+
+ The options to 'egrep' are as follows:
+
+'-c'
+ Print a count of the lines that matched the pattern, instead of the
+ lines themselves.
+
+'-e PATTERN'
+ Use PATTERN as the regexp to match. The purpose of the '-e' option
+ is to allow patterns that start with a '-'.
+
+'-i'
+ Ignore case distinctions in both the pattern and the input data.
+
+'-l'
+ Only print (list) the names of the files that matched, not the
+ lines that matched.
+
+'-q'
+ Be quiet. No output is produced and the exit value indicates
+ whether the pattern was matched.
+
+'-s'
+ Be silent. Do not print error messages for files that could not be
+ opened.
+
+'-v'
+ Invert the sense of the test. 'egrep' prints the lines that do
+ _not_ match the pattern and exits successfully if the pattern is
+ not matched.
+
+'-x'
+ Match the entire input line in order to consider the match as
+ having succeeded.
+
+ This version uses the 'getopt()' library function (*note Getopt
+Function::) and 'gawk''s 'BEGINFILE' and 'ENDFILE' special patterns
+(*note BEGINFILE/ENDFILE::).
+
+ The program begins with descriptive comments and then a 'BEGIN' rule
+that processes the command-line arguments with 'getopt()'. The '-i'
+(ignore case) option is particularly easy with 'gawk'; we just use the
+'IGNORECASE' predefined variable (*note Built-in Variables::):
+
+ # egrep.awk --- simulate egrep in awk
+ #
+ # Options:
+ # -c count of lines
+ # -e argument is pattern
+ # -i ignore case
+ # -l print filenames only
+ # -n add line number to output
+ # -q quiet - use exit value
+ # -s silent - don't print errors
+ # -v invert test, success if no match
+ # -x the entire line must match
+ #
+ # Requires getopt library function
+ # Uses IGNORECASE, BEGINFILE and ENDFILE
+ # Invoke using gawk -f egrep.awk -- options ...
+
+ BEGIN {
+ while ((c = getopt(ARGC, ARGV, "ce:ilnqsvx")) != -1) {
+ if (c == "c")
+ count_only++
+ else if (c == "e")
+ pattern = Optarg
+ else if (c == "i")
+ IGNORECASE = 1
+ else if (c == "l")
+ filenames_only++
+ else if (c == "n")
+ line_numbers++
+ else if (c == "q")
+ no_print++
+ else if (c == "s")
+ no_errors++
+ else if (c == "v")
+ invert++
+ else if (c == "x")
+ full_line++
+ else
+ usage()
+ }
+
+Note the comment about invocation: Because several of the options
+overlap with 'gawk''s, a '--' is needed to tell 'gawk' to stop looking
+for options.
+
+ Next comes the code that handles the 'egrep'-specific behavior.
+'egrep' uses the first nonoption on the command line if no pattern is
+supplied with '-e'. If the pattern is empty, that means no pattern was
+supplied, so it's necessary to print an error message and exit. The
+'awk' command-line arguments up to 'ARGV[Optind]' are cleared, so that
+'awk' won't try to process them as files. If no files are specified,
+the standard input is used, and if multiple files are specified, we make
+sure to note this so that the file names can precede the matched lines
+in the output:
+
+ if (pattern == "")
+ pattern = ARGV[Optind++]
+
+ if (pattern == "")
+ usage()
+
+ for (i = 1; i < Optind; i++)
+ ARGV[i] = ""
+
+ if (Optind >= ARGC) {
+ ARGV[1] = "-"
+ ARGC = 2
+ } else if (ARGC - Optind > 1)
+ do_filenames++
+ }
+
+ The 'BEGINFILE' rule executes when each new file is processed. In
+this case, it is fairly simple; it initializes a variable 'fcount' to
+zero. 'fcount' tracks how many lines in the current file matched the
+pattern.
+
+ Here also is where we implement the '-s' option. We check if 'ERRNO'
+has been set, and if '-s' was supplied. In that case, it's necessary to
+move on to the next file. Otherwise 'gawk' would exit with an error:
+
+ BEGINFILE {
+ fcount = 0
+ if (ERRNO && no_errors)
+ nextfile
+ }
+
+ The 'ENDFILE' rule executes after each file has been processed. It
+affects the output only when the user wants a count of the number of
+lines that matched. 'no_print' is true only if the exit status is
+desired. 'count_only' is true if line counts are desired. 'egrep'
+therefore only prints line counts if printing and counting are enabled.
+The output format must be adjusted depending upon the number of files to
+process. Finally, 'fcount' is added to 'total', so that we know the
+total number of lines that matched the pattern:
+
+ ENDFILE {
+ if (! no_print && count_only) {
+ if (do_filenames)
+ print file ":" fcount
+ else
+ print fcount
+ }
+
+ total += fcount
+ }
+
+ The following rule does most of the work of matching lines. The
+variable 'matches' is true (non-zero) if the line matched the pattern.
+If the user specified that the entire line must match (with '-x'), the
+code checks this condition by looking at the values of 'RSTART' and
+'RLENGTH'. If those indicate that the match is not over the full line,
+'matches' is set to zero (false).
+
+ If the user wants lines that did not match, we invert the sense of
+'matches' using the '!' operator. We then increment 'fcount' with the
+value of 'matches', which is either one or zero, depending upon a
+successful or unsuccessful match. If the line does not match, the
+'next' statement just moves on to the next input line.
+
+ We make a number of additional tests, but only if we are not counting
+lines. First, if the user only wants the exit status ('no_print' is
+true), then it is enough to know that _one_ line in this file matched,
+and we can skip on to the next file with 'nextfile'. Similarly, if we
+are only printing file names, we can print the file name, and then skip
+to the next file with 'nextfile'. Finally, each line is printed, with a
+leading file name, optional colon and line number, and the final colon
+if necessary:
+
+ {
+ matches = match($0, pattern)
+ if (matches && full_line && (RSTART != 1 || RLENGTH != length()))
+ matches = 0
+
+ if (invert)
+ matches = ! matches
+
+ fcount += matches # 1 or 0
+
+ if (! matches)
+ next
+
+ if (! count_only) {
+ if (no_print)
+ nextfile
+
+ if (filenames_only) {
+ print FILENAME
+ nextfile
+ }
+
+ if (do_filenames)
+ if (line_numbers)
+ print FILENAME ":" FNR ":" $0
+ else
+ print FILENAME ":" $0
+ else
+ print
+ }
+ }
+
+ The 'END' rule takes care of producing the correct exit status. If
+there are no matches, the exit status is one; otherwise, it is zero:
+
+ END {
+ exit (total == 0)
+ }
+
+ The 'usage()' function prints a usage message in case of invalid
+options, and then exits:
+
+ function usage()
+ {
+ print("Usage:\tegrep [-cilnqsvx] [-e pat] [files ...]") >
"/dev/stderr"
+ print("\tegrep [-cilnqsvx] pat [files ...]") > "/dev/stderr"
+ exit 1
+ }
+
+
+File: gawk.info, Node: Id Program, Next: Split Program, Prev: Egrep
Program, Up: Clones
+
+11.2.3 Printing Out User Information
+------------------------------------
+
+The 'id' utility lists a user's real and effective user ID numbers, real
+and effective group ID numbers, and the user's group set, if any. 'id'
+only prints the effective user ID and group ID if they are different
+from the real ones. If possible, 'id' also supplies the corresponding
+user and group names. The output might look like this:
+
+ $ id
+ -| uid=1000(arnold) gid=1000(arnold)
groups=1000(arnold),4(adm),7(lp),27(sudo)
+
+ This information is part of what is provided by 'gawk''s 'PROCINFO'
+array (*note Built-in Variables::). However, the 'id' utility provides
+a more palatable output than just individual numbers.
+
+ The POSIX version of 'id' takes several options that give you control
+over the output's format, such as printing only real ids, or printing
+only numbers or only names. Additionally, you can print the information
+for a specific user, instead of that of the current user.
+
+ Here is a version of POSIX 'id' written in 'awk'. It uses the
+'getopt()' library function (*note Getopt Function::), the user database
+library functions (*note Passwd Functions::), and the group database
+library functions (*note Group Functions::) from *note Library
+Functions::.
+
+ The program is moderately straightforward. All the work is done in
+the 'BEGIN' rule. It starts with explanatory comments, a list of
+options, and then a 'usage()' function:
+
+ # id.awk --- implement id in awk
+ #
+ # Requires user and group library functions and getopt
+ # output is:
+ # uid=12(foo) euid=34(bar) gid=3(baz) \
+ # egid=5(blat) groups=9(nine),2(two),1(one)
+
+ # Options:
+ # -G Output all group ids as space separated numbers (ruid, euid,
groups)
+ # -g Output only the euid as a number
+ # -n Output name instead of the numeric value (with -g/-G/-u)
+ # -r Output ruid/rguid instead of effective id
+ # -u Output only effective user id, as a number
+
+ function usage()
+ {
+ printf("Usage:\n" \
+ "\tid [user]\n" \
+ "\tid -G [-n] [user]\n" \
+ "\tid -g [-nr] [user]\n" \
+ "\tid -u [-nr] [user]\n") > "/dev/stderr"
+
+ exit 1
+ }
+
+ The first step is to parse the options using 'getopt()', and to set
+various flag variables according to the options given:
+
+ BEGIN {
+ # parse args
+ while ((c = getopt(ARGC, ARGV, "Ggnru")) != -1) {
+ if (c == "G")
+ groupset_only++
+ else if (c == "g")
+ egid_only++
+ else if (c == "n")
+ names_not_groups++
+ else if (c == "r")
+ real_ids_only++
+ else if (c == "u")
+ euid_only++
+ else
+ usage()
+ }
+
+ The next step is to check that no conflicting options were provided.
+'-G' and '-r' are mutually exclusive. It is also not allowed to provide
+more than one user name on the command line:
+
+ if (groupset_only && real_ids_only)
+ usage()
+ else if (ARGC - Optind > 1)
+ usage()
+
+ The user and group ID numbers are obtained from 'PROCINFO' for the
+current user, or from the user and password databases for a user
+supplied on the command line. In the latter case, 'real_ids_only' is
+set, since it's not possible to print information about the effective
+user and group IDs:
+
+ if (ARGC - Optind == 0) {
+ # gather info for current user
+ uid = PROCINFO["uid"]
+ euid = PROCINFO["euid"]
+ gid = PROCINFO["gid"]
+ egid = PROCINFO["egid"]
+ for (i = 1; ("group" i) in PROCINFO; i++)
+ groupset[i] = PROCINFO["group" i]
+ } else {
+ fill_info_for_user(ARGV[ARGC-1])
+ real_ids_only++
+ }
+
+ The test in the 'for' loop is worth noting. Any supplementary groups
+in the 'PROCINFO' array have the indices '"group1"' through '"groupN"'
+for some N (i.e., the total number of supplementary groups). However,
+we don't know in advance how many of these groups there are.
+
+ This loop works by starting at one, concatenating the value with
+'"group"', and then using 'in' to see if that value is in the array
+(*note Reference to Elements::). Eventually, 'i' increments past the
+last group in the array and the loop exits.
+
+ The loop is also correct if there are _no_ supplementary groups; then
+the condition is false the first time it's tested, and the loop body
+never executes.
+
+ Now, based on the options, we decide what information to print. For
+'-G' (print just the group set), we then select whether to print names
+or numbers. In either case, when done we exit:
+
+ if (groupset_only) {
+ if (names_not_groups) {
+ for (i = 1; i in groupset; i++) {
+ entry = getgrgid(groupset[i])
+ name = get_first_field(entry)
+ printf("%s", name)
+ if ((i + 1) in groupset)
+ printf(" ")
+ }
+ } else {
+ for (i = 1; i in groupset; i++) {
+ printf("%u", groupset[i])
+ if ((i + 1) in groupset)
+ printf(" ")
+ }
+ }
+
+ print "" # final newline
+ exit 0
+ }
+
+ Otherwise, for '-g' (effective group ID only), we check if '-r' was
+also provided, in which case we use the real group ID. Then based on
+'-n', we decide whether to print names or numbers. Here too, when done,
+we exit:
+
+ else if (egid_only) {
+ id = real_ids_only ? gid : egid
+ if (names_not_groups) {
+ entry = getgrgid(id)
+ name = get_first_field(entry)
+ printf("%s\n", name)
+ } else {
+ printf("%u\n", id)
+ }
+
+ exit 0
+ }
+
+ The 'get_first_field()' function extracts the group name from the
+group database entry for the given group ID.
+
+ Similar processing logic applies to '-u' (effective user ID only),
+combined with '-r' and '-n':
+
+ else if (euid_only) {
+ id = real_ids_only ? uid : euid
+ if (names_not_groups) {
+ entry = getpwuid(id)
+ name = get_first_field(entry)
+ printf("%s\n", name)
+ } else {
+ printf("%u\n", id)
+ }
+
+ exit 0
+ }
+
+ At this point, we haven't exited yet, so we print the regular,
+default output, based either on the current user's information, or that
+of the user whose name was provided on the command line. We start with
+the real user ID:
+
+ printf("uid=%d", uid)
+ pw = getpwuid(uid)
+ print_first_field(pw)
+
+ The 'print_first_field()' function prints the user's login name from
+the password file entry, surrounded by parentheses. It is shown soon.
+Printing the effective user ID is next:
+
+ if (euid != uid && ! real_ids_only) {
+ printf(" euid=%d", euid)
+ pw = getpwuid(euid)
+ print_first_field(pw)
+ }
+
+ Similar logic applies to the real and effective group IDs:
+
+ printf(" gid=%d", gid)
+ pw = getgrgid(gid)
+ print_first_field(pw)
+
+ if (egid != gid && ! real_ids_only) {
+ printf(" egid=%d", egid)
+ pw = getgrgid(egid)
+ print_first_field(pw)
+ }
+
+ Finally, we print the group set and the terminating newline:
+
+ for (i = 1; i in groupset; i++) {
+ if (i == 1)
+ printf(" groups=")
+ group = groupset[i]
+ printf("%d", group)
+ pw = getgrgid(group)
+ print_first_field(pw)
+ if ((i + 1) in groupset)
+ printf(",")
+ }
+
+ print ""
+ }
+
+ The 'get_first_field()' function extracts the first field from a
+password or group file entry for use as a user or group name. Fields
+are separated by ':' characters:
+
+ function get_first_field(str, a)
+ {
+ if (str != "") {
+ split(str, a, ":")
+ return a[1]
+ }
+ }
+
+ This function is then used by 'print_first_field()' to output the
+given name surrounded by parentheses:
+
+ function print_first_field(str)
+ {
+ first = get_first_field(str)
+ printf("(%s)", first)
+ }
+
+ These two functions simply isolate out some code that is used
+repeatedly, making the whole program shorter and cleaner. In
+particular, moving the check for the empty string into
+'get_first_field()' saves several lines of code.
+
+ Finally, 'fill_info_for_user()' fetches user, group, and group set
+information for the user named on the command. The code is fairly
+straightforward, merely requiring that we exit if the given user doesn't
+exist:
+
+ function fill_info_for_user(user,
+ pwent, fields, groupnames, grent, groups, i)
+ {
+ pwent = getpwnam(user)
+ if (pwent == "") {
+ printf("id: '%s': no such user\n", user) > "/dev/stderr"
+ exit 1
+ }
+
+ split(pwent, fields, ":")
+ uid = fields[3] + 0
+ gid = fields[4] + 0
+
+ Getting the group set is a little awkward. The library routine
+'getgruser()' returns a list of group _names_. These have to be gone
+through and turned back into group numbers, so that the rest of the code
+will work as expected:
+
+ groupnames = getgruser(user)
+ split(groupnames, groups, " ")
+ for (i = 1; i in groups; i++) {
+ grent = getgrnam(groups[i])
+ split(grent, fields, ":")
+ groupset[i] = fields[3] + 0
+ }
+ }
+
+
+File: gawk.info, Node: Split Program, Next: Tee Program, Prev: Id Program,
Up: Clones
+
+11.2.4 Splitting a Large File into Pieces
+-----------------------------------------
+
+The 'split' utility splits large text files into smaller pieces. The
+usage follows the POSIX standard for 'split' and is as follows:
+
+ 'split' ['-l' COUNT] ['-a' SUFFIX-LEN] [FILE [OUTNAME]]
+ 'split' '-b' N['k'|'m']] ['-a' SUFFIX-LEN] [FILE [OUTNAME]]
+
+ By default, the output files are named 'xaa', 'xab', and so on. Each
+file has 1,000 lines in it, with the likely exception of the last file.
+
+ The 'split' program has evolved over time, and the current POSIX
+version is more complicated than the original Unix version. The options
+and what they do are as follows:
+
+'-a' SUFFIX-LEN
+ Use SUFFIX-LEN characters for the suffix. For example, if
+ SUFFIX-LEN is four, the output files would range from 'xaaaa' to
+ 'xzzzz'.
+
+'-b' N['k'|'m']]
+ Instead of each file containing a specified number of lines, each
+ file should have (at most) N bytes. Supplying a trailing 'k'
+ multiplies N by 1,024, yielding kilobytes. Supplying a trailing
+ 'm' multiplies N by 1,048,576 (1,024 * 1,024) yielding megabytes.
+ (This option is mutually exclusive with '-l').
+
+'-l' COUNT
+ Each file should have at most COUNT lines, instead of the default
+ 1,000. (This option is mutually exclusive with '-b').
+
+ If supplied, FILE is the input file to read. Otherwise standard
+input is processed. If supplied, OUTNAME is the leading prefix to use
+for file names, instead of 'x'.
+
+ In order to use the '-b' option, 'gawk' should be invoked with its
+'-b' option (*note Options::), or with the environment variable 'LC_ALL'
+set to 'C', so that each input byte is treated as a separate
+character.(1)
+
+ Here is an implementation of 'split' in 'awk'. It uses the
+'getopt()' function presented in *note Getopt Function::.
+
+ The program begins with a standard descriptive comment and then a
+'usage()' function describing the options. The variable 'common' keeps
+the function's lines short so that they look nice on the page:
+
+ # split.awk --- do split in awk
+ #
+ # Requires getopt() library function.
+
+ function usage( common)
+ {
+ common = "[-a suffix-len] [file [outname]]"
+ printf("usage: split [-l count] %s\n", common) > "/dev/stderr"
+ printf(" split [-b N[k|m]] %s\n", common) > "/dev/stderr"
+ exit 1
+ }
+
+ Next, in a 'BEGIN' rule we set the default values and parse the
+arguments. After that we initialize the data structures used to cycle
+the suffix from 'aa...' to 'zz...'. Finally we set the name of the
+first output file:
+
+ BEGIN {
+ # Set defaults:
+ Suffix_length = 2
+ Line_count = 1000
+ Byte_count = 0
+ Outfile = "x"
+
+ parse_arguments()
+
+ init_suffix_data()
+
+ Output = (Outfile compute_suffix())
+ }
+
+ Parsing the arguments is straightforward. The program follows our
+convention (*note Library Names::) of having important global variables
+start with an uppercase letter:
+
+ function parse_arguments( i, c, l, modifier)
+ {
+ while ((c = getopt(ARGC, ARGV, "a:b:l:")) != -1) {
+ if (c == "a")
+ Suffix_length = Optarg + 0
+ else if (c == "b") {
+ Byte_count = Optarg + 0
+ Line_count = 0
+
+ l = length(Optarg)
+ modifier = substr(Optarg, l, 1)
+ if (modifier == "k")
+ Byte_count *= 1024
+ else if (modifier == "m")
+ Byte_count *= 1024 * 1024
+ } else if (c == "l") {
+ Line_count = Optarg + 0
+ Byte_count = 0
+ } else
+ usage()
+ }
+
+ # Clear out options
+ for (i = 1; i < Optind; i++)
+ ARGV[i] = ""
+
+ # Check for filename
+ if (ARGV[Optind]) {
+ Optind++
+
+ # Check for different prefix
+ if (ARGV[Optind]) {
+ Outfile = ARGV[Optind]
+ ARGV[Optind] = ""
+
+ if (++Optind < ARGC)
+ usage()
+ }
+ }
+ }
+
+ Managing the file name suffix is interesting. Given a suffix of
+length three, say, the values go from 'aaa', 'aab', 'aac' and so on, all
+the way to 'zzx', 'zzy', and finally 'zzz'. There are two important
+aspects to this:
+
+ * We have to be able to easily generate these suffixes, and in
+ particular easily handle "rolling over"; for example, going from
+ 'abz' to 'aca'.
+
+ * We have to tell when we've finished with the last file, so that if
+ we still have more input data we can print an error message and
+ exit. The trick is to handle this _after_ using the last suffix,
+ and not when the final suffix is created.
+
+ The computation is handled by 'compute_suffix()'. This function is
+called every time a new file is opened.
+
+ The flow here is messy, because we want to generate 'zzzz' (say), and
+use it, and only produce an error after all the file name suffixes have
+been used up. The logical steps are as follows:
+
+ 1. Generate the suffix, saving the value in 'result' to return. To do
+ this, the supplementary array 'Suffix_ind' contains one element for
+ each letter in the suffix. Each element ranges from 1 to 26,
+ acting as the index into a string containing all the lowercase
+ letters of the English alphabet. It is initialized by
+ 'init_suffix_data()'. 'result' is built up one letter at a time,
+ using each 'substr()'.
+
+ 2. Prepare the data structures for the next time 'compute_suffix()' is
+ called. To do this, we loop over 'Suffix_ind', _backwards_. If
+ the current element is less than 26, it's incremented and the loop
+ breaks ('abq' goes to 'abr'). Otherwise, the element is reset to
+ one and we move down the list ('abz' to 'aca'). Thus, the
+ 'Suffix_ind' array is always "one step ahead" of the actual file
+ name suffix to be returned.
+
+ 3. Check if we've gone past the limit of possible file names. If
+ 'Reached_last' is true, print a message and exit. Otherwise, check
+ if 'Suffix_ind' describes a suffix where all the letters are 'z'.
+ If that's the case we're about to return the final suffix. If so,
+ we set 'Reached_last' to true so that the _next_ call to
+ 'compute_suffix()' will cause a failure.
+
+ Physically, the steps in the function occur in the order 3, 1, 2:
+
+ function compute_suffix( i, result, letters)
+ {
+ # Logical step 3
+ if (Reached_last) {
+ printf("split: too many files!\n") > "/dev/stderr"
+ exit 1
+ } else if (on_last_file())
+ Reached_last = 1 # fail when wrapping after 'zzz'
+
+ # Logical step 1
+ result = ""
+ letters = "abcdefghijklmnopqrstuvwxyz"
+ for (i = 1; i <= Suffix_length; i++)
+ result = result substr(letters, Suffix_ind[i], 1)
+
+ # Logical step 2
+ for (i = Suffix_length; i >= 1; i--) {
+ if (++Suffix_ind[i] > 26) {
+ Suffix_ind[i] = 1
+ } else
+ break
+ }
+
+ return result
+ }
+
+ The 'Suffix_ind' array and 'Reached_last' are initialized by
+'init_suffix_data()':
+
+ function init_suffix_data( i)
+ {
+ for (i = 1; i <= Suffix_length; i++)
+ Suffix_ind[i] = 1
+
+ Reached_last = 0
+ }
+
+ The function 'on_last_file()' returns true if 'Suffix_ind' describes
+a suffix where all the letters are 'z' by checking that all the elements
+in the array are equal to 26:
+
+ function on_last_file( i, on_last)
+ {
+ on_last = 1
+ for (i = 1; i <= Suffix_length; i++) {
+ on_last = on_last && (Suffix_ind[i] == 26)
+ }
+
+ return on_last
+ }
+
+ The actual work of splitting the input file is done by the next two
+rules. Since splitting by line count and splitting by byte count are
+mutually exclusive, we simply use two separate rules, one for when
+'Line_count' is greater than zero, and another for when 'Byte_count' is
+greater than zero.
+
+ The variable 'tcount' counts how many lines have been processed so
+far. When it exceeds 'Line_count', it's time to close the previous file
+and switch to a new one:
+
+ Line_count > 0 {
+ if (++tcount > Line_count) {
+ close(Output)
+ Output = (Outfile compute_suffix())
+ tcount = 1
+ }
+ print > Output
+ }
+
+ The rule for handling bytes is more complicated. Since lines most
+likely vary in length, the 'Byte_count' boundary may be hit in the
+middle of an input record. In that case, 'split' has to write enough of
+the first bytes of the input record to finish up 'Byte_count' bytes,
+close the file, open a new file, and write the rest of the record to the
+new file. The logic here does all that:
+
+ Byte_count > 0 {
+ # `+ 1' is for the final newline
+ if (tcount + length($0) + 1 > Byte_count) { # would overflow
+ # compute leading bytes
+ leading_bytes = Byte_count - tcount
+
+ # write leading bytes
+ printf("%s", substr($0, 1, leading_bytes)) > Output
+
+ # close old file, open new file
+ close(Output)
+ Output = (Outfile compute_suffix())
+
+ # set up first bytes for new file
+ $0 = substr($0, leading_bytes + 1) # trailing bytes
+ tcount = 0
+ }
+
+ # write full record or trailing bytes
+ tcount += length($0) + 1
+ print > Output
+ }
+
+ Finally, the 'END' rule cleans up by closing the last output file:
+
+ END {
+ close(Output)
+ }
+
+ ---------- Footnotes ----------
+
+ (1) Using '-b' twice requires separating 'gawk''s options from those
+of the program. For example: 'gawk -f getopt.awk -f split.awk -b -- -b
+42m large-file.txt split-'.
+
+
+File: gawk.info, Node: Tee Program, Next: Uniq Program, Prev: Split
Program, Up: Clones
+
+11.2.5 Duplicating Output into Multiple Files
+---------------------------------------------
+
+The 'tee' program is known as a "pipe fitting." 'tee' copies its
+standard input to its standard output and also duplicates it to the
+files named on the command line. Its usage is as follows:
+
+ 'tee' ['-a'] FILE ...
+
+ The '-a' option tells 'tee' to append to the named files, instead of
+truncating them and starting over.
+
+ The 'BEGIN' rule first makes a copy of all the command-line arguments
+into an array named 'copy'. 'ARGV[0]' is not needed, so it is not
+copied. 'tee' cannot use 'ARGV' directly, because 'awk' attempts to
+process each file name in 'ARGV' as input data.
+
+ If the first argument is '-a', then the flag variable 'append' is set
+to true, and both 'ARGV[1]' and 'copy[1]' are deleted. If 'ARGC' is
+less than two, then no file names were supplied and 'tee' prints a usage
+message and exits. Finally, 'awk' is forced to read the standard input
+by setting 'ARGV[1]' to '"-"' and 'ARGC' to two:
+
+ # tee.awk --- tee in awk
+ #
+ # Copy standard input to all named output files.
+ # Append content if -a option is supplied.
+ #
+ BEGIN {
+ for (i = 1; i < ARGC; i++)
+ copy[i] = ARGV[i]
+
+ if (ARGV[1] == "-a") {
+ append = 1
+ delete ARGV[1]
+ delete copy[1]
+ ARGC--
+ }
+ if (ARGC < 2) {
+ print "usage: tee [-a] file ..." > "/dev/stderr"
+ exit 1
+ }
+ ARGV[1] = "-"
+ ARGC = 2
+ }
+
+ The following single rule does all the work. Because there is no
+pattern, it is executed for each line of input. The body of the rule
+simply prints the line into each file on the command line, and then to
+the standard output:
+
+ {
+ # moving the if outside the loop makes it run faster
+ if (append)
+ for (i in copy)
+ print >> copy[i]
+ else
+ for (i in copy)
+ print > copy[i]
+ print
+ }
+
+It is also possible to write the loop this way:
+
+ for (i in copy)
+ if (append)
+ print >> copy[i]
+ else
+ print > copy[i]
+
+This is more concise, but it is also less efficient. The 'if' is tested
+for each record and for each output file. By duplicating the loop body,
+the 'if' is only tested once for each input record. If there are N
+input records and M output files, the first method only executes N 'if'
+statements, while the second executes N'*'M 'if' statements.
+
+ Finally, the 'END' rule cleans up by closing all the output files:
+
+ END {
+ for (i in copy)
+ close(copy[i])
+ }
+
+
+File: gawk.info, Node: Uniq Program, Next: Wc Program, Prev: Tee Program,
Up: Clones
+
+11.2.6 Printing Nonduplicated Lines of Text
+-------------------------------------------
+
+The 'uniq' utility reads sorted lines of data on its standard input, and
+by default removes duplicate lines. In other words, it only prints
+unique lines--hence the name. 'uniq' has a number of options. The
+usage is as follows:
+
+ 'uniq' ['-udc' ['-f N'] ['-s N']] [INPUTFILE [OUTPUTFILE]]
+
+ The options for 'uniq' are:
+
+'-d'
+ Print only repeated (duplicated) lines.
+
+'-u'
+ Print only nonrepeated (unique) lines.
+
+'-c'
+ Count lines. This option overrides '-d' and '-u'. Both repeated
+ and nonrepeated lines are counted.
+
+'-f N'
+ Skip N fields before comparing lines. The definition of fields is
+ similar to 'awk''s default: nonwhitespace characters separated by
+ runs of spaces and/or TABs.
+
+'-s N'
+ Skip N characters before comparing lines. Any fields specified
+ with '-f' are skipped first.
+
+'INPUTFILE'
+ Data is read from the input file named on the command line, instead
+ of from the standard input.
+
+'OUTPUTFILE'
+ The generated output is sent to the named output file, instead of
+ to the standard output.
+
+ Normally 'uniq' behaves as if both the '-d' and '-u' options are
+provided.
+
+ 'uniq' uses the 'getopt()' library function (*note Getopt Function::)
+and the 'join()' library function (*note Join Function::).
+
+ The program begins with a 'usage()' function and then a brief outline
+of the options and their meanings in comments:
+
+ # uniq.awk --- do uniq in awk
+ #
+ # Requires getopt() and join() library functions
+
+ function usage()
+ {
+ print("Usage: uniq [-udc [-f fields] [-s chars]] " \
+ "[ in [ out ]]") > "/dev/stderr"
+ exit 1
+ }
+
+ # -c count lines. overrides -d and -u
+ # -d only repeated lines
+ # -u only nonrepeated lines
+ # -f n skip n fields
+ # -s n skip n characters, skip fields first
+
+ The POSIX standard for 'uniq' allows options to start with '+' as
+well as with '-'. An initial 'BEGIN' rule traverses the arguments
+changing any leading '+' to '-' so that the 'getopt()' function can
+parse the options:
+
+ # As of 2020, '+' can be used as the option character in addition to '-'
+ # Previously allowed use of -N to skip fields and +N to skip
+ # characters is no longer allowed, and not supported by this version.
+
+ BEGIN {
+ # Convert + to - so getopt can handle things
+ for (i = 1; i < ARGC; i++) {
+ first = substr(ARGV[i], 1, 1)
+ if (ARGV[i] == "--" || (first != "-" && first != "+"))
+ break
+ else if (first == "+")
+ # Replace "+" with "-"
+ ARGV[i] = "-" substr(ARGV[i], 2)
+ }
+ }
+
+ The next 'BEGIN' rule deals with the command-line arguments and
+options. If no options are supplied, then the default is taken, to
+print both repeated and nonrepeated lines. The output file, if
+provided, is assigned to 'outputfile'. Early on, 'outputfile' is
+initialized to the standard output, '/dev/stdout':
+
+ BEGIN {
+ count = 1
+ outputfile = "/dev/stdout"
+ opts = "udcf:s:"
+ while ((c = getopt(ARGC, ARGV, opts)) != -1) {
+ if (c == "u")
+ non_repeated_only++
+ else if (c == "d")
+ repeated_only++
+ else if (c == "c")
+ do_count++
+ else if (c == "f")
+ fcount = Optarg + 0
+ else if (c == "s")
+ charcount = Optarg + 0
+ else
+ usage()
+ }
+
+ for (i = 1; i < Optind; i++)
+ ARGV[i] = ""
+
+ if (repeated_only == 0 && non_repeated_only == 0)
+ repeated_only = non_repeated_only = 1
+
+ if (ARGC - Optind == 2) {
+ outputfile = ARGV[ARGC - 1]
+ ARGV[ARGC - 1] = ""
+ }
+ }
+
+ The following function, 'are_equal()', compares the current line,
+'$0', to the previous line, 'last'. It handles skipping fields and
+characters. If no field count and no character count are specified,
+'are_equal()' returns one or zero depending upon the result of a simple
+string comparison of 'last' and '$0'.
+
+ Otherwise, things get more complicated. If fields have to be
+skipped, each line is broken into an array using 'split()' (*note String
+Functions::); the desired fields are then joined back into a line using
+'join()'. The joined lines are stored in 'clast' and 'cline'. If no
+fields are skipped, 'clast' and 'cline' are set to 'last' and '$0',
+respectively. Finally, if characters are skipped, 'substr()' is used to
+strip off the leading 'charcount' characters in 'clast' and 'cline'.
+The two strings are then compared and 'are_equal()' returns the result:
+
+ function are_equal( n, m, clast, cline, alast, aline)
+ {
+ if (fcount == 0 && charcount == 0)
+ return (last == $0)
+
+ if (fcount > 0) {
+ n = split(last, alast)
+ m = split($0, aline)
+ clast = join(alast, fcount+1, n)
+ cline = join(aline, fcount+1, m)
+ } else {
+ clast = last
+ cline = $0
+ }
+ if (charcount) {
+ clast = substr(clast, charcount + 1)
+ cline = substr(cline, charcount + 1)
+ }
+
+ return (clast == cline)
+ }
+
+ The following two rules are the body of the program. The first one
+is executed only for the very first line of data. It sets 'last' equal
+to '$0', so that subsequent lines of text have something to be compared
+to.
+
+ The second rule does the work. The variable 'equal' is one or zero,
+depending upon the results of 'are_equal()''s comparison. If 'uniq' is
+counting repeated lines, and the lines are equal, then it increments the
+'count' variable. Otherwise, it prints the line and resets 'count',
+because the two lines are not equal.
+
+ If 'uniq' is not counting, and if the lines are equal, 'count' is
+incremented. Nothing is printed, as the point is to remove duplicates.
+Otherwise, if 'uniq' is counting repeated lines and more than one line
+is seen, or if 'uniq' is counting nonrepeated lines and only one line is
+seen, then the line is printed, and 'count' is reset.
+
+ Finally, similar logic is used in the 'END' rule to print the final
+line of input data:
+
+ NR == 1 {
+ last = $0
+ next
+ }
+
+ {
+ equal = are_equal()
+
+ if (do_count) { # overrides -d and -u
+ if (equal)
+ count++
+ else {
+ printf("%4d %s\n", count, last) > outputfile
+ last = $0
+ count = 1 # reset
+ }
+ next
+ }
+
+ if (equal)
+ count++
+ else {
+ if ((repeated_only && count > 1) ||
+ (non_repeated_only && count == 1))
+ print last > outputfile
+ last = $0
+ count = 1
+ }
+ }
+
+ END {
+ if (do_count)
+ printf("%4d %s\n", count, last) > outputfile
+ else if ((repeated_only && count > 1) ||
+ (non_repeated_only && count == 1))
+ print last > outputfile
+ close(outputfile)
+ }
+
+ As a side note, this program does not follow our recommended
+convention of naming global variables with a leading capital letter.
+Doing that would make the program a little easier to follow.
+
+
+File: gawk.info, Node: Wc Program, Prev: Uniq Program, Up: Clones
+
+11.2.7 Counting Things
+----------------------
+
+The 'wc' (word count) utility counts lines, words, characters and bytes
+in one or more input files.
+
+* Menu:
+
+* Bytes vs. Characters:: Modern character sets.
+* Using extensions:: A brief intro to extensions.
+* wc program:: Code for 'wc.awk'.
+
+
+File: gawk.info, Node: Bytes vs. Characters, Next: Using extensions, Up: Wc
Program
+
+11.2.7.1 Modern Character Sets
+..............................
+
+In the early days of computing, single bytes were used for storing
+characters. The most common character sets were ASCII and EBCDIC, which
+each provided all the English upper- and lowercase letters, the 10
+Hindu-Arabic numerals from 0 through 9, and a number of other standard
+punctuation and control characters.
+
+ Today, the most popular character set in use is Unicode (of which
+ASCII is a pure subset). Unicode provides tens of thousands of unique
+characters (called "code points") to cover most existing human languages
+(living and dead) and a number of nonhuman ones as well (such as Klingon
+and J.R.R. Tolkien's elvish languages).
+
+ To save space in files, Unicode code points are "encoded", where each
+character takes from one to four bytes in the file. UTF-8 is possibly
+the most popular of such "multibyte encodings".
+
+ The POSIX standard requires that 'awk' function in terms of
+characters, not bytes. Thus in 'gawk', 'length()', 'substr()',
+'split()', 'match()' and the other string functions (*note String
+Functions::) all work in terms of characters in the local character set,
+and not in terms of bytes. (Not all 'awk' implementations do so,
+though).
+
+ There is no standard, built-in way to distinguish characters from
+bytes in an 'awk' program. For an 'awk' implementation of 'wc', which
+needs to make such a distinction, we will have to use an external
+extension.
+
+
+File: gawk.info, Node: Using extensions, Next: wc program, Prev: Bytes vs.
Characters, Up: Wc Program
+
+11.2.7.2 A Brief Introduction To Extensions
+...........................................
+
+Loadable extensions are presented in full detail in *note Dynamic
+Extensions::. They provide a way to add functions to 'gawk' which can
+call out to other facilities written in C or C++.
+
+ For the purposes of 'wc.awk', it's enough to know that the extension
+is loaded with the '@load' directive, and the additional function we
+will use is called 'mbs_length()'. This function returns the number of
+bytes in a string, not the number of characters.
+
+ The '"mbs"' extension comes from the 'gawkextlib' project. *Note
+gawkextlib:: for more information.
+
+
+File: gawk.info, Node: wc program, Prev: Using extensions, Up: Wc Program
+
+11.2.7.3 Code for 'wc.awk'
+..........................
+
+The usage for 'wc' is as follows:
+
+ 'wc' ['-lwcm'] [FILES ...]
+
+ If no files are specified on the command line, 'wc' reads its
+standard input. If there are multiple files, it also prints total
+counts for all the files. The options and their meanings are as
+follows:
+
+'-c'
+ Count only bytes. Once upon a time, the 'c' in this option stood
+ for "characters." But, as explained earlier, bytes and character
+ are no longer synonymous with each other.
+
+'-l'
+ Count only lines.
+
+'-m'
+ Count only characters.
+
+'-w'
+ Count only words. A "word" is a contiguous sequence of
+ nonwhitespace characters, separated by spaces and/or TABs.
+ Luckily, this is the normal way 'awk' separates fields in its input
+ data.
+
+ Implementing 'wc' in 'awk' is particularly elegant, because 'awk'
+does a lot of the work for us; it splits lines into words (i.e., fields)
+and counts them, it counts lines (i.e., records), and it can easily tell
+us how long a line is in characters.
+
+ This program uses the 'getopt()' library function (*note Getopt
+Function::) and the file-transition functions (*note Filetrans
+Function::).
+
+ This version has one notable difference from older versions of 'wc':
+it always prints the counts in the order lines, words, characters and
+bytes. Older versions note the order of the '-l', '-w', and '-c'
+options on the command line, and print the counts in that order. POSIX
+does not mandate this behavior, though.
+
+ The 'BEGIN' rule does the argument processing. The variable
+'print_total' is true if more than one file is named on the command
+line:
+
+ # wc.awk --- count lines, words, characters, bytes
+
+ # Options:
+ # -l only count lines
+ # -w only count words
+ # -c only count bytes
+ # -m only count characters
+ #
+ # Default is to count lines, words, bytes
+ #
+ # Requires getopt() and file transition library functions
+ # Requires mbs extension from gawkextlib
+
+ @load "mbs"
+
+ BEGIN {
+ # let getopt() print a message about
+ # invalid options. we ignore them
+ while ((c = getopt(ARGC, ARGV, "lwcm")) != -1) {
+ if (c == "l")
+ do_lines = 1
+ else if (c == "w")
+ do_words = 1
+ else if (c == "c")
+ do_bytes = 1
+ else if (c == "m")
+ do_chars = 1
+ }
+ for (i = 1; i < Optind; i++)
+ ARGV[i] = ""
+
+ # if no options, do lines, words, bytes
+ if (! do_lines && ! do_words && ! do_chars && ! do_bytes)
+ do_lines = do_words = do_bytes = 1
+
+ print_total = (ARGC - i > 1)
+ }
+
+ The 'beginfile()' function is simple; it just resets the counts of
+lines, words, characters and bytes to zero, and saves the current file
+name in 'fname':
+
+ function beginfile(file)
+ {
+ lines = words = chars = bytes = 0
+ fname = FILENAME
+ }
+
+ The 'endfile()' function adds the current file's numbers to the
+running totals of lines, words, and characters. It then prints out
+those numbers for the file that was just read. It relies on
+'beginfile()' to reset the numbers for the following data file:
+
+ function endfile(file)
+ {
+ tlines += lines
+ twords += words
+ tchars += chars
+ tbytes += bytes
+ if (do_lines)
+ printf "\t%d", lines
+ if (do_words)
+ printf "\t%d", words
+ if (do_chars)
+ printf "\t%d", chars
+ if (do_bytes)
+ printf "\t%d", bytes
+ printf "\t%s\n", fname
+ }
+
+ There is one rule that is executed for each line. It adds the length
+of the record, plus one, to 'chars'. Adding one plus the record length
+is needed because the newline character separating records (the value of
+'RS') is not part of the record itself, and thus not included in its
+length. Similarly, it adds the length of the record in bytes, plus one,
+to 'bytes'. Next, 'lines' is incremented for each line read, and
+'words' is incremented by the value of 'NF', which is the number of
+"words" on this line:
+
+ # do per line
+ {
+ chars += length($0) + 1 # get newline
+ bytes += mbs_length($0) + 1
+ lines++
+ words += NF
+ }
+
+ Finally, the 'END' rule simply prints the totals for all the files:
+
+ END {
+ if (print_total) {
+ if (do_lines)
+ printf "\t%d", tlines
+ if (do_words)
+ printf "\t%d", twords
+ if (do_chars)
+ printf "\t%d", tchars
+ if (do_bytes)
+ printf "\t%d", tbytes
+ print "\ttotal"
+ }
+ }
+
+
+File: gawk.info, Node: Miscellaneous Programs, Next: Programs Summary,
Prev: Clones, Up: Sample Programs
+
+11.3 A Grab Bag of 'awk' Programs
+=================================
+
+This minor node is a large "grab bag" of miscellaneous programs. We
+hope you find them both interesting and enjoyable.
+
+* Menu:
+
+* Dupword Program:: Finding duplicated words in a document.
+* Alarm Program:: An alarm clock.
+* Translate Program:: A program similar to the 'tr' utility.
+* Labels Program:: Printing mailing labels.
+* Word Sorting:: A program to produce a word usage count.
+* History Sorting:: Eliminating duplicate entries from a history
+ file.
+* Extract Program:: Pulling out programs from Texinfo source
+ files.
+* Simple Sed:: A Simple Stream Editor.
+* Igawk Program:: A wrapper for 'awk' that includes
+ files.
+* Anagram Program:: Finding anagrams from a dictionary.
+* Signature Program:: People do amazing things with too much time on
+ their hands.
+
+
+File: gawk.info, Node: Dupword Program, Next: Alarm Program, Up:
Miscellaneous Programs
+
+11.3.1 Finding Duplicated Words in a Document
+---------------------------------------------
+
+A common error when writing large amounts of prose is to accidentally
+duplicate words. Typically you will see this in text as something like
+"the the program does the following..." When the text is online, often
+the duplicated words occur at the end of one line and the beginning of
+another, making them very difficult to spot.
+
+ This program, 'dupword.awk', scans through a file one line at a time
+and looks for adjacent occurrences of the same word. It also saves the
+last word on a line (in the variable 'prev') for comparison with the
+first word on the next line.
+
+ The first two statements make sure that the line is all lowercase, so
+that, for example, "The" and "the" compare equal to each other. The
+next statement replaces nonalphanumeric and nonwhitespace characters
+with spaces, so that punctuation does not affect the comparison either.
+The characters are replaced with spaces so that formatting controls
+don't create nonsense words (e.g., the Texinfo '@code{NF}' becomes
+'codeNF' if punctuation is simply deleted). The record is then resplit
+into fields, yielding just the actual words on the line, and ensuring
+that there are no empty fields.
+
+ If there are no fields left after removing all the punctuation, the
+current record is skipped. Otherwise, the program loops through each
+word, comparing it to the previous one:
+
+ # dupword.awk --- find duplicate words in text
+ {
+ $0 = tolower($0)
+ gsub(/[^[:alnum:][:blank:]]/, " ");
+ $0 = $0 # re-split
+ if (NF == 0)
+ next
+ if ($1 == prev)
+ printf("%s:%d: duplicate %s\n",
+ FILENAME, FNR, $1)
+ for (i = 2; i <= NF; i++)
+ if ($i == $(i-1))
+ printf("%s:%d: duplicate %s\n",
+ FILENAME, FNR, $i)
+ prev = $NF
+ }
+
+
+File: gawk.info, Node: Alarm Program, Next: Translate Program, Prev:
Dupword Program, Up: Miscellaneous Programs
+
+11.3.2 An Alarm Clock Program
+-----------------------------
+
+ Nothing cures insomnia like a ringing alarm clock.
+ -- _Arnold Robbins_
+ Sleep is for web developers.
+ -- _Erik Quanstrom_
+
+ The following program is a simple "alarm clock" program. You give it
+a time of day and an optional message. At the specified time, it prints
+the message on the standard output. In addition, you can give it the
+number of times to repeat the message as well as a delay between
+repetitions.
+
+ This program uses the 'getlocaltime()' function from *note
+Getlocaltime Function::.
+
+ All the work is done in the 'BEGIN' rule. The first part is argument
+checking and setting of defaults: the delay, the count, and the message
+to print. If the user supplied a message without the ASCII BEL
+character (known as the "alert" character, '"\a"'), then it is added to
+the message. (On many systems, printing the ASCII BEL generates an
+audible alert. Thus, when the alarm goes off, the system calls
+attention to itself in case the user is not looking at the computer.)
+Just for a change, this program uses a 'switch' statement (*note Switch
+Statement::), but the processing could be done with a series of
+'if'-'else' statements instead. Here is the program:
+
+ # alarm.awk --- set an alarm
+ #
+ # Requires getlocaltime() library function
+ # usage: alarm time [ "message" [ count [ delay ] ] ]
+
+ BEGIN {
+ # Initial argument sanity checking
+ usage1 = "usage: alarm time ['message' [count [delay]]]"
+ usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])
+
+ if (ARGC < 2) {
+ print usage1 > "/dev/stderr"
+ print usage2 > "/dev/stderr"
+ exit 1
+ }
+ switch (ARGC) {
+ case 5:
+ delay = ARGV[4] + 0
+ # fall through
+ case 4:
+ count = ARGV[3] + 0
+ # fall through
+ case 3:
+ message = ARGV[2]
+ break
+ default:
+ if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]{2}/) {
+ print usage1 > "/dev/stderr"
+ print usage2 > "/dev/stderr"
+ exit 1
+ }
+ break
+ }
+
+ # set defaults for once we reach the desired time
+ if (delay == 0)
+ delay = 180 # 3 minutes
+ if (count == 0)
+ count = 5
+ if (message == "")
+ message = sprintf("\aIt is now %s!\a", ARGV[1])
+ else if (index(message, "\a") == 0)
+ message = "\a" message "\a"
+
+ The next minor node of code turns the alarm time into hours and
+minutes, converts it (if necessary) to a 24-hour clock, and then turns
+that time into a count of the seconds since midnight. Next it turns the
+current time into a count of seconds since midnight. The difference
+between the two is how long to wait before setting off the alarm:
+
+ # split up alarm time
+ split(ARGV[1], atime, ":")
+ hour = atime[1] + 0 # force numeric
+ minute = atime[2] + 0 # force numeric
+
+ # get current broken down time
+ getlocaltime(now)
+
+ # if time given is 12-hour hours and it's after that
+ # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
+ # then add 12 to real hour
+ if (hour < 12 && now["hour"] > hour)
+ hour += 12
+
+ # set target time in seconds since midnight
+ target = (hour * 60 * 60) + (minute * 60)
+
+ # get current time in seconds since midnight
+ current = (now["hour"] * 60 * 60) + \
+ (now["minute"] * 60) + now["second"]
+
+ # how long to sleep for
+ naptime = target - current
+ if (naptime <= 0) {
+ print "alarm: time is in the past!" > "/dev/stderr"
+ exit 1
+ }
+
+ Finally, the program uses the 'system()' function (*note I/O
+Functions::) to call the 'sleep' utility. The 'sleep' utility simply
+pauses for the given number of seconds. If the exit status is not zero,
+the program assumes that 'sleep' was interrupted and exits. If 'sleep'
+exited with an OK status (zero), then the program prints the message in
+a loop, again using 'sleep' to delay for however many seconds are
+necessary:
+
+ # zzzzzz..... go away if interrupted
+ if (system(sprintf("sleep %d", naptime)) != 0)
+ exit 1
+
+ # time to notify!
+ command = sprintf("sleep %d", delay)
+ for (i = 1; i <= count; i++) {
+ print message
+ # if sleep command interrupted, go away
+ if (system(command) != 0)
+ break
+ }
+
+ exit 0
+ }
+
+
+File: gawk.info, Node: Translate Program, Next: Labels Program, Prev: Alarm
Program, Up: Miscellaneous Programs
+
+11.3.3 Transliterating Characters
+---------------------------------
+
+The system 'tr' utility transliterates characters. For example, it is
+often used to map uppercase letters into lowercase for further
+processing:
+
+ GENERATE DATA | tr 'A-Z' 'a-z' | PROCESS DATA ...
+
+ 'tr' requires two lists of characters.(1) When processing the input,
+the first character in the first list is replaced with the first
+character in the second list, the second character in the first list is
+replaced with the second character in the second list, and so on. If
+there are more characters in the "from" list than in the "to" list, the
+last character of the "to" list is used for the remaining characters in
+the "from" list.
+
+ Once upon a time, a user proposed adding a transliteration function
+to 'gawk'. The following program was written to prove that character
+transliteration could be done with a user-level function. This program
+is not as complete as the system 'tr' utility, but it does most of the
+job.
+
+ The 'translate' program was written long before 'gawk' acquired the
+ability to split each character in a string into separate array
+elements. Thus, it makes repeated use of the 'substr()', 'index()', and
+'gsub()' built-in functions (*note String Functions::). There are two
+functions. The first, 'stranslate()', takes three arguments:
+
+'from'
+ A list of characters from which to translate
+
+'to'
+ A list of characters to which to translate
+
+'target'
+ The string on which to do the translation
+
+ Associative arrays make the translation part fairly easy. 't_ar'
+holds the "to" characters, indexed by the "from" characters. Then a
+simple loop goes through 'from', one character at a time. For each
+character in 'from', if the character appears in 'target', it is
+replaced with the corresponding 'to' character.
+
+ The 'translate()' function calls 'stranslate()', using '$0' as the
+target. The main program sets two global variables, 'FROM' and 'TO',
+from the command line, and then changes 'ARGV' so that 'awk' reads from
+the standard input.
+
+ Finally, the processing rule simply calls 'translate()' for each
+record:
+
+ # translate.awk --- do tr-like stuff
+ # Bugs: does not handle things like tr A-Z a-z; it has
+ # to be spelled out. However, if `to' is shorter than `from',
+ # the last character in `to' is used for the rest of `from'.
+
+ function stranslate(from, to, target, lf, lt, ltarget, t_ar, i, c,
+ result)
+ {
+ lf = length(from)
+ lt = length(to)
+ ltarget = length(target)
+ for (i = 1; i <= lt; i++)
+ t_ar[substr(from, i, 1)] = substr(to, i, 1)
+ if (lt < lf)
+ for (; i <= lf; i++)
+ t_ar[substr(from, i, 1)] = substr(to, lt, 1)
+ for (i = 1; i <= ltarget; i++) {
+ c = substr(target, i, 1)
+ if (c in t_ar)
+ c = t_ar[c]
+ result = result c
+ }
+ return result
+ }
+
+ function translate(from, to)
+ {
+ return $0 = stranslate(from, to, $0)
+ }
+
+ # main program
+ BEGIN {
+ if (ARGC < 3) {
+ print "usage: translate from to" > "/dev/stderr"
+ exit
+ }
+ FROM = ARGV[1]
+ TO = ARGV[2]
+ ARGC = 2
+ ARGV[1] = "-"
+ }
+
+ {
+ translate(FROM, TO)
+ print
+ }
+
+ It is possible to do character transliteration in a user-level
+function, but it is not necessarily efficient, and we (the 'gawk'
+developers) started to consider adding a built-in function. However,
+shortly after writing this program, we learned that Brian Kernighan had
+added the 'toupper()' and 'tolower()' functions to his 'awk' (*note
+String Functions::). These functions handle the vast majority of the
+cases where character transliteration is necessary, and so we chose to
+simply add those functions to 'gawk' as well and then leave well enough
+alone.
+
+ An obvious improvement to this program would be to set up the 't_ar'
+array only once, in a 'BEGIN' rule. However, this assumes that the
+"from" and "to" lists will never change throughout the lifetime of the
+program.
+
+ Another obvious improvement is to enable the use of ranges, such as
+'a-z', as allowed by the 'tr' utility. Look at the code for 'cut.awk'
+(*note Cut Program::) for inspiration.
+
+ ---------- Footnotes ----------
+
+ (1) On some older systems, including Solaris, the system version of
+'tr' may require that the lists be written as range expressions enclosed
+in square brackets ('[a-z]') and quoted, to prevent the shell from
+attempting a file name expansion. This is not a feature.
+
+
+File: gawk.info, Node: Labels Program, Next: Word Sorting, Prev: Translate
Program, Up: Miscellaneous Programs
+
+11.3.4 Printing Mailing Labels
+------------------------------
+
+Here is a "real-world"(1) program. This script reads lists of names and
+addresses and generates mailing labels. Each page of labels has 20
+labels on it, two across and 10 down. The addresses are guaranteed to
+be no more than five lines of data. Each address is separated from the
+next by a blank line.
+
+ The basic idea is to read 20 labels' worth of data. Each line of
+each label is stored in the 'line' array. The single rule takes care of
+filling the 'line' array and printing the page when 20 labels have been
+read.
+
+ The 'BEGIN' rule simply sets 'RS' to the empty string, so that 'awk'
+splits records at blank lines (*note Records::). It sets 'MAXLINES' to
+100, because 100 is the maximum number of lines on the page (20 * 5 =
+100).
+
+ Most of the work is done in the 'printpage()' function. The label
+lines are stored sequentially in the 'line' array. But they have to
+print horizontally: 'line[1]' next to 'line[6]', 'line[2]' next to
+'line[7]', and so on. Two loops accomplish this. The outer loop,
+controlled by 'i', steps through every 10 lines of data; this is each
+row of labels. The inner loop, controlled by 'j', goes through the
+lines within the row. As 'j' goes from 0 to 4, 'i+j' is the 'j'th line
+in the row, and 'i+j+5' is the entry next to it. The output ends up
+looking something like this:
+
+ line 1 line 6
+ line 2 line 7
+ line 3 line 8
+ line 4 line 9
+ line 5 line 10
+ ...
+
+The 'printf' format string '%-41s' left-aligns the data and prints it
+within a fixed-width field.
+
+ As a final note, an extra blank line is printed at lines 21 and 61,
+to keep the output lined up on the labels. This is dependent on the
+particular brand of labels in use when the program was written. You
+will also note that there are two blank lines at the top and two blank
+lines at the bottom.
+
+ The 'END' rule arranges to flush the final page of labels; there may
+not have been an even multiple of 20 labels in the data:
+
+ # labels.awk --- print mailing labels
+
+ # Each label is 5 lines of data that may have blank lines.
+ # The label sheets have 2 blank lines at the top and 2 at
+ # the bottom.
+
+ BEGIN { RS = "" ; MAXLINES = 100 }
+
+ function printpage( i, j)
+ {
+ if (Nlines <= 0)
+ return
+
+ printf "\n\n" # header
+
+ for (i = 1; i <= Nlines; i += 10) {
+ if (i == 21 || i == 61)
+ print ""
+ for (j = 0; j < 5; j++) {
+ if (i + j > MAXLINES)
+ break
+ printf " %-41s %s\n", line[i+j], line[i+j+5]
+ }
+ print ""
+ }
+
+ printf "\n\n" # footer
+
+ delete line
+ }
+
+ # main rule
+ {
+ if (Count >= 20) {
+ printpage()
+ Count = 0
+ Nlines = 0
+ }
+ n = split($0, a, "\n")
+ for (i = 1; i <= n; i++)
+ line[++Nlines] = a[i]
+ for (; i <= 5; i++)
+ line[++Nlines] = ""
+ Count++
+ }
+
+ END {
+ printpage()
+ }
+
+ ---------- Footnotes ----------
+
+ (1) "Real world" is defined as "a program actually used to get
+something done."
+
+
+File: gawk.info, Node: Word Sorting, Next: History Sorting, Prev: Labels
Program, Up: Miscellaneous Programs
+
+11.3.5 Generating Word-Usage Counts
+-----------------------------------
+
+When working with large amounts of text, it can be interesting to know
+how often different words appear. For example, an author may overuse
+certain words, in which case he or she might wish to find synonyms to
+substitute for words that appear too often. This node develops a
+program for counting words and presenting the frequency information in a
+useful format.
+
+ At first glance, a program like this would seem to do the job:
+
+ # wordfreq-first-try.awk --- print list of word frequencies
+
+ {
+ for (i = 1; i <= NF; i++)
+ freq[$i]++
+ }
+
+ END {
+ for (word in freq)
+ printf "%s\t%d\n", word, freq[word]
+ }
+
+ The program relies on 'awk''s default field-splitting mechanism to
+break each line up into "words" and uses an associative array named
+'freq', indexed by each word, to count the number of times the word
+occurs. In the 'END' rule, it prints the counts.
+
+ This program has several problems that prevent it from being useful
+on real text files:
+
+ * The 'awk' language considers upper- and lowercase characters to be
+ distinct. Therefore, "bartender" and "Bartender" are not treated
+ as the same word. This is undesirable, because words are
+ capitalized if they begin sentences in normal text, and a frequency
+ analyzer should not be sensitive to capitalization.
+
+ * Words are detected using the 'awk' convention that fields are
+ separated just by whitespace. Other characters in the input
+ (except newlines) don't have any special meaning to 'awk'. This
+ means that punctuation characters count as part of words.
+
+ * The output does not come out in any useful order. You're more
+ likely to be interested in which words occur most frequently or in
+ having an alphabetized table of how frequently each word occurs.
+
+ The first problem can be solved by using 'tolower()' to remove case
+distinctions. The second problem can be solved by using 'gsub()' to
+remove punctuation characters. Finally, we solve the third problem by
+using the system 'sort' utility to process the output of the 'awk'
+script. Here is the new version of the program:
+
+ # wordfreq.awk --- print list of word frequencies
+
+ {
+ $0 = tolower($0) # remove case distinctions
+ # remove punctuation
+ gsub(/[^[:alnum:]_[:blank:]]/, "", $0)
+ for (i = 1; i <= NF; i++)
+ freq[$i]++
+ }
+
+ END {
+ for (word in freq)
+ printf "%s\t%d\n", word, freq[word]
+ }
+
+ The regexp '/[^[:alnum:]_[:blank:]]/' might have been written
+'/[[:punct:]]/', but then underscores would also be removed, and we want
+to keep them.
+
+ Assuming we have saved this program in a file named 'wordfreq.awk',
+and that the data is in 'file1', the following pipeline:
+
+ awk -f wordfreq.awk file1 | sort -k 2nr
+
+produces a table of the words appearing in 'file1' in order of
+decreasing frequency.
+
+ The 'awk' program suitably massages the data and produces a word
+frequency table, which is not ordered. The 'awk' script's output is
+then sorted by the 'sort' utility and printed on the screen.
+
+ The options given to 'sort' specify a sort that uses the second field
+of each input line (skipping one field), that the sort keys should be
+treated as numeric quantities (otherwise '15' would come before '5'),
+and that the sorting should be done in descending (reverse) order.
+
+ The 'sort' could even be done from within the program, by changing
+the 'END' action to:
+
+ END {
+ sort = "sort -k 2nr"
+ for (word in freq)
+ printf "%s\t%d\n", word, freq[word] | sort
+ close(sort)
+ }
+
+ This way of sorting must be used on systems that do not have true
+pipes at the command-line (or batch-file) level. See the general
+operating system documentation for more information on how to use the
+'sort' program.
+
+
+File: gawk.info, Node: History Sorting, Next: Extract Program, Prev: Word
Sorting, Up: Miscellaneous Programs
+
+11.3.6 Removing Duplicates from Unsorted Text
+---------------------------------------------
+
+The 'uniq' program (*note Uniq Program::) removes duplicate lines from
+_sorted_ data.
+
+ Suppose, however, you need to remove duplicate lines from a data file
+but that you want to preserve the order the lines are in. A good
+example of this might be a shell history file. The history file keeps a
+copy of all the commands you have entered, and it is not unusual to
+repeat a command several times in a row. Occasionally you might want to
+compact the history by removing duplicate entries. Yet it is desirable
+to maintain the order of the original commands.
+
+ This simple program does the job. It uses two arrays. The 'data'
+array is indexed by the text of each line. For each line, 'data[$0]' is
+incremented. If a particular line has not been seen before, then
+'data[$0]' is zero. In this case, the text of the line is stored in
+'lines[count]'. Each element of 'lines' is a unique command, and the
+indices of 'lines' indicate the order in which those lines are
+encountered. The 'END' rule simply prints out the lines, in order:
+
+ # histsort.awk --- compact a shell history file
+ # Thanks to Byron Rakitzis for the general idea
+
+ {
+ if (data[$0]++ == 0)
+ lines[++count] = $0
+ }
+
+ END {
+ for (i = 1; i <= count; i++)
+ print lines[i]
+ }
+
+ This program also provides a foundation for generating other useful
+information. For example, using the following 'print' statement in the
+'END' rule indicates how often a particular command is used:
+
+ print data[lines[i]], lines[i]
+
+This works because 'data[$0]' is incremented each time a line is seen.
+
+ Rick van Rein offers the following one-liner to do the same job of
+removing duplicates from unsorted text:
+
+ awk '{ if (! seen[$0]++) print }'
+
+ This can be simplified even further, at the risk of becoming almost
+too obscure:
+
+ awk '! seen[$0]++'
+
+This version uses the expression as a pattern, relying on 'awk''s
+default action of printing the line when the pattern is true.
+
+
+File: gawk.info, Node: Extract Program, Next: Simple Sed, Prev: History
Sorting, Up: Miscellaneous Programs
+
+11.3.7 Extracting Programs from Texinfo Source Files
+----------------------------------------------------
+
+The nodes *note Library Functions::, and *note Sample Programs::, are
+the top level nodes for a large number of 'awk' programs. If you want
+to experiment with these programs, it is tedious to type them in by
+hand. Here we present a program that can extract parts of a Texinfo
+input file into separate files.
+
+ This Info file is written in Texinfo
+(https://www.gnu.org/software/texinfo/), the GNU Project's document
+formatting language. A single Texinfo source file can be used to
+produce both printed documentation, with TeX, and online documentation.
+(The Texinfo language is described fully, starting with *note Texinfo:
+(texinfo)Top.)
+
+ For our purposes, it is enough to know three things about Texinfo
+input files:
+
+ * The "at" symbol ('@') is special in Texinfo, much as the backslash
+ ('\') is in C or 'awk'. Literal '@' symbols are represented in
+ Texinfo source files as '@@'.
+
+ * Comments start with either '@c' or '@comment'. The file-extraction
+ program works by using special comments that start at the beginning
+ of a line.
+
+ * Lines containing '@group' and '@end group' commands bracket example
+ text that should not be split across a page boundary.
+ (Unfortunately, TeX isn't always smart enough to do things exactly
+ right, so we have to give it some help.)
+
+ The following program, 'extract.awk', reads through a Texinfo source
+file and does two things, based on the special comments. Upon seeing
+'@c system ...', it runs a command, by extracting the command text from
+the control line and passing it on to the 'system()' function (*note I/O
+Functions::). Upon seeing '@c file FILENAME', each subsequent line is
+sent to the file FILENAME, until '@c endfile' is encountered. The rules
+in 'extract.awk' match either '@c' or '@comment' by letting the 'omment'
+part be optional. Lines containing '@group' and '@end group' are simply
+removed. 'extract.awk' uses the 'join()' library function (*note Join
+Function::).
+
+ The example programs in the online Texinfo source for 'GAWK:
+Effective AWK Programming' ('gawktexi.in') have all been bracketed
+inside 'file' and 'endfile' lines. The 'gawk' distribution uses a copy
+of 'extract.awk' to extract the sample programs and install many of them
+in a standard directory where 'gawk' can find them. The Texinfo file
+looks something like this:
+
+ ...
+ This program has a @code{BEGIN} rule
+ that prints a nice message:
+
+ @example
+ @c file examples/messages.awk
+ BEGIN @{ print "Don't panic!" @}
+ @c endfile
+ @end example
+
+ It also prints some final advice:
+
+ @example
+ @c file examples/messages.awk
+ END @{ print "Always avoid bored archaeologists!" @}
+ @c endfile
+ @end example
+ ...
+
+ 'extract.awk' begins by setting 'IGNORECASE' to one, so that mixed
+upper- and lowercase letters in the directives won't matter.
+
+ The first rule handles calling 'system()', checking that a command is
+given ('NF' is at least three) and also checking that the command exits
+with a zero exit status, signifying OK:
+
+ # extract.awk --- extract files and run programs from Texinfo files
+
+ BEGIN { IGNORECASE = 1 }
+
+ /^@c(omment)?[ \t]+system/ {
+ if (NF < 3) {
+ e = ("extract: " FILENAME ":" FNR)
+ e = (e ": badly formed `system' line")
+ print e > "/dev/stderr"
+ next
+ }
+ $1 = ""
+ $2 = ""
+ stat = system($0)
+ if (stat != 0) {
+ e = ("extract: " FILENAME ":" FNR)
+ e = (e ": warning: system returned " stat)
+ print e > "/dev/stderr"
+ }
+ }
+
+The variable 'e' is used so that the rule fits nicely on the screen.
+
+ The second rule handles moving data into files. It verifies that a
+file name is given in the directive. If the file named is not the
+current file, then the current file is closed. Keeping the current file
+open until a new file is encountered allows the use of the '>'
+redirection for printing the contents, keeping open-file management
+simple.
+
+ The 'for' loop does the work. It reads lines using 'getline' (*note
+Getline::). For an unexpected end-of-file, it calls the
+'unexpected_eof()' function. If the line is an "endfile" line, then it
+breaks out of the loop. If the line is an '@group' or '@end group'
+line, then it ignores it and goes on to the next line. Similarly,
+comments within examples are also ignored.
+
+ Most of the work is in the following few lines. If the line has no
+'@' symbols, the program can print it directly. Otherwise, each leading
+'@' must be stripped off. To remove the '@' symbols, the line is split
+into separate elements of the array 'a', using the 'split()' function
+(*note String Functions::). The '@' symbol is used as the separator
+character. Each element of 'a' that is empty indicates two successive
+'@' symbols in the original line. For each two empty elements ('@@' in
+the original file), we have to add a single '@' symbol back in.
+
+ When the processing of the array is finished, 'join()' is called with
+the value of 'SUBSEP' (*note Multidimensional::), to rejoin the pieces
+back into a single line. That line is then printed to the output file:
+
+ /^@c(omment)?[ \t]+file/ {
+ if (NF != 3) {
+ e = ("extract: " FILENAME ":" FNR ": badly formed `file' line")
+ print e > "/dev/stderr"
+ next
+ }
+ if ($3 != curfile) {
+ if (curfile != "")
+ filelist[curfile] = 1 # save to close later
+ curfile = $3
+ }
+
+ for (;;) {
+ if ((getline line) <= 0)
+ unexpected_eof()
+ if (line ~ /^@c(omment)?[ \t]+endfile/)
+ break
+ else if (line ~ /^@(end[ \t]+)?group/)
+ continue
+ else if (line ~ /^@c(omment+)?[ \t]+/)
+ continue
+ if (index(line, "@") == 0) {
+ print line > curfile
+ continue
+ }
+ n = split(line, a, "@")
+ # if a[1] == "", means leading @,
+ # don't add one back in.
+ for (i = 2; i <= n; i++) {
+ if (a[i] == "") { # was an @@
+ a[i] = "@"
+ if (a[i+1] == "")
+ i++
+ }
+ }
+ print join(a, 1, n, SUBSEP) > curfile
+ }
+ }
+
+ An important thing to note is the use of the '>' redirection. Output
+done with '>' only opens the file once; it stays open and subsequent
+output is appended to the file (*note Redirection::). This makes it
+easy to mix program text and explanatory prose for the same sample
+source file (as has been done here!) without any hassle. The file is
+only closed when a new data file name is encountered or at the end of
+the input file.
+
+ When a new file name is encountered, instead of closing the file, the
+program saves the name of the current file in 'filelist'. This makes it
+possible to interleave the code for more than one file in the Texinfo
+input file. (Previous versions of this program _did_ close the file.
+But because of the '>' redirection, a file whose parts were not all one
+after the other ended up getting clobbered.) An 'END' rule then closes
+all the open files when processing is finished:
+
+ END {
+ close(curfile) # close the last one
+ for (f in filelist) # close all the rest
+ close(f)
+ }
+
+ Finally, the function 'unexpected_eof()' prints an appropriate error
+message and then exits:
+
+ function unexpected_eof()
+ {
+ printf("extract: %s:%d: unexpected EOF or error\n",
+ FILENAME, FNR) > "/dev/stderr"
+ exit 1
+ }
+
+
+File: gawk.info, Node: Simple Sed, Next: Igawk Program, Prev: Extract
Program, Up: Miscellaneous Programs
+
+11.3.8 A Simple Stream Editor
+-----------------------------
+
+The 'sed' utility is a "stream editor", a program that reads a stream of
+data, makes changes to it, and passes it on. It is often used to make
+global changes to a large file or to a stream of data generated by a
+pipeline of commands. Although 'sed' is a complicated program in its
+own right, its most common use is to perform global substitutions in the
+middle of a pipeline:
+
+ COMMAND1 < orig.data | sed 's/old/new/g' | COMMAND2 > result
+
+ Here, 's/old/new/g' tells 'sed' to look for the regexp 'old' on each
+input line and globally replace it with the text 'new' (i.e., all the
+occurrences on a line). This is similar to 'awk''s 'gsub()' function
+(*note String Functions::).
+
+ The following program, 'awksed.awk', accepts at least two
+command-line arguments: the pattern to look for and the text to replace
+it with. Any additional arguments are treated as data file names to
+process. If none are provided, the standard input is used:
+
+ # awksed.awk --- do s/foo/bar/g using just print
+ # Thanks to Michael Brennan for the idea
+
+ function usage()
+ {
+ print "usage: awksed pat repl [files...]" > "/dev/stderr"
+ exit 1
+ }
+
+ BEGIN {
+ # validate arguments
+ if (ARGC < 3)
+ usage()
+
+ RS = ARGV[1]
+ ORS = ARGV[2]
+
+ # don't use arguments as files
+ ARGV[1] = ARGV[2] = ""
+ }
+
+ # look ma, no hands!
+ {
+ if (RT == "")
+ printf "%s", $0
+ else
+ print
+ }
+
+ The program relies on 'gawk''s ability to have 'RS' be a regexp, as
+well as on the setting of 'RT' to the actual text that terminates the
+record (*note Records::).
+
+ The idea is to have 'RS' be the pattern to look for. 'gawk'
+automatically sets '$0' to the text between matches of the pattern.
+This is text that we want to keep, unmodified. Then, by setting 'ORS'
+to the replacement text, a simple 'print' statement outputs the text we
+want to keep, followed by the replacement text.
+
+ There is one wrinkle to this scheme, which is what to do if the last
+record doesn't end with text that matches 'RS'. Using a 'print'
+statement unconditionally prints the replacement text, which is not
+correct. However, if the file did not end in text that matches 'RS',
+'RT' is set to the null string. In this case, we can print '$0' using
+'printf' (*note Printf::).
+
+ The 'BEGIN' rule handles the setup, checking for the right number of
+arguments and calling 'usage()' if there is a problem. Then it sets
+'RS' and 'ORS' from the command-line arguments and sets 'ARGV[1]' and
+'ARGV[2]' to the null string, so that they are not treated as file names
+(*note ARGC and ARGV::).
+
+ The 'usage()' function prints an error message and exits. Finally,
+the single rule handles the printing scheme outlined earlier, using
+'print' or 'printf' as appropriate, depending upon the value of 'RT'.
+
+
+File: gawk.info, Node: Igawk Program, Next: Anagram Program, Prev: Simple
Sed, Up: Miscellaneous Programs
+
+11.3.9 An Easy Way to Use Library Functions
+-------------------------------------------
+
+In *note Include Files::, we saw how 'gawk' provides a built-in
+file-inclusion capability. However, this is a 'gawk' extension. This
+minor node provides the motivation for making file inclusion available
+for standard 'awk', and shows how to do it using a combination of shell
+and 'awk' programming.
+
+ Using library functions in 'awk' can be very beneficial. It
+encourages code reuse and the writing of general functions. Programs
+are smaller and therefore clearer. However, using library functions is
+only easy when writing 'awk' programs; it is painful when running them,
+requiring multiple '-f' options. If 'gawk' is unavailable, then so too
+is the 'AWKPATH' environment variable and the ability to put 'awk'
+functions into a library directory (*note Options::). It would be nice
+to be able to write programs in the following manner:
+
+ # library functions
+ @include getopt.awk
+ @include join.awk
+ ...
+
+ # main program
+ BEGIN {
+ while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
+ ...
+ ...
+ }
+
+ The following program, 'igawk.sh', provides this service. It
+simulates 'gawk''s searching of the 'AWKPATH' variable and also allows
+"nested" includes (i.e., a file that is included with '@include' can
+contain further '@include' statements). 'igawk' makes an effort to only
+include files once, so that nested includes don't accidentally include a
+library function twice.
+
+ 'igawk' should behave just like 'gawk' externally. This means it
+should accept all of 'gawk''s command-line arguments, including the
+ability to have multiple source files specified via '-f' and the ability
+to mix command-line and library source files.
+
+ The program is written using the POSIX Shell ('sh') command
+language.(1) It works as follows:
+
+ 1. Loop through the arguments, saving anything that doesn't represent
+ 'awk' source code for later, when the expanded program is run.
+
+ 2. For any arguments that do represent 'awk' text, put the arguments
+ into a shell variable that will be expanded. There are two cases:
+
+ a. Literal text, provided with '-e' or '--source'. This text is
+ just appended directly.
+
+ b. Source file names, provided with '-f'. We use a neat trick
+ and append '@include FILENAME' to the shell variable's
+ contents. Because the file-inclusion program works the way
+ 'gawk' does, this gets the text of the file included in the
+ program at the correct point.
+
+ 3. Run an 'awk' program (naturally) over the shell variable's contents
+ to expand '@include' statements. The expanded program is placed in
+ a second shell variable.
+
+ 4. Run the expanded program with 'gawk' and any other original
+ command-line arguments that the user supplied (such as the data
+ file names).
+
+ This program uses shell variables extensively: for storing
+command-line arguments and the text of the 'awk' program that will
+expand the user's program, for the user's original program, and for the
+expanded program. Doing so removes some potential problems that might
+arise were we to use temporary files instead, at the cost of making the
+script somewhat more complicated.
+
+ The initial part of the program turns on shell tracing if the first
+argument is 'debug'.
+
+ The next part loops through all the command-line arguments. There
+are several cases of interest:
+
+'--'
+ This ends the arguments to 'igawk'. Anything else should be passed
+ on to the user's 'awk' program without being evaluated.
+
+'-W'
+ This indicates that the next option is specific to 'gawk'. To make
+ argument processing easier, the '-W' is appended to the front of
+ the remaining arguments and the loop continues. (This is an 'sh'
+ programming trick. Don't worry about it if you are not familiar
+ with 'sh'.)
+
+'-v', '-F'
+ These are saved and passed on to 'gawk'.
+
+'-f', '--file', '--file=', '-Wfile='
+ The file name is appended to the shell variable 'program' with an
+ '@include' statement. The 'expr' utility is used to remove the
+ leading option part of the argument (e.g., '--file='). (Typical
+ 'sh' usage would be to use the 'echo' and 'sed' utilities to do
+ this work. Unfortunately, some versions of 'echo' evaluate escape
+ sequences in their arguments, possibly mangling the program text.
+ Using 'expr' avoids this problem.)
+
+'--source', '--source=', '-Wsource='
+ The source text is appended to 'program'.
+
+'--version', '-Wversion'
+ 'igawk' prints its version number, runs 'gawk --version' to get the
+ 'gawk' version information, and then exits.
+
+ If none of the '-f', '--file', '-Wfile', '--source', or '-Wsource'
+arguments are supplied, then the first nonoption argument should be the
+'awk' program. If there are no command-line arguments left, 'igawk'
+prints an error message and exits. Otherwise, the first argument is
+appended to 'program'. In any case, after the arguments have been
+processed, the shell variable 'program' contains the complete text of
+the original 'awk' program.
+
+ The program is as follows:
+
+ #! /bin/sh
+ # igawk --- like gawk but do @include processing
+
+ if [ "$1" = debug ]
+ then
+ set -x
+ shift
+ fi
+
+ # A literal newline, so that program text is formatted correctly
+ n='
+ '
+
+ # Initialize variables to empty
+ program=
+ opts=
+
+ while [ $# -ne 0 ] # loop over arguments
+ do
+ case $1 in
+ --) shift
+ break ;;
+
+ -W) shift
+ # The ${x?'message here'} construct prints a
+ # diagnostic if $x is the null string
+ set -- -W"${@?'missing operand'}"
+ continue ;;
+
+ -[vF]) opts="$opts $1 '${2?'missing operand'}'"
+ shift ;;
+
+ -[vF]*) opts="$opts '$1'" ;;
+
+ -f) program="$program$n@include ${2?'missing operand'}"
+ shift ;;
+
+ -f*) f=$(expr "$1" : '-f\(.*\)')
+ program="$program$n@include $f" ;;
+
+ -[W-]file=*)
+ f=$(expr "$1" : '-.file=\(.*\)')
+ program="$program$n@include $f" ;;
+
+ -[W-]file)
+ program="$program$n@include ${2?'missing operand'}"
+ shift ;;
+
+ -[W-]source=*)
+ t=$(expr "$1" : '-.source=\(.*\)')
+ program="$program$n$t" ;;
+
+ -[W-]source)
+ program="$program$n${2?'missing operand'}"
+ shift ;;
+
+ -[W-]version)
+ echo igawk: version 3.0 1>&2
+ gawk --version
+ exit 0 ;;
+
+ -[W-]*) opts="$opts '$1'" ;;
+
+ *) break ;;
+ esac
+ shift
+ done
+
+ if [ -z "$program" ]
+ then
+ program=${1?'missing program'}
+ shift
+ fi
+
+ # At this point, `program' has the program.
+
+ The 'awk' program to process '@include' directives is stored in the
+shell variable 'expand_prog'. Doing this keeps the shell script
+readable. The 'awk' program reads through the user's program, one line
+at a time, using 'getline' (*note Getline::). The input file names and
+'@include' statements are managed using a stack. As each '@include' is
+encountered, the current file name is "pushed" onto the stack and the
+file named in the '@include' directive becomes the current file name.
+As each file is finished, the stack is "popped," and the previous input
+file becomes the current input file again. The process is started by
+making the original file the first one on the stack.
+
+ The 'pathto()' function does the work of finding the full path to a
+file. It simulates 'gawk''s behavior when searching the 'AWKPATH'
+environment variable (*note AWKPATH Variable::). If a file name has a
+'/' in it, no path search is done. Similarly, if the file name is
+'"-"', then that string is used as-is. Otherwise, the file name is
+concatenated with the name of each directory in the path, and an attempt
+is made to open the generated file name. The only way to test if a file
+can be read in 'awk' is to go ahead and try to read it with 'getline';
+this is what 'pathto()' does.(2) If the file can be read, it is closed
+and the file name is returned:
+
+ expand_prog='
+
+ function pathto(file, i, t, junk)
+ {
+ if (index(file, "/") != 0)
+ return file
+
+ if (file == "-")
+ return file
+
+ for (i = 1; i <= ndirs; i++) {
+ t = (pathlist[i] "/" file)
+ if ((getline junk < t) > 0) {
+ # found it
+ close(t)
+ return t
+ }
+ }
+ return ""
+ }
+
+ The main program is contained inside one 'BEGIN' rule. The first
+thing it does is set up the 'pathlist' array that 'pathto()' uses.
+After splitting the path on ':', null elements are replaced with '"."',
+which represents the current directory:
+
+ BEGIN {
+ path = ENVIRON["AWKPATH"]
+ ndirs = split(path, pathlist, ":")
+ for (i = 1; i <= ndirs; i++) {
+ if (pathlist[i] == "")
+ pathlist[i] = "."
+ }
+
+ The stack is initialized with 'ARGV[1]', which will be
+'"/dev/stdin"'. The main loop comes next. Input lines are read in
+succession. Lines that do not start with '@include' are printed
+verbatim. If the line does start with '@include', the file name is in
+'$2'. 'pathto()' is called to generate the full path. If it cannot,
+then the program prints an error message and continues.
+
+ The next thing to check is if the file is included already. The
+'processed' array is indexed by the full file name of each included file
+and it tracks this information for us. If the file is seen again, a
+warning message is printed. Otherwise, the new file name is pushed onto
+the stack and processing continues.
+
+ Finally, when 'getline' encounters the end of the input file, the
+file is closed and the stack is popped. When 'stackptr' is less than
+zero, the program is done:
+
+ stackptr = 0
+ input[stackptr] = ARGV[1] # ARGV[1] is first file
+
+ for (; stackptr >= 0; stackptr--) {
+ while ((getline < input[stackptr]) > 0) {
+ if (tolower($1) != "@include") {
+ print
+ continue
+ }
+ fpath = pathto($2)
+ if (fpath == "") {
+ printf("igawk: %s:%d: cannot find %s\n",
+ input[stackptr], FNR, $2) > "/dev/stderr"
+ continue
+ }
+ if (! (fpath in processed)) {
+ processed[fpath] = input[stackptr]
+ input[++stackptr] = fpath # push onto stack
+ } else
+ print $2, "included in", input[stackptr],
+ "already included in",
+ processed[fpath] > "/dev/stderr"
+ }
+ close(input[stackptr])
+ }
+ }' # close quote ends `expand_prog' variable
+
+ processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF
+ $program
+ EOF
+ )
+
+ The shell construct 'COMMAND << MARKER' is called a "here document".
+Everything in the shell script up to the MARKER is fed to COMMAND as
+input. The shell processes the contents of the here document for
+variable and command substitution (and possibly other things as well,
+depending upon the shell).
+
+ The shell construct '$(...)' is called "command substitution". The
+output of the command inside the parentheses is substituted into the
+command line. Because the result is used in a variable assignment, it
+is saved as a single string, even if the results contain whitespace.
+
+ The expanded program is saved in the variable 'processed_program'.
+It's done in these steps:
+
+ 1. Run 'gawk' with the '@include'-processing program (the value of the
+ 'expand_prog' shell variable) reading standard input.
+
+ 2. Standard input is the contents of the user's program, from the
+ shell variable 'program'. Feed its contents to 'gawk' via a here
+ document.
+
+ 3. Save the results of this processing in the shell variable
+ 'processed_program' by using command substitution.
+
+ The last step is to call 'gawk' with the expanded program, along with
+the original options and command-line arguments that the user supplied:
+
+ eval gawk $opts -- '"$processed_program"' '"$@"'
+
+ The 'eval' command is a shell construct that reruns the shell's
+parsing process. This keeps things properly quoted.
+
+ This version of 'igawk' represents the fifth version of this program.
+There are four key simplifications that make the program work better:
+
+ * Using '@include' even for the files named with '-f' makes building
+ the initial collected 'awk' program much simpler; all the
+ '@include' processing can be done once.
+
+ * Not trying to save the line read with 'getline' in the 'pathto()'
+ function when testing for the file's accessibility for use with the
+ main program simplifies things considerably.
+
+ * Using a 'getline' loop in the 'BEGIN' rule does it all in one
+ place. It is not necessary to call out to a separate loop for
+ processing nested '@include' statements.
+
+ * Instead of saving the expanded program in a temporary file, putting
+ it in a shell variable avoids some potential security problems.
+ This has the disadvantage that the script relies upon more features
+ of the 'sh' language, making it harder to follow for those who
+ aren't familiar with 'sh'.
+
+ Also, this program illustrates that it is often worthwhile to combine
+'sh' and 'awk' programming together. You can usually accomplish quite a
+lot, without having to resort to low-level programming in C or C++, and
+it is frequently easier to do certain kinds of string and argument
+manipulation using the shell than it is in 'awk'.
+
+ Finally, 'igawk' shows that it is not always necessary to add new
+features to a program; they can often be layered on top.(3)
+
+ ---------- Footnotes ----------
+
+ (1) Fully explaining the 'sh' language is beyond the scope of this
+book. We provide some minimal explanations, but see a good shell
+programming book if you wish to understand things in more depth.
+
+ (2) On some very old versions of 'awk', the test 'getline junk < t'
+can loop forever if the file exists but is empty.
+
+ (3) 'gawk' does '@include' processing itself in order to support the
+use of 'awk' programs as Web CGI scripts.
+
+
+File: gawk.info, Node: Anagram Program, Next: Signature Program, Prev:
Igawk Program, Up: Miscellaneous Programs
+
+11.3.10 Finding Anagrams from a Dictionary
+------------------------------------------
+
+An interesting programming challenge is to search for "anagrams" in a
+word list (such as '/usr/share/dict/words' on many GNU/Linux systems).
+One word is an anagram of another if both words contain the same letters
+(e.g., "babbling" and "blabbing").
+
+ Column 2, Problem C, of Jon Bentley's 'Programming Pearls', Second
+Edition, presents an elegant algorithm. The idea is to give words that
+are anagrams a common signature, sort all the words together by their
+signatures, and then print them. Dr. Bentley observes that taking the
+letters in each word and sorting them produces those common signatures.
+
+ The following program uses arrays of arrays to bring together words
+with the same signature and array sorting to print the words in sorted
+order:
+
+ # anagram.awk --- An implementation of the anagram-finding algorithm
+ # from Jon Bentley's "Programming Pearls," 2nd edition.
+ # Addison Wesley, 2000, ISBN 0-201-65788-0.
+ # Column 2, Problem C, section 2.8, pp 18-20.
+
+ /'s$/ { next } # Skip possessives
+
+ The program starts with a header, and then a rule to skip possessives
+in the dictionary file. The next rule builds up the data structure.
+The first dimension of the array is indexed by the signature; the second
+dimension is the word itself:
+
+ {
+ key = word2key($1) # Build signature
+ data[key][$1] = $1 # Store word with signature
+ }
+
+ The 'word2key()' function creates the signature. It splits the word
+apart into individual letters, sorts the letters, and then joins them
+back together:
+
+ # word2key --- split word apart into letters, sort, and join back together
+
+ function word2key(word, a, i, n, result)
+ {
+ n = split(word, a, "")
+ asort(a)
+
+ for (i = 1; i <= n; i++)
+ result = result a[i]
+
+ return result
+ }
+
+ Finally, the 'END' rule traverses the array and prints out the
+anagram lists. It sends the output to the system 'sort' command because
+otherwise the anagrams would appear in arbitrary order:
+
+ END {
+ sort = "sort"
+ for (key in data) {
+ # Sort words with same key
+ nwords = asorti(data[key], words)
+ if (nwords == 1)
+ continue
+
+ # And print. Minor glitch: trailing space at end of each line
+ for (j = 1; j <= nwords; j++)
+ printf("%s ", words[j]) | sort
+ print "" | sort
+ }
+ close(sort)
+ }
+
+ Here is some partial output when the program is run:
+
+ $ gawk -f anagram.awk /usr/share/dict/words | grep '^b'
+ ...
+ babbled blabbed
+ babbler blabber brabble
+ babblers blabbers brabbles
+ babbling blabbing
+ babbly blabby
+ babel bable
+ babels beslab
+ babery yabber
+ ...
+
+
+File: gawk.info, Node: Signature Program, Prev: Anagram Program, Up:
Miscellaneous Programs
+
+11.3.11 And Now for Something Completely Different
+--------------------------------------------------
+
+The following program was written by Davide Brini and is published on
+his website (http://backreference.org/2011/02/03/obfuscated-awk/). It
+serves as his signature in the Usenet group 'comp.lang.awk'. He
+supplies the following copyright terms:
+
+ Copyright (C) 2008 Davide Brini
+
+ Copying and distribution of the code published in this page, with
+ or without modification, are permitted in any medium without
+ royalty provided the copyright notice and this notice are
+ preserved.
+
+ Here is the program:
+
+ awk 'BEGIN{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c";
+ printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O,
+ X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O,
+ O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O}'
+
+ We leave it to you to determine what the program does. (If you are
+truly desperate to understand it, see Chris Johansen's explanation,
+which is embedded in the Texinfo source file for this Info file.)
+
+
+File: gawk.info, Node: Programs Summary, Next: Programs Exercises, Prev:
Miscellaneous Programs, Up: Sample Programs
+
+11.4 Summary
+============
+
+ * The programs provided in this major node continue on the theme that
+ reading programs is an excellent way to learn Good Programming.
+
+ * Using '#!' to make 'awk' programs directly runnable makes them
+ easier to use. Otherwise, invoke the program using 'awk -f ...'.
+
+ * Reimplementing standard POSIX programs in 'awk' is a pleasant
+ exercise; 'awk''s expressive power lets you write such programs in
+ relatively few lines of code, yet they are functionally complete
+ and usable.
+
+ * One of standard 'awk''s weaknesses is working with individual
+ characters. The ability to use 'split()' with the empty string as
+ the separator can considerably simplify such tasks.
+
+ * The examples here demonstrate the usefulness of the library
+ functions from *note Library Functions:: for a number of real (if
+ small) programs.
+
+ * Besides reinventing POSIX wheels, other programs solved a selection
+ of interesting problems, such as finding duplicate words in text,
+ printing mailing labels, and finding anagrams.
+
+
+File: gawk.info, Node: Programs Exercises, Prev: Programs Summary, Up:
Sample Programs
+
+11.5 Exercises
+==============
+
+ 1. Rewrite 'cut.awk' (*note Cut Program::) using 'split()' with '""'
+ as the separator.
+
+ 2. In *note Egrep Program::, we mentioned that 'egrep -i' could be
+ simulated in versions of 'awk' without 'IGNORECASE' by using
+ 'tolower()' on the line and the pattern. In a footnote there, we
+ also mentioned that this solution has a bug: the translated line is
+ output, and not the original one. Fix this problem.
+
+ 3. The POSIX version of 'id' takes options that control which
+ information is printed. Modify the 'awk' version (*note Id
+ Program::) to accept the same arguments and perform in the same
+ way.
+
+ 4. The 'split.awk' program (*note Split Program::) assumes that
+ letters are contiguous in the character set, which isn't true for
+ EBCDIC systems. Fix this problem. (Hint: Consider a different way
+ to work through the alphabet, without relying on 'ord()' and
+ 'chr()'.)
+
+ 5. In 'uniq.awk' (*note Uniq Program::, the logic for choosing which
+ lines to print represents a "state machine", which is "a device
+ which can be in one of a set number of stable conditions depending
+ on its previous condition and on the present values of its
+ inputs."(1) Brian Kernighan suggests that "an alternative approach
+ to state machines is to just read the input into an array, then use
+ indexing. It's almost always easier code, and for most inputs
+ where you would use this, just as fast." Rewrite the logic to
+ follow this suggestion.
+
+ 6. Why can't the 'wc.awk' program (*note Wc Program::) just use the
+ value of 'FNR' in 'endfile()'? Hint: Examine the code in *note
+ Filetrans Function::.
+
+ 7. Manipulation of individual characters in the 'translate' program
+ (*note Translate Program::) is painful using standard 'awk'
+ functions. Given that 'gawk' can split strings into individual
+ characters using '""' as the separator, how might you use this
+ feature to simplify the program?
+
+ 8. The 'extract.awk' program (*note Extract Program::) was written
+ before 'gawk' had the 'gensub()' function. Use it to simplify the
+ code.
+
+ 9. Compare the performance of the 'awksed.awk' program (*note Simple
+ Sed::) with the more straightforward:
+
+ BEGIN {
+ pat = ARGV[1]
+ repl = ARGV[2]
+ ARGV[1] = ARGV[2] = ""
+ }
+
+ { gsub(pat, repl); print }
+
+ 10. What are the advantages and disadvantages of 'awksed.awk' versus
+ the real 'sed' utility?
+
+ 11. In *note Igawk Program::, we mentioned that not trying to save the
+ line read with 'getline' in the 'pathto()' function when testing
+ for the file's accessibility for use with the main program
+ simplifies things considerably. What problem does this engender
+ though?
+
+ 12. As an additional example of the idea that it is not always
+ necessary to add new features to a program, consider the idea of
+ having two files in a directory in the search path:
+
+ 'default.awk'
+ This file contains a set of default library functions, such as
+ 'getopt()' and 'assert()'.
+
+ 'site.awk'
+ This file contains library functions that are specific to a
+ site or installation; i.e., locally developed functions.
+ Having a separate file allows 'default.awk' to change with new
+ 'gawk' releases, without requiring the system administrator to
+ update it each time by adding the local functions.
+
+ One user suggested that 'gawk' be modified to automatically read
+ these files upon startup. Instead, it would be very simple to
+ modify 'igawk' to do this. Since 'igawk' can process nested
+ '@include' directives, 'default.awk' could simply contain
+ '@include' statements for the desired library functions. Make this
+ change.
+
+ 13. Modify 'anagram.awk' (*note Anagram Program::), to avoid the use
+ of the external 'sort' utility.
+
+ ---------- Footnotes ----------
+
+ (1) This definition is from
+<https://www.lexico.com/en/definition/state_machine>.
+
+
+File: gawk.info, Node: Advanced Features, Next: Internationalization, Prev:
Sample Programs, Up: Top
+
+12 Advanced Features of 'gawk'
+******************************
+
+ Write documentation as if whoever reads it is a violent psychopath
+ who knows where you live.
+ -- _Steve English, as quoted by Peter Langston_
+
+ This major node discusses advanced features in 'gawk'. It's a bit of
+a "grab bag" of items that are otherwise unrelated to each other.
+First, we look at a command-line option that allows 'gawk' to recognize
+nondecimal numbers in input data, not just in 'awk' programs. Then,
+'gawk''s special features for sorting arrays are presented. Next,
+two-way I/O, discussed briefly in earlier parts of this Info file, is
+described in full detail, along with the basics of TCP/IP networking.
+Finally, we see how 'gawk' can "profile" an 'awk' program, making it
+possible to tune it for performance.
+
+ Additional advanced features are discussed in separate major nodes of
+their own:
+
+ * *note Internationalization::, discusses how to internationalize
+ your 'awk' programs, so that they can speak multiple national
+ languages.
+
+ * *note Debugger::, describes 'gawk''s built-in command-line debugger
+ for debugging 'awk' programs.
+
+ * *note Arbitrary Precision Arithmetic::, describes how you can use
+ 'gawk' to perform arbitrary-precision arithmetic.
+
+ * *note Dynamic Extensions::, discusses the ability to dynamically
+ add new built-in functions to 'gawk'.
+
+* Menu:
+
+* Nondecimal Data:: Allowing nondecimal input data.
+* Boolean Typed Values:: Values with 'number|bool' type.
+* Array Sorting:: Facilities for controlling array traversal and
+ sorting arrays.
+* Two-way I/O:: Two-way communications with another process.
+* TCP/IP Networking:: Using 'gawk' for network programming.
+* Profiling:: Profiling your 'awk' programs.
+* Extension Philosophy:: What should be built-in and what should not.
+* Advanced Features Summary:: Summary of advanced features.
+
+
+File: gawk.info, Node: Nondecimal Data, Next: Boolean Typed Values, Up:
Advanced Features
+
+12.1 Allowing Nondecimal Input Data
+===================================
+
+If you run 'gawk' with the '--non-decimal-data' option, you can have
+nondecimal values in your input data:
+
+ $ echo 0123 123 0x123 |
+ > gawk --non-decimal-data '{ printf "%d, %d, %d\n", $1, $2, $3 }'
+ -| 83, 123, 291
+
+ For this feature to work, write your program so that 'gawk' treats
+your data as numeric:
+
+ $ echo 0123 123 0x123 | gawk '{ print $1, $2, $3 }'
+ -| 0123 123 0x123
+
+The 'print' statement treats its expressions as strings. Although the
+fields can act as numbers when necessary, they are still strings, so
+'print' does not try to treat them numerically. You need to add zero to
+a field to force it to be treated as a number. For example:
+
+ $ echo 0123 123 0x123 | gawk --non-decimal-data '
+ > { print $1, $2, $3
+ > print $1 + 0, $2 + 0, $3 + 0 }'
+ -| 0123 123 0x123
+ -| 83 123 291
+
+ Because it is common to have decimal data with leading zeros, and
+because using this facility could lead to surprising results, the
+default is to leave it disabled. If you want it, you must explicitly
+request it.
+
+ CAUTION: _Use of this option is not recommended._ It can break old
+ programs very badly. Instead, use the 'strtonum()' function to
+ convert your data (*note String Functions::). This makes your
+ programs easier to write and easier to read, and leads to less
+ surprising results.
+
+ This option may disappear in a future version of 'gawk'.
+
+
+File: gawk.info, Node: Boolean Typed Values, Next: Array Sorting, Prev:
Nondecimal Data, Up: Advanced Features
+
+12.2 Boolean Typed Values
+=========================
+
+Scalar values in 'awk' are either numbers or strings. 'gawk' also
+supports values of type 'regexp' (*note Strong Regexp Constants::).
+
+ As described in *note Truth Values::, Boolean values in 'awk' don't
+have a separate type: a value counts as "true" if it is nonzero or
+non-null, and as "false" otherwise.
+
+ When interchanging data with languages that do have a real Boolean
+type, using a standard format such as JSON or XML, the lack of a true
+Boolean type in 'awk' is problematic. (See, for example, the 'json'
+extension provided by the 'gawkextlib' project
+(https://sourceforge.net/projects/gawkextlib).)
+
+ It's easy to import Boolean data into 'awk', but then the fact that
+it was originally Boolean is lost. Exporting data is even harder;
+there's no way to indicate that a value is really Boolean.
+
+ To solve this problem, 'gawk' provides a function named 'mkbool()'.
+It takes one argument, which is any 'awk' expression, and it returns a
+value of Boolean type.
+
+ The returned values are normal 'awk' numeric values, with values of
+either one or zero, depending upon the truth value of the original
+expression passed in the call to 'mkbool()'.
+
+ The 'typeof()' function (*note Type Functions::) returns
+'"number|bool"' for these values.
+
+ Thus Boolean-typed values _are_ numbers as far as 'gawk' is
+concerned, except that extension code can treat them as Booleans if
+desired.
+
+ While it would have been possible to add two new built-in variables
+of Boolean type named 'TRUE' and 'FALSE', doing so would undoubtedly
+have broken many existing 'awk' programs. Instead, having a "generator"
+function that creates Boolean values gives flexibility, without breaking
+as much existing code.
+
+
+File: gawk.info, Node: Array Sorting, Next: Two-way I/O, Prev: Boolean
Typed Values, Up: Advanced Features
+
+12.3 Controlling Array Traversal and Array Sorting
+==================================================
+
+'gawk' lets you control the order in which a 'for (INDX in ARRAY)' loop
+traverses an array.
+
+ In addition, two built-in functions, 'asort()' and 'asorti()', let
+you sort arrays based on the array values and indices, respectively.
+These two functions also provide control over the sorting criteria used
+to order the elements during sorting.
+
+* Menu:
+
+* Controlling Array Traversal:: How to use PROCINFO["sorted_in"].
+* Array Sorting Functions:: How to use 'asort()' and 'asorti()'.
+
+
+File: gawk.info, Node: Controlling Array Traversal, Next: Array Sorting
Functions, Up: Array Sorting
+
+12.3.1 Controlling Array Traversal
+----------------------------------
+
+By default, the order in which a 'for (INDX in ARRAY)' loop scans an
+array is not defined; it is generally based upon the internal
+implementation of arrays inside 'awk'.
+
+ Often, though, it is desirable to be able to loop over the elements
+in a particular order that you, the programmer, choose. 'gawk' lets you
+do this.
+
+ *note Controlling Scanning:: describes how you can assign special,
+predefined values to 'PROCINFO["sorted_in"]' in order to control the
+order in which 'gawk' traverses an array during a 'for' loop.
+
+ In addition, the value of 'PROCINFO["sorted_in"]' can be a function
+name.(1) This lets you traverse an array based on any custom criterion.
+The array elements are ordered according to the return value of this
+function. The comparison function should be defined with at least four
+arguments:
+
+ function comp_func(i1, v1, i2, v2)
+ {
+ COMPARE ELEMENTS 1 AND 2 IN SOME FASHION
+ RETURN < 0; 0; OR > 0
+ }
+
+ Here, 'i1' and 'i2' are the indices, and 'v1' and 'v2' are the
+corresponding values of the two elements being compared. Either 'v1' or
+'v2', or both, can be arrays if the array being traversed contains
+subarrays as values. (*Note Arrays of Arrays:: for more information
+about subarrays.) The three possible return values are interpreted as
+follows:
+
+'comp_func(i1, v1, i2, v2) < 0'
+ Index 'i1' comes before index 'i2' during loop traversal.
+
+'comp_func(i1, v1, i2, v2) == 0'
+ Indices 'i1' and 'i2' come together, but the relative order with
+ respect to each other is undefined.
+
+'comp_func(i1, v1, i2, v2) > 0'
+ Index 'i1' comes after index 'i2' during loop traversal.
+
+ Our first comparison function can be used to scan an array in
+numerical order of the indices:
+
+ function cmp_num_idx(i1, v1, i2, v2)
+ {
+ # numerical index comparison, ascending order
+ return (i1 - i2)
+ }
+
+ Our second function traverses an array based on the string order of
+the element values rather than by indices:
+
+ function cmp_str_val(i1, v1, i2, v2)
+ {
+ # string value comparison, ascending order
+ v1 = v1 ""
+ v2 = v2 ""
+ if (v1 < v2)
+ return -1
+ return (v1 != v2)
+ }
+
+ The third comparison function makes all numbers, and numeric strings
+without any leading or trailing spaces, come out first during loop
+traversal:
+
+ function cmp_num_str_val(i1, v1, i2, v2, n1, n2)
+ {
+ # numbers before string value comparison, ascending order
+ n1 = v1 + 0
+ n2 = v2 + 0
+ if (n1 == v1)
+ return (n2 == v2) ? (n1 - n2) : -1
+ else if (n2 == v2)
+ return 1
+ return (v1 < v2) ? -1 : (v1 != v2)
+ }
+
+ Here is a main program to demonstrate how 'gawk' behaves using each
+of the previous functions:
+
+ BEGIN {
+ data["one"] = 10
+ data["two"] = 20
+ data[10] = "one"
+ data[100] = 100
+ data[20] = "two"
+
+ f[1] = "cmp_num_idx"
+ f[2] = "cmp_str_val"
+ f[3] = "cmp_num_str_val"
+ for (i = 1; i <= 3; i++) {
+ printf("Sort function: %s\n", f[i])
+ PROCINFO["sorted_in"] = f[i]
+ for (j in data)
+ printf("\tdata[%s] = %s\n", j, data[j])
+ print ""
+ }
+ }
+
+ Here are the results when the program is run:
+
+ $ gawk -f compdemo.awk
+ -| Sort function: cmp_num_idx Sort by numeric index
+ -| data[two] = 20
+ -| data[one] = 10 Both strings are numerically zero
+ -| data[10] = one
+ -| data[20] = two
+ -| data[100] = 100
+ -|
+ -| Sort function: cmp_str_val Sort by element values as strings
+ -| data[one] = 10
+ -| data[100] = 100 String 100 is less than string 20
+ -| data[two] = 20
+ -| data[10] = one
+ -| data[20] = two
+ -|
+ -| Sort function: cmp_num_str_val Sort all numeric values before all
strings
+ -| data[one] = 10
+ -| data[two] = 20
+ -| data[100] = 100
+ -| data[10] = one
+ -| data[20] = two
+
+ Consider sorting the entries of a GNU/Linux system password file
+according to login name. The following program sorts records by a
+specific field position and can be used for this purpose:
+
+ # passwd-sort.awk --- simple program to sort by field position
+ # field position is specified by the global variable POS
+
+ function cmp_field(i1, v1, i2, v2)
+ {
+ # comparison by value, as string, and ascending order
+ return v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS])
+ }
+
+ {
+ for (i = 1; i <= NF; i++)
+ a[NR][i] = $i
+ }
+
+ END {
+ PROCINFO["sorted_in"] = "cmp_field"
+ if (POS < 1 || POS > NF)
+ POS = 1
+
+ for (i in a) {
+ for (j = 1; j <= NF; j++)
+ printf("%s%c", a[i][j], j < NF ? ":" : "")
+ print ""
+ }
+ }
+
+ The first field in each entry of the password file is the user's
+login name, and the fields are separated by colons. Each record defines
+a subarray, with each field as an element in the subarray. Running the
+program produces the following output:
+
+ $ gawk -v POS=1 -F: -f sort.awk /etc/passwd
+ -| adm:x:3:4:adm:/var/adm:/sbin/nologin
+ -| apache:x:48:48:Apache:/var/www:/sbin/nologin
+ -| avahi:x:70:70:Avahi daemon:/:/sbin/nologin
+ ...
+
+ The comparison should normally always return the same value when
+given a specific pair of array elements as its arguments. If
+inconsistent results are returned, then the order is undefined. This
+behavior can be exploited to introduce random order into otherwise
+seemingly ordered data:
+
+ function cmp_randomize(i1, v1, i2, v2)
+ {
+ # random order (caution: this may never terminate!)
+ return (2 - 4 * rand())
+ }
+
+ As already mentioned, the order of the indices is arbitrary if two
+elements compare equal. This is usually not a problem, but letting the
+tied elements come out in arbitrary order can be an issue, especially
+when comparing item values. The partial ordering of the equal elements
+may change the next time the array is traversed, if other elements are
+added to or removed from the array. One way to resolve ties when
+comparing elements with otherwise equal values is to include the indices
+in the comparison rules. Note that doing this may make the loop
+traversal less efficient, so consider it only if necessary. The
+following comparison functions force a deterministic order, and are
+based on the fact that the (string) indices of two elements are never
+equal:
+
+ function cmp_numeric(i1, v1, i2, v2)
+ {
+ # numerical value (and index) comparison, descending order
+ return (v1 != v2) ? (v2 - v1) : (i2 - i1)
+ }
+
+ function cmp_string(i1, v1, i2, v2)
+ {
+ # string value (and index) comparison, descending order
+ v1 = v1 i1
+ v2 = v2 i2
+ return (v1 > v2) ? -1 : (v1 != v2)
+ }
+
+ A custom comparison function can often simplify ordered loop
+traversal, and the sky is really the limit when it comes to designing
+such a function.
+
+ When string comparisons are made during a sort, either for element
+values where one or both aren't numbers, or for element indices handled
+as strings, the value of 'IGNORECASE' (*note Built-in Variables::)
+controls whether the comparisons treat corresponding upper- and
+lowercase letters as equivalent or distinct.
+
+ Another point to keep in mind is that in the case of subarrays, the
+element values can themselves be arrays; a production comparison
+function should use the 'isarray()' function (*note Type Functions::) to
+check for this, and choose a defined sorting order for subarrays.
+
+ All sorting based on 'PROCINFO["sorted_in"]' is disabled in POSIX
+mode, because the 'PROCINFO' array is not special in that case.
+
+ As a side note, sorting the array indices before traversing the array
+has been reported to add a 15% to 20% overhead to the execution time of
+'awk' programs. For this reason, sorted array traversal is not the
+default.
+
+ ---------- Footnotes ----------
+
+ (1) This is why the predefined sorting orders start with an '@'
+character, which cannot be part of an identifier.
+
+
+File: gawk.info, Node: Array Sorting Functions, Prev: Controlling Array
Traversal, Up: Array Sorting
+
+12.3.2 Sorting Array Values and Indices with 'gawk'
+---------------------------------------------------
+
+In most 'awk' implementations, sorting an array requires writing a
+'sort()' function. This can be educational for exploring different
+sorting algorithms, but usually that's not the point of the program.
+'gawk' provides the built-in 'asort()' and 'asorti()' functions (*note
+String Functions::) for sorting arrays. For example:
+
+ POPULATE THE ARRAY data
+ n = asort(data)
+ for (i = 1; i <= n; i++)
+ DO SOMETHING WITH data[i]
+
+ After the call to 'asort()', the array 'data' is indexed from 1 to
+some number N, the total number of elements in 'data'. (This count is
+'asort()''s return value.) 'data[1]' <= 'data[2]' <= 'data[3]', and so
+on. The default comparison is based on the type of the elements (*note
+Typing and Comparison::). All numeric values come before all string
+values, which in turn come before all subarrays.
+
+ An important side effect of calling 'asort()' is that _the array's
+original indices are irrevocably lost_. As this isn't always desirable,
+'asort()' accepts a second argument:
+
+ POPULATE THE ARRAY source
+ n = asort(source, dest)
+ for (i = 1; i <= n; i++)
+ DO SOMETHING WITH dest[i]
+
+ In this case, 'gawk' copies the 'source' array into the 'dest' array
+and then sorts 'dest', destroying its indices. However, the 'source'
+array is not affected.
+
+ Often, what's needed is to sort on the values of the _indices_
+instead of the values of the elements. To do that, use the 'asorti()'
+function. The interface and behavior are identical to that of
+'asort()', except that the index values are used for sorting and become
+the values of the result array:
+
+ { source[$0] = some_func($0) }
+
+ END {
+ n = asorti(source, dest)
+ for (i = 1; i <= n; i++) {
+ Work with sorted indices directly:
+ DO SOMETHING WITH dest[i]
+ ...
+ Access original array via sorted indices:
+ DO SOMETHING WITH source[dest[i]]
+ }
+ }
+
+ So far, so good. Now it starts to get interesting. Both 'asort()'
+and 'asorti()' accept a third string argument to control comparison of
+array elements. When we introduced 'asort()' and 'asorti()' in *note
+String Functions::, we ignored this third argument; however, now is the
+time to describe how this argument affects these two functions.
+
+ Basically, the third argument specifies how the array is to be
+sorted. There are two possibilities. As with 'PROCINFO["sorted_in"]',
+this argument may be one of the predefined names that 'gawk' provides
+(*note Controlling Scanning::), or it may be the name of a user-defined
+function (*note Controlling Array Traversal::).
+
+ In the latter case, _the function can compare elements in any way it
+chooses_, taking into account just the indices, just the values, or
+both. This is extremely powerful.
+
+ Once the array is sorted, 'asort()' takes the _values_ in their final
+order and uses them to fill in the result array, whereas 'asorti()'
+takes the _indices_ in their final order and uses them to fill in the
+result array.
+
+ NOTE: Copying array indices and elements isn't expensive in terms
+ of memory. Internally, 'gawk' maintains "reference counts" to
+ data. For example, when 'asort()' copies the first array to the
+ second one, there is only one copy of the original array elements'
+ data, even though both arrays use the values.
+
+ You may use the same array for both the first and second arguments to
+'asort()' and 'asorti()'. Doing so only makes sense if you are also
+supplying the third argument, since 'awk' doesn't provide a way to pass
+that third argument without also passing the first and second ones.
+
+ Because 'IGNORECASE' affects string comparisons, the value of
+'IGNORECASE' also affects sorting for both 'asort()' and 'asorti()'.
+Note also that the locale's sorting order does _not_ come into play;
+comparisons are based on character values only.(1)
+
+ The following example demonstrates the use of a comparison function
+with 'asort()'. The comparison function, 'case_fold_compare()', maps
+both values to lowercase in order to compare them ignoring case.
+
+ # case_fold_compare --- compare as strings, ignoring case
+
+ function case_fold_compare(i1, v1, i2, v2, l, r)
+ {
+ l = tolower(v1)
+ r = tolower(v2)
+
+ if (l < r)
+ return -1
+ else if (l == r)
+ return 0
+ else
+ return 1
+ }
+
+ And here is the test program for it:
+
+ # Test program
+
+ BEGIN {
+ Letters = "abcdefghijklmnopqrstuvwxyz" \
+ "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+ split(Letters, data, "")
+
+ asort(data, result, "case_fold_compare")
+
+ j = length(result)
+ for (i = 1; i <= j; i++) {
+ printf("%s", result[i])
+ if (i % (j/2) == 0)
+ printf("\n")
+ else
+ printf(" ")
+ }
+ }
+
+ When run, we get the following:
+
+ $ gawk -f case_fold_compare.awk
+ -| A a B b c C D d e E F f g G H h i I J j k K l L M m
+ -| n N O o p P Q q r R S s t T u U V v w W X x y Y z Z
+
+ ---------- Footnotes ----------
+
+ (1) This is true because locale-based comparison occurs only when in
+POSIX-compatibility mode, and because 'asort()' and 'asorti()' are
+'gawk' extensions, they are not available in that case.
+
+
+File: gawk.info, Node: Two-way I/O, Next: TCP/IP Networking, Prev: Array
Sorting, Up: Advanced Features
+
+12.4 Two-Way Communications with Another Process
+================================================
+
+It is often useful to be able to send data to a separate program for
+processing and then read the result. This can always be done with
+temporary files:
+
+ # Write the data for processing
+ tempfile = ("mydata." PROCINFO["pid"])
+ while (NOT DONE WITH DATA)
+ print DATA | ("subprogram > " tempfile)
+ close("subprogram > " tempfile)
+
+ # Read the results, remove tempfile when done
+ while ((getline newdata < tempfile) > 0)
+ PROCESS newdata APPROPRIATELY
+ close(tempfile)
+ system("rm " tempfile)
+
+This works, but not elegantly. Among other things, it requires that the
+program be run in a directory that cannot be shared among users; for
+example, '/tmp' will not do, as another user might happen to be using a
+temporary file with the same name.(1)
+
+ However, with 'gawk', it is possible to open a _two-way_ pipe to
+another process. The second process is termed a "coprocess", as it runs
+in parallel with 'gawk'. The two-way connection is created using the
+'|&' operator (borrowed from the Korn shell, 'ksh'):(2)
+
+ do {
+ print DATA |& "subprogram"
+ "subprogram" |& getline results
+ } while (DATA LEFT TO PROCESS)
+ close("subprogram")
+
+ The first time an I/O operation is executed using the '|&' operator,
+'gawk' creates a two-way pipeline to a child process that runs the other
+program. Output created with 'print' or 'printf' is written to the
+program's standard input, and output from the program's standard output
+can be read by the 'gawk' program using 'getline'. As is the case with
+processes started by '|', the subprogram can be any program, or pipeline
+of programs, that can be started by the shell.
+
+ There are some cautionary items to be aware of:
+
+ * As the code inside 'gawk' currently stands, the coprocess's
+ standard error goes to the same place that the parent 'gawk''s
+ standard error goes. It is not possible to read the child's
+ standard error separately.
+
+ * I/O buffering may be a problem. 'gawk' automatically flushes all
+ output down the pipe to the coprocess. However, if the coprocess
+ does not flush its output, 'gawk' may hang when doing a 'getline'
+ in order to read the coprocess's results. This could lead to a
+ situation known as "deadlock", where each process is waiting for
+ the other one to do something.
+
+ It is possible to close just one end of the two-way pipe to a
+coprocess, by supplying a second argument to the 'close()' function of
+either '"to"' or '"from"' (*note Close Files And Pipes::). These
+strings tell 'gawk' to close the end of the pipe that sends data to the
+coprocess or the end that reads from it, respectively.
+
+ This is particularly necessary in order to use the system 'sort'
+utility as part of a coprocess; 'sort' must read _all_ of its input data
+before it can produce any output. The 'sort' program does not receive
+an end-of-file indication until 'gawk' closes the write end of the pipe.
+
+ When you have finished writing data to the 'sort' utility, you can
+close the '"to"' end of the pipe, and then start reading sorted data via
+'getline'. For example:
+
+ BEGIN {
+ command = "LC_ALL=C sort"
+ n = split("abcdefghijklmnopqrstuvwxyz", a, "")
+
+ for (i = n; i > 0; i--)
+ print a[i] |& command
+ close(command, "to")
+
+ while ((command |& getline line) > 0)
+ print "got", line
+ close(command)
+ }
+
+ This program writes the letters of the alphabet in reverse order, one
+per line, down the two-way pipe to 'sort'. It then closes the write end
+of the pipe, so that 'sort' receives an end-of-file indication. This
+causes 'sort' to sort the data and write the sorted data back to the
+'gawk' program. Once all of the data has been read, 'gawk' terminates
+the coprocess and exits.
+
+ As a side note, the assignment 'LC_ALL=C' in the 'sort' command
+ensures traditional Unix (ASCII) sorting from 'sort'. This is not
+strictly necessary here, but it's good to know how to do this.
+
+ Be careful when closing the '"from"' end of a two-way pipe; in this
+case 'gawk' waits for the child process to exit, which may cause your
+program to hang. (Thus, this particular feature is of much less use in
+practice than being able to close the '"to"' end.)
+
+ CAUTION: Normally, it is a fatal error to write to the '"to"' end
+ of a two-way pipe which has been closed, and it is also a fatal
+ error to read from the '"from"' end of a two-way pipe that has been
+ closed.
+
+ You may set 'PROCINFO["COMMAND", "NONFATAL"]' to make such
+ operations become nonfatal. If you do so, you then need to check
+ 'ERRNO' after each 'print', 'printf', or 'getline'. *Note
+ Nonfatal::, for more information.
+
+ You may also use pseudo-ttys (ptys) for two-way communication instead
+of pipes, if your system supports them. This is done on a per-command
+basis, by setting a special element in the 'PROCINFO' array (*note
+Auto-set::), like so:
+
+ command = "sort -nr" # command, save in convenience variable
+ PROCINFO[command, "pty"] = 1 # update PROCINFO
+ print ... |& command # start two-way pipe
+ ...
+
+If your system does not have ptys, or if all the system's ptys are in
+use, 'gawk' automatically falls back to using regular pipes.
+
+ Using ptys usually avoids the buffer deadlock issues described
+earlier, at some loss in performance. This is because the tty driver
+buffers and sends data line-by-line. On systems with the 'stdbuf' (part
+of the GNU Coreutils package
+(https://www.gnu.org/software/coreutils/coreutils.html)), you can use
+that program instead of ptys.
+
+ Note also that ptys are not fully transparent. Certain binary
+control codes, such 'Ctrl-d' for end-of-file, are interpreted by the tty
+driver and not passed through.
+
+ CAUTION: Finally, coprocesses open up the possibility of "deadlock"
+ between 'gawk' and the program running in the coprocess. This can
+ occur if you send "too much" data to the coprocess before reading
+ any back; each process is blocked writing data with no one
+ available to read what they've already written. There is no
+ workaround for deadlock; careful programming and knowledge of the
+ behavior of the coprocess are required.
+
+ The following example, due to Andrew Schorr, demonstrates how using
+ptys can help deal with buffering deadlocks.
+
+ Suppose 'gawk' were unable to add numbers. You could use a coprocess
+to do it. Here's an exceedingly simple program written for that
+purpose:
+
+ $ cat add.c
+ #include <stdio.h>
+
+ int
+ main(void)
+ {
+ int x, y;
+ while (scanf("%d %d", & x, & y) == 2)
+ printf("%d\n", x + y);
+ return 0;
+ }
+ $ cc -O add.c -o add Compile the program
+
+ You could then write an exceedingly simple 'gawk' program to add
+numbers by passing them to the coprocess:
+
+ $ echo 1 2 |
+ > gawk -v cmd=./add '{ print |& cmd; cmd |& getline x; print x }'
+
+ And it would deadlock, because 'add.c' fails to call
+'setlinebuf(stdout)'. The 'add' program freezes.
+
+ Now try instead:
+
+ $ echo 1 2 |
+ > gawk -v cmd=add 'BEGIN { PROCINFO[cmd, "pty"] = 1 }
+ > { print |& cmd; cmd |& getline x; print x }'
+ -| 3
+
+ By using a pty, 'gawk' fools the standard I/O library into thinking
+it has an interactive session, so it defaults to line buffering. And
+now, magically, it works!
+
+ ---------- Footnotes ----------
+
+ (1) Michael Brennan suggests the use of 'rand()' to generate unique
+file names. This is a valid point; nevertheless, temporary files remain
+more difficult to use than two-way pipes.
+
+ (2) This is very different from the same operator in the C shell and
+in Bash.
+
+
+File: gawk.info, Node: TCP/IP Networking, Next: Profiling, Prev: Two-way
I/O, Up: Advanced Features
+
+12.5 Using 'gawk' for Network Programming
+=========================================
+
+ 'EMRED':
+ A host is a host from coast to coast,
+ and nobody talks to a host that's close,
+ unless the host that isn't close
+ is busy, hung, or dead.
+ -- _Mike O'Brien (aka Mr. Protocol)_
+
+ In addition to being able to open a two-way pipeline to a coprocess
+on the same system (*note Two-way I/O::), it is possible to make a
+two-way connection to another process on another system across an IP
+network connection.
+
+ You can think of this as just a _very long_ two-way pipeline to a
+coprocess. The way 'gawk' decides that you want to use TCP/IP
+networking is by recognizing special file names that begin with one of
+'/inet/', '/inet4/', or '/inet6/'.
+
+ The full syntax of the special file name is
+'/NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT'. The components
+are:
+
+NET-TYPE
+ Specifies the kind of Internet connection to make. Use '/inet4/'
+ to force IPv4, and '/inet6/' to force IPv6. Plain '/inet/' (which
+ used to be the only option) uses the system default, most likely
+ IPv4.
+
+PROTOCOL
+ The protocol to use over IP. This must be either 'tcp', or 'udp',
+ for a TCP or UDP IP connection, respectively. TCP should be used
+ for most applications.
+
+LOCAL-PORT
+ The local TCP or UDP port number to use. Use a port number of '0'
+ when you want the system to pick a port. This is what you should
+ do when writing a TCP or UDP client. You may also use a well-known
+ service name, such as 'smtp' or 'http', in which case 'gawk'
+ attempts to determine the predefined port number using the C
+ 'getaddrinfo()' function.
+
+REMOTE-HOST
+ The IP address or fully qualified domain name of the Internet host
+ to which you want to connect.
+
+REMOTE-PORT
+ The TCP or UDP port number to use on the given REMOTE-HOST. Again,
+ use '0' if you don't care, or else a well-known service name.
+
+ NOTE: Failure in opening a two-way socket will result in a nonfatal
+ error being returned to the calling code. The value of 'ERRNO'
+ indicates the error (*note Auto-set::).
+
+ Consider the following very simple example:
+
+ BEGIN {
+ Service = "/inet/tcp/0/localhost/daytime"
+ Service |& getline
+ print $0
+ close(Service)
+ }
+
+ This program reads the current date and time from the local system's
+TCP 'daytime' server. It then prints the results and closes the
+connection.
+
+ Because this topic is extensive, the use of 'gawk' for TCP/IP
+programming is documented separately. *Note General Introduction:
+(gawkinet)Top, for a much more complete introduction and discussion, as
+well as extensive examples.
+
+ NOTE: 'gawk' can only open direct sockets. There is currently no
+ way to access services available over Secure Socket Layer (SSL);
+ this includes any web service whose URL starts with 'https://'.
+
+
+File: gawk.info, Node: Profiling, Next: Extension Philosophy, Prev: TCP/IP
Networking, Up: Advanced Features
+
+12.6 Profiling Your 'awk' Programs
+==================================
+
+You may produce execution traces of your 'awk' programs. This is done
+by passing the option '--profile' to 'gawk'. When 'gawk' has finished
+running, it creates a profile of your program in a file named
+'awkprof.out'. Because it is profiling, it also executes up to 45%
+slower than 'gawk' normally does.
+
+ As shown in the following example, the '--profile' option can be used
+to change the name of the file where 'gawk' will write the profile:
+
+ gawk --profile=myprog.prof -f myprog.awk data1 data2
+
+In the preceding example, 'gawk' places the profile in 'myprog.prof'
+instead of in 'awkprof.out'.
+
+ Here is a sample session showing a simple 'awk' program, its input
+data, and the results from running 'gawk' with the '--profile' option.
+First, the 'awk' program:
+
+ BEGIN { print "First BEGIN rule" }
+
+ END { print "First END rule" }
+
+ /foo/ {
+ print "matched /foo/, gosh"
+ for (i = 1; i <= 3; i++)
+ sing()
+ }
+
+ {
+ if (/foo/)
+ print "if is true"
+ else
+ print "else is true"
+ }
+
+ BEGIN { print "Second BEGIN rule" }
+
+ END { print "Second END rule" }
+
+ function sing( dummy)
+ {
+ print "I gotta be me!"
+ }
+
+ Following is the input data:
+
+ foo
+ bar
+ baz
+ foo
+ junk
+
+ Here is the 'awkprof.out' that results from running the 'gawk'
+profiler on this program and data (this example also illustrates that
+'awk' programmers sometimes get up very early in the morning to work):
+
+ # gawk profile, created Mon Sep 29 05:16:21 2014
+
+ # BEGIN rule(s)
+
+ BEGIN {
+ 1 print "First BEGIN rule"
+ }
+
+ BEGIN {
+ 1 print "Second BEGIN rule"
+ }
+
+ # Rule(s)
+
+ 5 /foo/ { # 2
+ 2 print "matched /foo/, gosh"
+ 6 for (i = 1; i <= 3; i++) {
+ 6 sing()
+ }
+ }
+
+ 5 {
+ 5 if (/foo/) { # 2
+ 2 print "if is true"
+ 3 } else {
+ 3 print "else is true"
+ }
+ }
+
+ # END rule(s)
+
+ END {
+ 1 print "First END rule"
+ }
+
+ END {
+ 1 print "Second END rule"
+ }
+
+
+ # Functions, listed alphabetically
+
+ 6 function sing(dummy)
+ {
+ 6 print "I gotta be me!"
+ }
+
+ This example illustrates many of the basic features of profiling
+output. They are as follows:
+
+ * The program is printed in the order 'BEGIN' rules, 'BEGINFILE'
+ rules, pattern-action rules, 'ENDFILE' rules, 'END' rules, and
+ functions, listed alphabetically. Multiple 'BEGIN' and 'END' rules
+ retain their separate identities, as do multiple 'BEGINFILE' and
+ 'ENDFILE' rules.
+
+ * Pattern-action rules have two counts. The first count, to the left
+ of the rule, shows how many times the rule's pattern was _tested_.
+ The second count, to the right of the rule's opening left brace in
+ a comment, shows how many times the rule's action was _executed_.
+ The difference between the two indicates how many times the rule's
+ pattern evaluated to false.
+
+ * Similarly, the count for an 'if'-'else' statement shows how many
+ times the condition was tested. To the right of the opening left
+ brace for the 'if''s body is a count showing how many times the
+ condition was true. The count for the 'else' indicates how many
+ times the test failed.
+
+ * The count for a loop header (such as 'for' or 'while') shows how
+ many times the loop test was executed. (Because of this, you can't
+ just look at the count on the first statement in a rule to
+ determine how many times the rule was executed. If the first
+ statement is a loop, the count is misleading.)
+
+ * For user-defined functions, the count next to the 'function'
+ keyword indicates how many times the function was called. The
+ counts next to the statements in the body show how many times those
+ statements were executed.
+
+ * The layout uses "K&R" style with TABs. Braces are used everywhere,
+ even when the body of an 'if', 'else', or loop is only a single
+ statement.
+
+ * Parentheses are used only where needed, as indicated by the
+ structure of the program and the precedence rules. For example,
+ '(3 + 5) * 4' means add three and five, then multiply the total by
+ four. However, '3 + 5 * 4' has no parentheses, and means '3 + (5 *
+ 4)'. However, explicit parentheses in the source program are
+ retained.
+
+ * Parentheses are used around the arguments to 'print' and 'printf'
+ only when the 'print' or 'printf' statement is followed by a
+ redirection. Similarly, if the target of a redirection isn't a
+ scalar, it gets parenthesized.
+
+ * 'gawk' supplies leading comments in front of the 'BEGIN' and 'END'
+ rules, the 'BEGINFILE' and 'ENDFILE' rules, the pattern-action
+ rules, and the functions.
+
+ * Functions are listed alphabetically. All functions in the 'awk'
+ namespace are listed first, in alphabetical order. Then come the
+ functions in namespaces. The namespaces are listed in alphabetical
+ order, and the functions within each namespace are listed
+ alphabetically.
+
+ The profiled version of your program may not look exactly like what
+you typed when you wrote it. This is because 'gawk' creates the
+profiled version by "pretty-printing" its internal representation of the
+program. The advantage to this is that 'gawk' can produce a standard
+representation. Also, things such as:
+
+ /foo/
+
+come out as:
+
+ /foo/ {
+ print
+ }
+
+which is correct, but possibly unexpected. (If a program uses both
+'print $0' and plain 'print', that distinction is retained.)
+
+ Besides creating profiles when a program has completed, 'gawk' can
+produce a profile while it is running. This is useful if your 'awk'
+program goes into an infinite loop and you want to see what has been
+executed. To use this feature, run 'gawk' with the '--profile' option
+in the background:
+
+ $ gawk --profile -f myprog &
+ [1] 13992
+
+The shell prints a job number and process ID number; in this case,
+13992. Use the 'kill' command to send the 'USR1' signal to 'gawk':
+
+ $ kill -USR1 13992
+
+As usual, the profiled version of the program is written to
+'awkprof.out', or to a different file if one was specified with the
+'--profile' option.
+
+ Along with the regular profile, as shown earlier, the profile file
+includes a trace of any active functions:
+
+ # Function Call Stack:
+
+ # 3. baz
+ # 2. bar
+ # 1. foo
+ # -- main --
+
+ You may send 'gawk' the 'USR1' signal as many times as you like.
+Each time, the profile and function call trace are appended to the
+output profile file.
+
+ If you use the 'HUP' signal instead of the 'USR1' signal, 'gawk'
+produces the profile and the function call trace and then exits.
+
+ When 'gawk' runs on MS-Windows systems, it uses the 'INT' and 'QUIT'
+signals for producing the profile, and in the case of the 'INT' signal,
+'gawk' exits. This is because these systems don't support the 'kill'
+command, so the only signals you can deliver to a program are those
+generated by the keyboard. The 'INT' signal is generated by the
+'Ctrl-c' or 'Ctrl-BREAK' key, while the 'QUIT' signal is generated by
+the 'Ctrl-\' key.
+
+ Finally, 'gawk' also accepts another option, '--pretty-print'. When
+called this way, 'gawk' "pretty-prints" the program into 'awkprof.out',
+without any execution counts.
+
+ NOTE: Once upon a time, the '--pretty-print' option would also run
+ your program. This is no longer the case.
+
+ There is a significant difference between the output created when
+profiling, and that created when pretty-printing. Pretty-printed output
+preserves the original comments that were in the program, although their
+placement may not correspond exactly to their original locations in the
+source code. However, no comments should be lost. Also, 'gawk' does
+the best it can to preserve the distinction between comments at the end
+of a statement and comments on lines by themselves. This isn't always
+perfect, though.
+
+ However, as a deliberate design decision, profiling output _omits_
+the original program's comments. This allows you to focus on the
+execution count data and helps you avoid the temptation to use the
+profiler for pretty-printing.
+
+ Additionally, pretty-printed output does not have the leading
+indentation that the profiling output does. This makes it easy to
+pretty-print your code once development is completed, and then use the
+result as the final version of your program.
+
+ Because the internal representation of your program is formatted to
+recreate an 'awk' program, profiling and pretty-printing automatically
+disable 'gawk''s default optimizations.
+
+ Profiling and pretty-printing also preserve the original format of
+numeric constants; if you used an octal or hexadecimal value in your
+source code, it will appear that way in the output.
+
+
+File: gawk.info, Node: Extension Philosophy, Next: Advanced Features
Summary, Prev: Profiling, Up: Advanced Features
+
+12.7 Builtin Features versus Extensions
+=======================================
+
+As this and subsequent major nodes show, 'gawk' has a large number of
+extensions over standard 'awk' built-in to the program. These have
+developed over time. More recently, the focus has moved to using the
+extension mechanism (*note Dynamic Extensions::) for adding features.
+This minor node discusses the "guiding philosophy" behind what should be
+added to the interpreter as a built-in feature versus what should be
+done in extensions.
+
+ There are several goals:
+
+ 1. Keep the language 'awk'; it should not become unrecognizable, even
+ if programs in it will only run on 'gawk'.
+
+ 2. Keep the core from getting any larger unless absolutely necessary.
+
+ 3. Add new functionality either in 'awk' scripts ('-f', '@include') or
+ in loadable extensions written in C or C++ ('-l', '@load').
+
+ 4. Extend the core interpreter only if some feature is:
+
+ A. Truly desirable.
+ B. Cannot be done via library files or loadable extensions.
+ C. Can be implemented without too much pain in the core.
+ Combining modules with 'awk' files is a powerful technique. Some of
+the sample extensions demonstrate this.
+
+ Loading extensions and library files should not be done
+automatically, because then there's overhead that most users don't want
+or need.
+
+
+File: gawk.info, Node: Advanced Features Summary, Prev: Extension
Philosophy, Up: Advanced Features
+
+12.8 Summary
+============
+
+ * The '--non-decimal-data' option causes 'gawk' to treat octal- and
+ hexadecimal-looking input data as octal and hexadecimal. This
+ option should be used with caution or not at all; use of
+ 'strtonum()' is preferable. Note that this option may disappear in
+ a future version of 'gawk'.
+
+ * You can take over complete control of sorting in 'for (INDX in
+ ARRAY)' array traversal by setting 'PROCINFO["sorted_in"]' to the
+ name of a user-defined function that does the comparison of array
+ elements based on index and value.
+
+ * Similarly, you can supply the name of a user-defined comparison
+ function as the third argument to either 'asort()' or 'asorti()' to
+ control how those functions sort arrays. Or you may provide one of
+ the predefined control strings that work for
+ 'PROCINFO["sorted_in"]'.
+
+ * You can use the '|&' operator to create a two-way pipe to a
+ coprocess. You read from the coprocess with 'getline' and write to
+ it with 'print' or 'printf'. Use 'close()' to close off the
+ coprocess completely, or optionally, close off one side of the
+ two-way communications.
+
+ * By using special file names with the '|&' operator, you can open a
+ TCP/IP (or UDP/IP) connection to remote hosts on the Internet.
+ 'gawk' supports both IPv4 and IPv6.
+
+ * You can generate statement count profiles of your program. This
+ can help you determine which parts of your program may be taking
+ the most time and let you tune them more easily. Sending the
+ 'USR1' signal while profiling causes 'gawk' to dump the profile and
+ keep going, including a function call stack.
+
+ * You can also just "pretty-print" the program.
+
+ * New features should be developed using the extension mechanism if
+ possible; they should be added to the core interpreter only as a
+ last resort.
+
+
+File: gawk.info, Node: Internationalization, Next: Debugger, Prev: Advanced
Features, Up: Top
+
+13 Internationalization with 'gawk'
+***********************************
+
+ Moon... Gorgeous... MEDITATION!
+ -- _Pretty Guardian Sailor Moon Eternal, The Movie_
+
+ It probably sounded better in Japanese.
+ -- _Malka Robbins_
+
+ Once upon a time, computer makers wrote software that worked only in
+English. Eventually, hardware and software vendors noticed that if
+their systems worked in the native languages of non-English-speaking
+countries, they were able to sell more systems. As a result,
+internationalization and localization of programs and software systems
+became a common practice.
+
+ For many years, the ability to provide internationalization was
+largely restricted to programs written in C and C++. This major node
+describes the underlying library 'gawk' uses for internationalization,
+as well as how 'gawk' makes internationalization features available at
+the 'awk' program level. Having internationalization available at the
+'awk' level gives software developers additional flexibility--they are
+no longer forced to write in C or C++ when internationalization is a
+requirement.
+
+* Menu:
+
+* I18N and L10N:: Internationalization and Localization.
+* Explaining gettext:: How GNU 'gettext' works.
+* Programmer i18n:: Features for the programmer.
+* Translator i18n:: Features for the translator.
+* I18N Example:: A simple i18n example.
+* Gawk I18N:: 'gawk' is also internationalized.
+* I18N Summary:: Summary of I18N stuff.
+
+
+File: gawk.info, Node: I18N and L10N, Next: Explaining gettext, Up:
Internationalization
+
+13.1 Internationalization and Localization
+==========================================
+
+"Internationalization" means writing (or modifying) a program once, in
+such a way that it can use multiple languages without requiring further
+source code changes. "Localization" means providing the data necessary
+for an internationalized program to work in a particular language. Most
+typically, these terms refer to features such as the language used for
+printing error messages, the language used to read responses, and
+information related to how numerical and monetary values are printed and
+read.
+
+
+File: gawk.info, Node: Explaining gettext, Next: Programmer i18n, Prev:
I18N and L10N, Up: Internationalization
+
+13.2 GNU 'gettext'
+==================
+
+'gawk' uses GNU 'gettext' to provide its internationalization features.
+The facilities in GNU 'gettext' focus on messages: strings printed by a
+program, either directly or via formatting with 'printf' or
+'sprintf()'.(1)
+
+ When using GNU 'gettext', each application has its own "text domain".
+This is a unique name, such as 'kpilot' or 'gawk', that identifies the
+application. A complete application may have multiple
+components--programs written in C or C++, as well as scripts written in
+'sh' or 'awk'. All of the components use the same text domain.
+
+ To make the discussion concrete, assume we're writing an application
+named 'guide'. Internationalization consists of the following steps, in
+this order:
+
+ 1. The programmer reviews the source for all of 'guide''s components
+ and marks each string that is a candidate for translation. For
+ example, '"`-F': option required"' is a good candidate for
+ translation. A table with strings of option names is not (e.g.,
+ 'gawk''s '--profile' option should remain the same, no matter what
+ the local language).
+
+ 2. The programmer indicates the application's text domain ('"guide"')
+ to the 'gettext' library, by calling the 'textdomain()' function.
+
+ 3. Messages from the application are extracted from the source code
+ and collected into a portable object template file ('guide.pot'),
+ which lists the strings and their translations. The translations
+ are initially empty. The original (usually English) messages serve
+ as the key for lookup of the translations.
+
+ 4. For each language with a translator, 'guide.pot' is copied to a
+ portable object file ('.po') and translations are created and
+ shipped with the application. For example, there might be a
+ 'fr.po' for a French translation.
+
+ 5. Each language's '.po' file is converted into a binary message
+ object ('.gmo') file. A message object file contains the original
+ messages and their translations in a binary format that allows fast
+ lookup of translations at runtime.
+
+ 6. When 'guide' is built and installed, the binary translation files
+ are installed in a standard place.
+
+ 7. For testing and development, it is possible to tell 'gettext' to
+ use '.gmo' files in a different directory than the standard one by
+ using the 'bindtextdomain()' function.
+
+ 8. At runtime, 'guide' looks up each string via a call to 'gettext()'.
+ The returned string is the translated string if available, or the
+ original string if not.
+
+ 9. If necessary, it is possible to access messages from a different
+ text domain than the one belonging to the application, without
+ having to switch the application's default text domain back and
+ forth.
+
+ In C (or C++), the string marking and dynamic translation lookup are
+accomplished by wrapping each string in a call to 'gettext()':
+
+ printf("%s", gettext("Don't Panic!\n"));
+
+ The tools that extract messages from source code pull out all strings
+enclosed in calls to 'gettext()'.
+
+ The GNU 'gettext' developers, recognizing that typing 'gettext(...)'
+over and over again is both painful and ugly to look at, use the macro
+'_' (an underscore) to make things easier:
+
+ /* In the standard header file: */
+ #define _(str) gettext(str)
+
+ /* In the program text: */
+ printf("%s", _("Don't Panic!\n"));
+
+This reduces the typing overhead to just three extra characters per
+string and is considerably easier to read as well.
+
+ There are locale "categories" for different types of locale-related
+information. The defined locale categories that 'gettext' knows about
+are:
+
+'LC_MESSAGES'
+ Text messages. This is the default category for 'gettext'
+ operations, but it is possible to supply a different one
+ explicitly, if necessary. (It is almost never necessary to supply
+ a different category.)
+
+'LC_COLLATE'
+ Text-collation information (i.e., how different characters and/or
+ groups of characters sort in a given language).
+
+'LC_CTYPE'
+ Character-type information (alphabetic, digit, upper- or lowercase,
+ and so on) as well as character encoding. This information is
+ accessed via the POSIX character classes in regular expressions,
+ such as '/[[:alnum:]]/' (*note Bracket Expressions::).
+
+'LC_MONETARY'
+ Monetary information, such as the currency symbol, and whether the
+ symbol goes before or after a number.
+
+'LC_NUMERIC'
+ Numeric information, such as which characters to use for the
+ decimal point and the thousands separator.(2)
+
+'LC_TIME'
+ Time- and date-related information, such as 12- or 24-hour clock,
+ month printed before or after the day in a date, local month
+ abbreviations, and so on.
+
+'LC_ALL'
+ All of the above. (Not too useful in the context of 'gettext'.)
+
+ NOTE: As described in *note Locales::, environment variables with
+ the same name as the locale categories ('LC_CTYPE', 'LC_ALL', etc.)
+ influence 'gawk''s behavior (and that of other utilities).
+
+ Normally, these variables also affect how the 'gettext' library
+ finds translations. However, the 'LANGUAGE' environment variable
+ overrides the 'LC_XXX' variables. Many GNU/Linux systems may
+ define this variable without your knowledge, causing 'gawk' to not
+ find the correct translations. If this happens to you, look to see
+ if 'LANGUAGE' is defined, and if so, use the shell's 'unset'
+ command to remove it.
+
+ For testing translations of 'gawk' itself, you can set the
+'GAWK_LOCALE_DIR' environment variable. See the documentation for the C
+'bindtextdomain()' function and also see *note Other Environment
+Variables::.
+
+ ---------- Footnotes ----------
+
+ (1) For some operating systems, the 'gawk' port doesn't support GNU
+'gettext'. Therefore, these features are not available if you are using
+one of those operating systems. Sorry.
+
+ (2) Americans use a comma every three decimal places and a period for
+the decimal point, while many Europeans do exactly the opposite:
+1,234.56 versus 1.234,56.
+
+
+File: gawk.info, Node: Programmer i18n, Next: Translator i18n, Prev:
Explaining gettext, Up: Internationalization
+
+13.3 Internationalizing 'awk' Programs
+======================================
+
+'gawk' provides the following variables for internationalization:
+
+'TEXTDOMAIN'
+ This variable indicates the application's text domain. For
+ compatibility with GNU 'gettext', the default value is
+ '"messages"'.
+
+'_"your message here"'
+ String constants marked with a leading underscore are candidates
+ for translation at runtime. String constants without a leading
+ underscore are not translated.
+
+ 'gawk' provides the following functions for internationalization:
+
+'dcgettext(STRING [, DOMAIN [, CATEGORY]])'
+ Return the translation of STRING in text domain DOMAIN for locale
+ category CATEGORY. The default value for DOMAIN is the current
+ value of 'TEXTDOMAIN'. The default value for CATEGORY is
+ '"LC_MESSAGES"'.
+
+ If you supply a value for CATEGORY, it must be a string equal to
+ one of the known locale categories described in *note Explaining
+ gettext::. You must also supply a text domain. Use 'TEXTDOMAIN'
+ if you want to use the current domain.
+
+ CAUTION: The order of arguments to the 'awk' version of the
+ 'dcgettext()' function is purposely different from the order
+ for the C version. The 'awk' version's order was chosen to be
+ simple and to allow for reasonable 'awk'-style default
+ arguments.
+
+'dcngettext(STRING1, STRING2, NUMBER [, DOMAIN [, CATEGORY]])'
+ Return the plural form used for NUMBER of the translation of
+ STRING1 and STRING2 in text domain DOMAIN for locale category
+ CATEGORY. STRING1 is the English singular variant of a message,
+ and STRING2 is the English plural variant of the same message. The
+ default value for DOMAIN is the current value of 'TEXTDOMAIN'. The
+ default value for CATEGORY is '"LC_MESSAGES"'.
+
+ The same remarks about argument order as for the 'dcgettext()'
+ function apply.
+
+'bindtextdomain(DIRECTORY [, DOMAIN ])'
+ Change the directory in which 'gettext' looks for '.gmo' files, in
+ case they will not or cannot be placed in the standard locations
+ (e.g., during testing). Return the directory in which DOMAIN is
+ "bound."
+
+ The default DOMAIN is the value of 'TEXTDOMAIN'. If DIRECTORY is
+ the null string ('""'), then 'bindtextdomain()' returns the current
+ binding for the given DOMAIN.
+
+ To use these facilities in your 'awk' program, follow these steps:
+
+ 1. Set the variable 'TEXTDOMAIN' to the text domain of your program.
+ This is best done in a 'BEGIN' rule (*note BEGIN/END::), or it can
+ also be done via the '-v' command-line option (*note Options::):
+
+ BEGIN {
+ TEXTDOMAIN = "guide"
+ ...
+ }
+
+ 2. Mark all translatable strings with a leading underscore ('_')
+ character. It _must_ be adjacent to the opening quote of the
+ string. For example:
+
+ print _"hello, world"
+ x = _"you goofed"
+ printf(_"Number of users is %d\n", nusers)
+
+ 3. If you are creating strings dynamically, you can still translate
+ them, using the 'dcgettext()' built-in function:(1)
+
+ if (groggy)
+ message = dcgettext("%d customers disturbing me\n", "adminprog")
+ else
+ message = dcgettext("enjoying %d customers\n", "adminprog")
+ printf(message, ncustomers)
+
+ Here, the call to 'dcgettext()' supplies a different text domain
+ ('"adminprog"') in which to find the message, but it uses the
+ default '"LC_MESSAGES"' category.
+
+ The previous example only works if 'ncustomers' is greater than
+ one. This example would be better done with 'dcngettext()':
+
+ if (groggy)
+ message = dcngettext("%d customer disturbing me\n",
+ "%d customers disturbing me\n",
+ ncustomers, "adminprog")
+ else
+ message = dcngettext("enjoying %d customer\n",
+ "enjoying %d customers\n",
+ ncustomers, "adminprog")
+ printf(message, ncustomers)
+
+ 4. During development, you might want to put the '.gmo' file in a
+ private directory for testing. This is done with the
+ 'bindtextdomain()' built-in function:
+
+ BEGIN {
+ TEXTDOMAIN = "guide" # our text domain
+ if (Testing) {
+ # where to find our files
+ bindtextdomain("testdir")
+ # joe is in charge of adminprog
+ bindtextdomain("../joe/testdir", "adminprog")
+ }
+ ...
+ }
+
+ *Note I18N Example:: for an example program showing the steps to
+create and use translations from 'awk'.
+
+ ---------- Footnotes ----------
+
+ (1) Thanks to Bruno Haible for this example.
+
+
+File: gawk.info, Node: Translator i18n, Next: I18N Example, Prev:
Programmer i18n, Up: Internationalization
+
+13.4 Translating 'awk' Programs
+===============================
+
+Once a program's translatable strings have been marked, they must be
+extracted to create the initial '.pot' file. As part of translation, it
+is often helpful to rearrange the order in which arguments to 'printf'
+are output.
+
+ 'gawk''s '--gen-pot' command-line option extracts the messages and is
+discussed next. After that, 'printf''s ability to rearrange the order
+for 'printf' arguments at runtime is covered.
+
+* Menu:
+
+* String Extraction:: Extracting marked strings.
+* Printf Ordering:: Rearranging 'printf' arguments.
+* I18N Portability:: 'awk'-level portability issues.
+
+
+File: gawk.info, Node: String Extraction, Next: Printf Ordering, Up:
Translator i18n
+
+13.4.1 Extracting Marked Strings
+--------------------------------
+
+Once your 'awk' program is working, and all the strings have been marked
+and you've set (and perhaps bound) the text domain, it is time to
+produce translations. First, use the '--gen-pot' command-line option to
+create the initial '.pot' file:
+
+ gawk --gen-pot -f guide.awk > guide.pot
+
+ When run with '--gen-pot', 'gawk' does not execute your program.
+Instead, it parses it as usual and prints all marked strings to standard
+output in the format of a GNU 'gettext' Portable Object file. Also
+included in the output are any constant strings that appear as the first
+argument to 'dcgettext()' or as the first and second argument to
+'dcngettext()'.(1) You should distribute the generated '.pot' file with
+your 'awk' program; translators will eventually use it to provide you
+translations that you can also then distribute. *Note I18N Example::
+for the full list of steps to go through to create and test translations
+for 'guide'.
+
+ ---------- Footnotes ----------
+
+ (1) The 'xgettext' utility that comes with GNU 'gettext' can handle
+'.awk' files.
+
+
+File: gawk.info, Node: Printf Ordering, Next: I18N Portability, Prev:
String Extraction, Up: Translator i18n
+
+13.4.2 Rearranging 'printf' Arguments
+-------------------------------------
+
+Format strings for 'printf' and 'sprintf()' (*note Printf::) present a
+special problem for translation. Consider the following:(1)
+
+ printf(_"String `%s' has %d characters\n",
+ string, length(string)))
+
+ A possible German translation for this might be:
+
+ "%d Zeichen lang ist die Zeichenkette `%s'\n"
+
+ The problem should be obvious: the order of the format specifications
+is different from the original! Even though 'gettext()' can return the
+translated string at runtime, it cannot change the argument order in the
+call to 'printf'.
+
+ To solve this problem, 'printf' format specifiers may have an
+additional optional element, which we call a "positional specifier".
+For example:
+
+ "%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"
+
+ Here, the positional specifier consists of an integer count, which
+indicates which argument to use, and a '$'. Counts are one-based, and
+the format string itself is _not_ included. Thus, in the following
+example, 'string' is the first argument and 'length(string)' is the
+second:
+
+ $ gawk 'BEGIN {
+ > string = "Don\47t Panic"
+ > printf "%2$d characters live in \"%1$s\"\n",
+ > string, length(string)
+ > }'
+ -| 11 characters live in "Don't Panic"
+
+ If present, positional specifiers come first in the format
+specification, before the flags, the field width, and/or the precision.
+
+ Positional specifiers can be used with the dynamic field width and
+precision capability:
+
+ $ gawk 'BEGIN {
+ > printf("%*.*s\n", 10, 20, "hello")
+ > printf("%3$*2$.*1$s\n", 20, 10, "hello")
+ > }'
+ -| hello
+ -| hello
+
+ NOTE: When using '*' with a positional specifier, the '*' comes
+ first, then the integer position, and then the '$'. This is
+ somewhat counterintuitive.
+
+ 'gawk' does not allow you to mix regular format specifiers and those
+with positional specifiers in the same string:
+
+ $ gawk 'BEGIN { printf "%d %3$s\n", 1, 2, "hi" }'
+ error-> gawk: cmd. line:1: fatal: must use `count$' on all formats or none
+
+ NOTE: There are some pathological cases that 'gawk' may fail to
+ diagnose. In such cases, the output may not be what you expect.
+ It's still a bad idea to try mixing them, even if 'gawk' doesn't
+ detect it.
+
+ Although positional specifiers can be used directly in 'awk'
+programs, their primary purpose is to help in producing correct
+translations of format strings into languages different from the one in
+which the program is first written.
+
+ ---------- Footnotes ----------
+
+ (1) This example is borrowed from the GNU 'gettext' manual.
+
+
+File: gawk.info, Node: I18N Portability, Prev: Printf Ordering, Up:
Translator i18n
+
+13.4.3 'awk' Portability Issues
+-------------------------------
+
+'gawk''s internationalization features were purposely chosen to have as
+little impact as possible on the portability of 'awk' programs that use
+them to other versions of 'awk'. Consider this program:
+
+ BEGIN {
+ TEXTDOMAIN = "guide"
+ if (Test_Guide) # set with -v
+ bindtextdomain("/test/guide/messages")
+ print _"don't panic!"
+ }
+
+As written, it won't work on other versions of 'awk'. However, it is
+actually almost portable, requiring very little change:
+
+ * Assignments to 'TEXTDOMAIN' won't have any effect, because
+ 'TEXTDOMAIN' is not special in other 'awk' implementations.
+
+ * Non-GNU versions of 'awk' treat marked strings as the concatenation
+ of a variable named '_' with the string following it.(1)
+ Typically, the variable '_' has the null string ('""') as its
+ value, leaving the original string constant as the result.
+
+ * By defining "dummy" functions to replace 'dcgettext()',
+ 'dcngettext()', and 'bindtextdomain()', the 'awk' program can be
+ made to run, but all the messages are output in the original
+ language. For example:
+
+ function bindtextdomain(dir, domain)
+ {
+ return dir
+ }
+
+ function dcgettext(string, domain, category)
+ {
+ return string
+ }
+
+ function dcngettext(string1, string2, number, domain, category)
+ {
+ return (number == 1 ? string1 : string2)
+ }
+
+ * The use of positional specifications in 'printf' or 'sprintf()' is
+ _not_ portable. To support 'gettext()' at the C level, many
+ systems' C versions of 'sprintf()' do support positional
+ specifiers. But it works only if enough arguments are supplied in
+ the function call. Many versions of 'awk' pass 'printf' formats
+ and arguments unchanged to the underlying C library version of
+ 'sprintf()', but only one format and argument at a time. What
+ happens if a positional specification is used is anybody's guess.
+ However, because the positional specifications are primarily for
+ use in _translated_ format strings, and because non-GNU 'awk's
+ never retrieve the translated string, this should not be a problem
+ in practice.
+
+ ---------- Footnotes ----------
+
+ (1) This is good fodder for an "Obfuscated 'awk'" contest.
+
+
+File: gawk.info, Node: I18N Example, Next: Gawk I18N, Prev: Translator
i18n, Up: Internationalization
+
+13.5 A Simple Internationalization Example
+==========================================
+
+Now let's look at a step-by-step example of how to internationalize and
+localize a simple 'awk' program, using 'guide.awk' as our original
+source:
+
+ BEGIN {
+ TEXTDOMAIN = "guide"
+ bindtextdomain(".") # for testing
+ print _"Don't Panic"
+ print _"The Answer Is", 42
+ print "Pardon me, Zaphod who?"
+ }
+
+Run 'gawk --gen-pot' to create the '.pot' file:
+
+ $ gawk --gen-pot -f guide.awk > guide.pot
+
+This produces:
+
+ #: guide.awk:4
+ msgid "Don't Panic"
+ msgstr ""
+
+ #: guide.awk:5
+ msgid "The Answer Is"
+ msgstr ""
+
+
+ This original portable object template file is saved and reused for
+each language into which the application is translated. The 'msgid' is
+the original string and the 'msgstr' is the translation.
+
+ NOTE: Strings not marked with a leading underscore do not appear in
+ the 'guide.pot' file.
+
+ Next, the messages must be translated. Here is a translation to a
+hypothetical dialect of English, called "Mellow":(1)
+
+ $ cp guide.pot guide-mellow.po
+ ADD TRANSLATIONS TO guide-mellow.po ...
+
+Following are the translations:
+
+ #: guide.awk:4
+ msgid "Don't Panic"
+ msgstr "Hey man, relax!"
+
+ #: guide.awk:5
+ msgid "The Answer Is"
+ msgstr "Like, the scoop is"
+
+
+ NOTE: The following instructions apply to GNU/Linux with the GNU C
+ Library. Be aware that the actual steps may change over time, that
+ the following description may not be accurate for all GNU/Linux
+ distributions, and that things may work entirely differently on
+ other operating systems.
+
+ The next step is to make the directory to hold the binary message
+object file and then to create the 'guide.mo' file. The directory has
+the form 'LOCALE/LC_MESSAGES', where LOCALE is a locale name known to
+the C 'gettext' routines.
+
+ How do we know which locale to use? It turns out that there are four
+different environment variables used by the C 'gettext' routines. In
+order, they are '$LANGUAGE', '$LC_ALL', '$LANG', and '$LC_MESSAGES'.(2)
+Thus, we check the value of '$LANGUAGE':
+
+ $ echo $LANGUAGE
+ -| en_US.UTF-8
+
+We next make the directories:
+
+ $ mkdir en_US.UTF-8 en_US.UTF-8/LC_MESSAGES
+
+ The 'msgfmt' utility converts the human-readable '.po' file into a
+machine-readable '.mo' file. By default, 'msgfmt' creates a file named
+'messages'. This file must be renamed and placed in the proper
+directory (using the '-o' option) so that 'gawk' can find it:
+
+ $ msgfmt guide-mellow.po -o en_US.UTF-8/LC_MESSAGES/guide.mo
+
+ Finally, we run the program to test it:
+
+ $ gawk -f guide.awk
+ -| Hey man, relax!
+ -| Like, the scoop is 42
+ -| Pardon me, Zaphod who?
+
+ If the three replacement functions for 'dcgettext()', 'dcngettext()',
+and 'bindtextdomain()' (*note I18N Portability::) are in a file named
+'libintl.awk', then we can run 'guide.awk' unchanged as follows:
+
+ $ gawk --posix -f guide.awk -f libintl.awk
+ -| Don't Panic
+ -| The Answer Is 42
+ -| Pardon me, Zaphod who?
+
+ ---------- Footnotes ----------
+
+ (1) Perhaps it would be better if it were called "Hippy." Ah, well.
+
+ (2) Well, sort of. It seems that if '$LC_ALL' is set to 'C', then no
+translations are done. Go figure.
+
+
+File: gawk.info, Node: Gawk I18N, Next: I18N Summary, Prev: I18N Example,
Up: Internationalization
+
+13.6 'gawk' Can Speak Your Language
+===================================
+
+'gawk' itself has been internationalized using the GNU 'gettext'
+package. (GNU 'gettext' is described in complete detail in *Note GNU
+'gettext' utilities: (gettext)Top.) As of this writing, the latest
+version of GNU 'gettext' is version 0.19.8.1
+(ftp://ftp.gnu.org/gnu/gettext/gettext-0.19.8.1.tar.gz).
+
+ If a translation of 'gawk''s messages exists, then 'gawk' produces
+usage messages, warnings, and fatal errors in the local language.
+
+
+File: gawk.info, Node: I18N Summary, Prev: Gawk I18N, Up:
Internationalization
+
+13.7 Summary
+============
+
+ * Internationalization means writing a program such that it can use
+ multiple languages without requiring source code changes.
+ Localization means providing the data necessary for an
+ internationalized program to work in a particular language.
+
+ * 'gawk' uses GNU 'gettext' to let you internationalize and localize
+ 'awk' programs. A program's text domain identifies the program for
+ grouping all messages and other data together.
+
+ * You mark a program's strings for translation by preceding them with
+ an underscore. Once that is done, the strings are extracted into a
+ '.pot' file. This file is copied for each language into a '.po'
+ file, and the '.po' files are compiled into '.gmo' files for use at
+ runtime.
+
+ * You can use positional specifications with 'sprintf()' and 'printf'
+ to rearrange the placement of argument values in formatted strings
+ and output. This is useful for the translation of format control
+ strings.
+
+ * The internationalization features have been designed so that they
+ can be easily worked around in a standard 'awk'.
+
+ * 'gawk' itself has been internationalized and ships with a number of
+ translations for its messages.
+
+
+File: gawk.info, Node: Debugger, Next: Namespaces, Prev:
Internationalization, Up: Top
+
+14 Debugging 'awk' Programs
+***************************
+
+It would be nice if computer programs worked perfectly the first time
+they were run, but in real life, this rarely happens for programs of any
+complexity. Thus, most programming languages have facilities available
+for "debugging" programs, and 'awk' is no exception.
+
+ The 'gawk' debugger is purposely modeled after the GNU Debugger (GDB)
+(https://www.gnu.org/software/gdb/) command-line debugger. If you are
+familiar with GDB, learning how to use 'gawk' for debugging your
+programs is easy.
+
+* Menu:
+
+* Debugging:: Introduction to 'gawk' debugger.
+* Sample Debugging Session:: Sample debugging session.
+* List of Debugger Commands:: Main debugger commands.
+* Readline Support:: Readline support.
+* Limitations:: Limitations and future plans.
+* Debugging Summary:: Debugging summary.
+
+
+File: gawk.info, Node: Debugging, Next: Sample Debugging Session, Up:
Debugger
+
+14.1 Introduction to the 'gawk' Debugger
+========================================
+
+This minor node introduces debugging in general and begins the
+discussion of debugging in 'gawk'.
+
+* Menu:
+
+* Debugging Concepts:: Debugging in General.
+* Debugging Terms:: Additional Debugging Concepts.
+* Awk Debugging:: Awk Debugging.
+
+
+File: gawk.info, Node: Debugging Concepts, Next: Debugging Terms, Up:
Debugging
+
+14.1.1 Debugging in General
+---------------------------
+
+(If you have used debuggers in other languages, you may want to skip
+ahead to *note Awk Debugging::.)
+
+ Of course, a debugging program cannot remove bugs for you, because it
+has no way of knowing what you or your users consider a "bug" versus a
+"feature." (Sometimes, we humans have a hard time with this ourselves.)
+In that case, what can you expect from such a tool? The answer to that
+depends on the language being debugged, but in general, you can expect
+at least the following:
+
+ * The ability to watch a program execute its instructions one by one,
+ giving you, the programmer, the opportunity to think about what is
+ happening on a time scale of seconds, minutes, or hours, rather
+ than the nanosecond time scale at which the code usually runs.
+
+ * The opportunity to not only passively observe the operation of your
+ program, but to control it and try different paths of execution,
+ without having to change your source files.
+
+ * The chance to see the values of data in the program at any point in
+ execution, and also to change that data on the fly, to see how that
+ affects what happens afterward. (This often includes the ability
+ to look at internal data structures besides the variables you
+ actually defined in your code.)
+
+ * The ability to obtain additional information about your program's
+ state or even its internal structure.
+
+ All of these tools provide a great amount of help in using your own
+skills and understanding of the goals of your program to find where it
+is going wrong (or, for that matter, to better comprehend a perfectly
+functional program that you or someone else wrote).
+
+
+File: gawk.info, Node: Debugging Terms, Next: Awk Debugging, Prev:
Debugging Concepts, Up: Debugging
+
+14.1.2 Debugging Concepts
+-------------------------
+
+Before diving in to the details, we need to introduce several important
+concepts that apply to just about all debuggers. The following list
+defines terms used throughout the rest of this major node:
+
+"Stack frame"
+ Programs generally call functions during the course of their
+ execution. One function can call another, or a function can call
+ itself (recursion). You can view the chain of called functions
+ (main program calls A, which calls B, which calls C), as a stack of
+ executing functions: the currently running function is the topmost
+ one on the stack, and when it finishes (returns), the next one down
+ then becomes the active function. Such a stack is termed a "call
+ stack".
+
+ For each function on the call stack, the system maintains a data
+ area that contains the function's parameters, local variables, and
+ return value, as well as any other "bookkeeping" information needed
+ to manage the call stack. This data area is termed a "stack
+ frame".
+
+ 'gawk' also follows this model, and gives you access to the call
+ stack and to each stack frame. You can see the call stack, as well
+ as from where each function on the stack was invoked. Commands
+ that print the call stack print information about each stack frame
+ (as detailed later on).
+
+"Breakpoint"
+ During debugging, you often wish to let the program run until it
+ reaches a certain point, and then continue execution from there one
+ statement (or instruction) at a time. The way to do this is to set
+ a "breakpoint" within the program. A breakpoint is where the
+ execution of the program should break off (stop), so that you can
+ take over control of the program's execution. You can add and
+ remove as many breakpoints as you like.
+
+"Watchpoint"
+ A watchpoint is similar to a breakpoint. The difference is that
+ breakpoints are oriented around the code: stop when a certain point
+ in the code is reached. A watchpoint, however, specifies that
+ program execution should stop when a _data value_ is changed. This
+ is useful, as sometimes it happens that a variable receives an
+ erroneous value, and it's hard to track down where this happens
+ just by looking at the code. By using a watchpoint, you can stop
+ whenever a variable is assigned to, and usually find the errant
+ code quite quickly.
+
+
+File: gawk.info, Node: Awk Debugging, Prev: Debugging Terms, Up: Debugging
+
+14.1.3 'awk' Debugging
+----------------------
+
+Debugging an 'awk' program has some specific aspects that are not shared
+with programs written in other languages.
+
+ First of all, the fact that 'awk' programs usually take input line by
+line from a file or files and operate on those lines using specific
+rules makes it especially useful to organize viewing the execution of
+the program in terms of these rules. As we will see, each 'awk' rule is
+treated almost like a function call, with its own specific block of
+instructions.
+
+ In addition, because 'awk' is by design a very concise language, it
+is easy to lose sight of everything that is going on "inside" each line
+of 'awk' code. The debugger provides the opportunity to look at the
+individual primitive instructions carried out by the higher-level 'awk'
+commands.(1)
+
+ ---------- Footnotes ----------
+
+ (1) The "primitive instructions" are defined by 'gawk' itself; the
+debugger does not work at the level of machine instructions.
+
+
+File: gawk.info, Node: Sample Debugging Session, Next: List of Debugger
Commands, Prev: Debugging, Up: Debugger
+
+14.2 Sample 'gawk' Debugging Session
+====================================
+
+In order to illustrate the use of 'gawk' as a debugger, let's look at a
+sample debugging session. We will use the 'awk' implementation of the
+POSIX 'uniq' command presented earlier (*note Uniq Program::) as our
+example.
+
+* Menu:
+
+* Debugger Invocation:: How to Start the Debugger.
+* Finding The Bug:: Finding the Bug.
+
+
+File: gawk.info, Node: Debugger Invocation, Next: Finding The Bug, Up:
Sample Debugging Session
+
+14.2.1 How to Start the Debugger
+--------------------------------
+
+Starting the debugger is almost exactly like running 'gawk' normally,
+except you have to pass an additional option, '--debug', or the
+corresponding short option, '-D'. The file(s) containing the program
+and any supporting code are given on the command line as arguments to
+one or more '-f' options. ('gawk' is not designed to debug command-line
+programs, only programs contained in files.) In our case, we invoke the
+debugger like this:
+
+ $ gawk -D -f getopt.awk -f join.awk -f uniq.awk -1 inputfile
+
+where both 'getopt.awk' and 'uniq.awk' are in '$AWKPATH'. (Experienced
+users of GDB or similar debuggers should note that this syntax is
+slightly different from what you are used to. With the 'gawk' debugger,
+you give the arguments for running the program in the command line to
+the debugger rather than as part of the 'run' command at the debugger
+prompt.) The '-1' is an option to 'uniq.awk'.
+
+ Instead of immediately running the program on 'inputfile', as 'gawk'
+would ordinarily do, the debugger merely loads all the program source
+files, compiles them internally, and then gives us a prompt:
+
+ gawk>
+
+from which we can issue commands to the debugger. At this point, no
+code has been executed.
+
+
+File: gawk.info, Node: Finding The Bug, Prev: Debugger Invocation, Up:
Sample Debugging Session
+
+14.2.2 Finding the Bug
+----------------------
+
+Let's say that we are having a problem using (a faulty version of)
+'uniq.awk' in "field-skipping" mode, and it doesn't seem to be catching
+lines which should be identical when skipping the first field, such as:
+
+ awk is a wonderful program!
+ gawk is a wonderful program!
+
+ This could happen if we were thinking (C-like) of the fields in a
+record as being numbered in a zero-based fashion, so instead of the
+lines:
+
+ clast = join(alast, fcount+1, n)
+ cline = join(aline, fcount+1, m)
+
+we wrote:
+
+ clast = join(alast, fcount, n)
+ cline = join(aline, fcount, m)
+
+ The first thing we usually want to do when trying to investigate a
+problem like this is to put a breakpoint in the program so that we can
+watch it at work and catch what it is doing wrong. A reasonable spot
+for a breakpoint in 'uniq.awk' is at the beginning of the function
+'are_equal()', which compares the current line with the previous one.
+To set the breakpoint, use the 'b' (breakpoint) command:
+
+ gawk> b are_equal
+ -| Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 63
+
+ The debugger tells us the file and line number where the breakpoint
+is. Now type 'r' or 'run' and the program runs until it hits the
+breakpoint for the first time:
+
+ gawk> r
+ -| Starting program:
+ -| Stopping in Rule ...
+ -| Breakpoint 1, are_equal(n, m, clast, cline, alast, aline)
+ at `awklib/eg/prog/uniq.awk':63
+ -| 63 if (fcount == 0 && charcount == 0)
+ gawk>
+
+ Now we can look at what's going on inside our program. First of all,
+let's see how we got to where we are. At the prompt, we type 'bt'
+(short for "backtrace"), and the debugger responds with a listing of the
+current stack frames:
+
+ gawk> bt
+ -| #0 are_equal(n, m, clast, cline, alast, aline)
+ at `awklib/eg/prog/uniq.awk':68
+ -| #1 in main() at `awklib/eg/prog/uniq.awk':88
+
+ This tells us that 'are_equal()' was called by the main program at
+line 88 of 'uniq.awk'. (This is not a big surprise, because this is the
+only call to 'are_equal()' in the program, but in more complex programs,
+knowing who called a function and with what parameters can be the key to
+finding the source of the problem.)
+
+ Now that we're in 'are_equal()', we can start looking at the values
+of some variables. Let's say we type 'p n' ('p' is short for "print").
+We would expect to see the value of 'n', a parameter to 'are_equal()'.
+Actually, the debugger gives us:
+
+ gawk> p n
+ -| n = untyped variable
+
+In this case, 'n' is an uninitialized local variable, because the
+function was called without arguments (*note Function Calls::).
+
+ A more useful variable to display might be the current record:
+
+ gawk> p $0
+ -| $0 = "gawk is a wonderful program!"
+
+This might be a bit puzzling at first, as this is the second line of our
+test input. Let's look at 'NR':
+
+ gawk> p NR
+ -| NR = 2
+
+So we can see that 'are_equal()' was only called for the second record
+of the file. Of course, this is because our program contains a rule for
+'NR == 1':
+
+ NR == 1 {
+ last = $0
+ next
+ }
+
+ OK, let's just check that that rule worked correctly:
+
+ gawk> p last
+ -| last = "awk is a wonderful program!"
+
+ Everything we have done so far has verified that the program has
+worked as planned, up to and including the call to 'are_equal()', so the
+problem must be inside this function. To investigate further, we must
+begin "stepping through" the lines of 'are_equal()'. We start by typing
+'n' (for "next"):
+
+ gawk> n
+ -| 66 if (fcount > 0) {
+
+ This tells us that 'gawk' is now ready to execute line 66, which
+decides whether to give the lines the special "field-skipping" treatment
+indicated by the '-1' command-line option. (Notice that we skipped from
+where we were before, at line 63, to here, because the condition in line
+63, 'if (fcount == 0 && charcount == 0)', was false.)
+
+ Continuing to step, we now get to the splitting of the current and
+last records:
+
+ gawk> n
+ -| 67 n = split(last, alast)
+ gawk> n
+ -| 68 m = split($0, aline)
+
+ At this point, we should be curious to see what our records were
+split into, so we try to look:
+
+ gawk> p n m alast aline
+ -| n = 5
+ -| m = untyped variable
+ -| alast = array, 5 elements
+ -| aline = untyped variable
+
+(The 'p' command can take more than one argument, similar to 'awk''s
+'print' statement.)
+
+ This is kind of disappointing, though. All we found out is that
+there are five elements in 'alast'; 'm' and 'aline' don't have values
+because we are at line 68 but haven't executed it yet. This information
+is useful enough (we now know that none of the words were accidentally
+left out), but what if we want to see inside the array?
+
+ The first choice would be to use subscripts:
+
+ gawk> p alast[0]
+ -| "0" not in array `alast'
+
+Oops!
+
+ gawk> p alast[1]
+ -| alast["1"] = "awk"
+
+ This would be kind of slow for a 100-member array, though, so 'gawk'
+provides a shortcut (reminiscent of another language not to be
+mentioned):
+
+ gawk> p @alast
+ -| alast["1"] = "awk"
+ -| alast["2"] = "is"
+ -| alast["3"] = "a"
+ -| alast["4"] = "wonderful"
+ -| alast["5"] = "program!"
+
+ It looks like we got this far OK. Let's take another step or two:
+
+ gawk> n
+ -| 69 clast = join(alast, fcount, n)
+ gawk> n
+ -| 70 cline = join(aline, fcount, m)
+
+ Well, here we are at our error (sorry to spoil the suspense). What
+we had in mind was to join the fields starting from the second one to
+make the virtual record to compare, and if the first field were numbered
+zero, this would work. Let's look at what we've got:
+
+ gawk> p cline clast
+ -| cline = "gawk is a wonderful program!"
+ -| clast = "awk is a wonderful program!"
+
+ Hey, those look pretty familiar! They're just our original,
+unaltered input records. A little thinking (the human brain is still
+the best debugging tool), and we realize that we were off by one!
+
+ We get out of the debugger:
+
+ gawk> q
+ -| The program is running. Exit anyway (y/n)? y
+
+Then we get into an editor:
+
+ clast = join(alast, fcount+1, n)
+ cline = join(aline, fcount+1, m)
+
+and problem solved!
+
+
+File: gawk.info, Node: List of Debugger Commands, Next: Readline Support,
Prev: Sample Debugging Session, Up: Debugger
+
+14.3 Main Debugger Commands
+===========================
+
+The 'gawk' debugger command set can be divided into the following
+categories:
+
+ * Breakpoint control
+
+ * Execution control
+
+ * Viewing and changing data
+
+ * Working with the stack
+
+ * Getting information
+
+ * Miscellaneous
+
+ Each of these are discussed in the following subsections. In the
+following descriptions, commands that may be abbreviated show the
+abbreviation on a second description line. A debugger command name may
+also be truncated if that partial name is unambiguous. The debugger has
+the built-in capability to automatically repeat the previous command
+just by hitting 'Enter'. This works for the commands 'list', 'next',
+'nexti', 'step', 'stepi', and 'continue' executed without any argument.
+
+* Menu:
+
+* Breakpoint Control:: Control of Breakpoints.
+* Debugger Execution Control:: Control of Execution.
+* Viewing And Changing Data:: Viewing and Changing Data.
+* Execution Stack:: Dealing with the Stack.
+* Debugger Info:: Obtaining Information about the Program and
+ the Debugger State.
+* Miscellaneous Debugger Commands:: Miscellaneous Commands.
+
+
+File: gawk.info, Node: Breakpoint Control, Next: Debugger Execution Control,
Up: List of Debugger Commands
+
+14.3.1 Control of Breakpoints
+-----------------------------
+
+As we saw earlier, the first thing you probably want to do in a
+debugging session is to get your breakpoints set up, because your
+program will otherwise just run as if it was not under the debugger.
+The commands for controlling breakpoints are:
+
+'break' [[FILENAME':']N | FUNCTION] ['"EXPRESSION"']
+'b' [[FILENAME':']N | FUNCTION] ['"EXPRESSION"']
+ Without any argument, set a breakpoint at the next instruction to
+ be executed in the selected stack frame. Arguments can be one of
+ the following:
+
+ N
+ Set a breakpoint at line number N in the current source file.
+
+ FILENAME':'N
+ Set a breakpoint at line number N in source file FILENAME.
+
+ FUNCTION
+ Set a breakpoint at entry to (the first instruction of)
+ function FUNCTION.
+
+ Each breakpoint is assigned a number that can be used to delete it
+ from the breakpoint list using the 'delete' command.
+
+ With a breakpoint, you may also supply a condition. This is an
+ 'awk' expression (enclosed in double quotes) that the debugger
+ evaluates whenever the breakpoint is reached. If the condition is
+ true, then the debugger stops execution and prompts for a command.
+ Otherwise, it continues executing the program.
+
+'clear' [[FILENAME':']N | FUNCTION]
+ Without any argument, delete any breakpoint at the next instruction
+ to be executed in the selected stack frame. If the program stops
+ at a breakpoint, this deletes that breakpoint so that the program
+ does not stop at that location again. Arguments can be one of the
+ following:
+
+ N
+ Delete breakpoint(s) set at line number N in the current
+ source file.
+
+ FILENAME':'N
+ Delete breakpoint(s) set at line number N in source file
+ FILENAME.
+
+ FUNCTION
+ Delete breakpoint(s) set at entry to function FUNCTION.
+
+'condition' N '"EXPRESSION"'
+ Add a condition to existing breakpoint or watchpoint N. The
+ condition is an 'awk' expression _enclosed in double quotes_ that
+ the debugger evaluates whenever the breakpoint or watchpoint is
+ reached. If the condition is true, then the debugger stops
+ execution and prompts for a command. Otherwise, the debugger
+ continues executing the program. If the condition expression is
+ not specified, any existing condition is removed (i.e., the
+ breakpoint or watchpoint is made unconditional).
+
+'delete' [N1 N2 ...] [N-M]
+'d' [N1 N2 ...] [N-M]
+ Delete specified breakpoints or a range of breakpoints. Delete all
+ defined breakpoints if no argument is supplied.
+
+'disable' [N1 N2 ... | N-M]
+ Disable specified breakpoints or a range of breakpoints. Without
+ any argument, disable all breakpoints.
+
+'enable' ['del' | 'once'] [N1 N2 ...] [N-M]
+'e' ['del' | 'once'] [N1 N2 ...] [N-M]
+ Enable specified breakpoints or a range of breakpoints. Without
+ any argument, enable all breakpoints. Optionally, you can specify
+ how to enable the breakpoints:
+
+ 'del'
+ Enable the breakpoints temporarily, then delete each one when
+ the program stops at it.
+
+ 'once'
+ Enable the breakpoints temporarily, then disable each one when
+ the program stops at it.
+
+'ignore' N COUNT
+ Ignore breakpoint number N the next COUNT times it is hit.
+
+'tbreak' [[FILENAME':']N | FUNCTION]
+'t' [[FILENAME':']N | FUNCTION]
+ Set a temporary breakpoint (enabled for only one stop). The
+ arguments are the same as for 'break'.
+
+
+File: gawk.info, Node: Debugger Execution Control, Next: Viewing And
Changing Data, Prev: Breakpoint Control, Up: List of Debugger Commands
+
+14.3.2 Control of Execution
+---------------------------
+
+Now that your breakpoints are ready, you can start running the program
+and observing its behavior. There are more commands for controlling
+execution of the program than we saw in our earlier example:
+
+'commands' [N]
+'silent'
+...
+'end'
+ Set a list of commands to be executed upon stopping at a breakpoint
+ or watchpoint. N is the breakpoint or watchpoint number. Without
+ a number, the last one set is used. The actual commands follow,
+ starting on the next line, and terminated by the 'end' command. If
+ the command 'silent' is in the list, the usual messages about
+ stopping at a breakpoint and the source line are not printed. Any
+ command in the list that resumes execution (e.g., 'continue')
+ terminates the list (an implicit 'end'), and subsequent commands
+ are ignored. For example:
+
+ gawk> commands
+ > silent
+ > printf "A silent breakpoint; i = %d\n", i
+ > info locals
+ > set i = 10
+ > continue
+ > end
+ gawk>
+
+'continue' [COUNT]
+'c' [COUNT]
+ Resume program execution. If continued from a breakpoint and COUNT
+ is specified, ignore the breakpoint at that location the next COUNT
+ times before stopping.
+
+'finish'
+ Execute until the selected stack frame returns. Print the returned
+ value.
+
+'next' [COUNT]
+'n' [COUNT]
+ Continue execution to the next source line, stepping over function
+ calls. The argument COUNT controls how many times to repeat the
+ action, as in 'step'.
+
+'nexti' [COUNT]
+'ni' [COUNT]
+ Execute one (or COUNT) instruction(s), stepping over function
+ calls.
+
+'return' [VALUE]
+ Cancel execution of a function call. If VALUE (either a string or
+ a number) is specified, it is used as the function's return value.
+ If used in a frame other than the innermost one (the currently
+ executing function; i.e., frame number 0), discard all inner frames
+ in addition to the selected one, and the caller of that frame
+ becomes the innermost frame.
+
+'run'
+'r'
+ Start/restart execution of the program. When restarting, the
+ debugger retains the current breakpoints, watchpoints, command
+ history, automatic display variables, and debugger options.
+
+'step' [COUNT]
+'s' [COUNT]
+ Continue execution until control reaches a different source line in
+ the current stack frame, stepping inside any function called within
+ the line. If the argument COUNT is supplied, steps that many times
+ before stopping, unless it encounters a breakpoint or watchpoint.
+
+'stepi' [COUNT]
+'si' [COUNT]
+ Execute one (or COUNT) instruction(s), stepping inside function
+ calls. (For illustration of what is meant by an "instruction" in
+ 'gawk', see the output shown under 'dump' in *note Miscellaneous
+ Debugger Commands::.)
+
+'until' [[FILENAME':']N | FUNCTION]
+'u' [[FILENAME':']N | FUNCTION]
+ Without any argument, continue execution until a line past the
+ current line in the current stack frame is reached. With an
+ argument, continue execution until the specified location is
+ reached, or the current stack frame returns.
+
+
+File: gawk.info, Node: Viewing And Changing Data, Next: Execution Stack,
Prev: Debugger Execution Control, Up: List of Debugger Commands
+
+14.3.3 Viewing and Changing Data
+--------------------------------
+
+The commands for viewing and changing variables inside of 'gawk' are:
+
+'display' [VAR | '$'N]
+ Add variable VAR (or field '$N') to the display list. The value of
+ the variable or field is displayed each time the program stops.
+ Each variable added to the list is identified by a unique number:
+
+ gawk> display x
+ -| 10: x = 1
+
+ This displays the assigned item number, the variable name, and its
+ current value. If the display variable refers to a function
+ parameter, it is silently deleted from the list as soon as the
+ execution reaches a context where no such variable of the given
+ name exists. Without argument, 'display' displays the current
+ values of items on the list.
+
+'eval "AWK STATEMENTS"'
+ Evaluate AWK STATEMENTS in the context of the running program. You
+ can do anything that an 'awk' program would do: assign values to
+ variables, call functions, and so on.
+
+ NOTE: You cannot use 'eval' to execute a statement containing
+ any of the following: 'exit', 'getline', 'next', 'nextfile',
+ or 'return'.
+
+'eval' PARAM, ...
+AWK STATEMENTS
+'end'
+ This form of 'eval' is similar, but it allows you to define "local
+ variables" that exist in the context of the AWK STATEMENTS, instead
+ of using variables or function parameters defined by the program.
+
+'print' VAR1[',' VAR2 ...]
+'p' VAR1[',' VAR2 ...]
+ Print the value of a 'gawk' variable or field. Fields must be
+ referenced by constants:
+
+ gawk> print $3
+
+ This prints the third field in the input record (if the specified
+ field does not exist, it prints 'Null field'). A variable can be
+ an array element, with the subscripts being constant string values.
+ To print the contents of an array, prefix the name of the array
+ with the '@' symbol:
+
+ gawk> print @a
+
+ This prints the indices and the corresponding values for all
+ elements in the array 'a'.
+
+'printf' FORMAT [',' ARG ...]
+ Print formatted text. The FORMAT may include escape sequences,
+ such as '\n' (*note Escape Sequences::). No newline is printed
+ unless one is specified.
+
+'set' VAR'='VALUE
+ Assign a constant (number or string) value to an 'awk' variable or
+ field. String values must be enclosed between double quotes
+ ('"'...'"').
+
+ You can also set special 'awk' variables, such as 'FS', 'NF', 'NR',
+ and so on.
+
+'watch' VAR | '$'N ['"EXPRESSION"']
+'w' VAR | '$'N ['"EXPRESSION"']
+ Add variable VAR (or field '$N') to the watch list. The debugger
+ then stops whenever the value of the variable or field changes.
+ Each watched item is assigned a number that can be used to delete
+ it from the watch list using the 'unwatch' command.
+
+ With a watchpoint, you may also supply a condition. This is an
+ 'awk' expression (enclosed in double quotes) that the debugger
+ evaluates whenever the watchpoint is reached. If the condition is
+ true, then the debugger stops execution and prompts for a command.
+ Otherwise, 'gawk' continues executing the program.
+
+'undisplay' [N]
+ Remove item number N (or all items, if no argument) from the
+ automatic display list.
+
+'unwatch' [N]
+ Remove item number N (or all items, if no argument) from the watch
+ list.
+
+
+File: gawk.info, Node: Execution Stack, Next: Debugger Info, Prev: Viewing
And Changing Data, Up: List of Debugger Commands
+
+14.3.4 Working with the Stack
+-----------------------------
+
+Whenever you run a program that contains any function calls, 'gawk'
+maintains a stack of all of the function calls leading up to where the
+program is right now. You can see how you got to where you are, and
+also move around in the stack to see what the state of things was in the
+functions that called the one you are in. The commands for doing this
+are:
+
+'backtrace' [COUNT]
+'bt' [COUNT]
+'where' [COUNT]
+ Print a backtrace of all function calls (stack frames), or
+ innermost COUNT frames if COUNT > 0. Print the outermost COUNT
+ frames if COUNT < 0. The backtrace displays the name and arguments
+ to each function, the source file name, and the line number. The
+ alias 'where' for 'backtrace' is provided for longtime GDB users
+ who may be used to that command.
+
+'down' [COUNT]
+ Move COUNT (default 1) frames down the stack toward the innermost
+ frame. Then select and print the frame.
+
+'frame' [N]
+'f' [N]
+ Select and print stack frame N. Frame 0 is the currently
+ executing, or "innermost", frame (function call); frame 1 is the
+ frame that called the innermost one. The highest-numbered frame is
+ the one for the main program. The printed information consists of
+ the frame number, function and argument names, source file, and the
+ source line.
+
+'up' [COUNT]
+ Move COUNT (default 1) frames up the stack toward the outermost
+ frame. Then select and print the frame.
+
+
+File: gawk.info, Node: Debugger Info, Next: Miscellaneous Debugger Commands,
Prev: Execution Stack, Up: List of Debugger Commands
+
+14.3.5 Obtaining Information About the Program and the Debugger State
+---------------------------------------------------------------------
+
+Besides looking at the values of variables, there is often a need to get
+other sorts of information about the state of your program and of the
+debugging environment itself. The 'gawk' debugger has one command that
+provides this information, appropriately called 'info'. 'info' is used
+with one of a number of arguments that tell it exactly what you want to
+know:
+
+'info' WHAT
+'i' WHAT
+ The value for WHAT should be one of the following:
+
+ 'args'
+ List arguments of the selected frame.
+
+ 'break'
+ List all currently set breakpoints.
+
+ 'display'
+ List all items in the automatic display list.
+
+ 'frame'
+ Give a description of the selected stack frame.
+
+ 'functions'
+ List all function definitions including source file names and
+ line numbers.
+
+ 'locals'
+ List local variables of the selected frame.
+
+ 'source'
+ Print the name of the current source file. Each time the
+ program stops, the current source file is the file containing
+ the current instruction. When the debugger first starts, the
+ current source file is the first file included via the '-f'
+ option. The 'list FILENAME:LINENO' command can be used at any
+ time to change the current source.
+
+ 'sources'
+ List all program sources.
+
+ 'variables'
+ List all global variables.
+
+ 'watch'
+ List all items in the watch list.
+
+ Additional commands give you control over the debugger, the ability
+to save the debugger's state, and the ability to run debugger commands
+from a file. The commands are:
+
+'option' [NAME['='VALUE]]
+'o' [NAME['='VALUE]]
+ Without an argument, display the available debugger options and
+ their current values. 'option NAME' shows the current value of the
+ named option. 'option NAME=VALUE' assigns a new value to the named
+ option. The available options are:
+
+ 'history_size'
+ Set the maximum number of lines to keep in the history file
+ './.gawk_history'. The default is 100.
+
+ 'listsize'
+ Specify the number of lines that 'list' prints. The default
+ is 15.
+
+ 'outfile'
+ Send 'gawk' output to a file; debugger output still goes to
+ standard output. An empty string ('""') resets output to
+ standard output.
+
+ 'prompt'
+ Change the debugger prompt. The default is 'gawk> '.
+
+ 'save_history' ['on' | 'off']
+ Save command history to file './.gawk_history'. The default
+ is 'on'.
+
+ 'save_options' ['on' | 'off']
+ Save current options to file './.gawkrc' upon exit. The
+ default is 'on'. Options are read back into the next session
+ upon startup.
+
+ 'trace' ['on' | 'off']
+ Turn instruction tracing on or off. The default is 'off'.
+
+'save' FILENAME
+ Save the commands from the current session to the given file name,
+ so that they can be replayed using the 'source' command.
+
+'source' FILENAME
+ Run command(s) from a file; an error in any command does not
+ terminate execution of subsequent commands. Comments (lines
+ starting with '#') are allowed in a command file. Empty lines are
+ ignored; they do _not_ repeat the last command. You can't restart
+ the program by having more than one 'run' command in the file.
+ Also, the list of commands may include additional 'source'
+ commands; however, the 'gawk' debugger will not source the same
+ file more than once in order to avoid infinite recursion.
+
+ In addition to, or instead of, the 'source' command, you can use
+ the '-D FILE' or '--debug=FILE' command-line options to execute
+ commands from a file non-interactively (*note Options::).
+
+
+File: gawk.info, Node: Miscellaneous Debugger Commands, Prev: Debugger Info,
Up: List of Debugger Commands
+
+14.3.6 Miscellaneous Commands
+-----------------------------
+
+There are a few more commands that do not fit into the previous
+categories, as follows:
+
+'dump' [FILENAME]
+ Dump byte code of the program to standard output or to the file
+ named in FILENAME. This prints a representation of the internal
+ instructions that 'gawk' executes to implement the 'awk' commands
+ in a program. This can be very enlightening, as the following
+ partial dump of Davide Brini's obfuscated code (*note Signature
+ Program::) demonstrates:
+
+ gawk> dump
+ -| # BEGIN
+ -|
+ -| [ 1:0xfcd340] Op_rule : [in_rule = BEGIN] [source_file
= brini.awk]
+ -| [ 1:0xfcc240] Op_push_i : "~" [MALLOC|STRING|STRCUR]
+ -| [ 1:0xfcc2a0] Op_push_i : "~" [MALLOC|STRING|STRCUR]
+ -| [ 1:0xfcc280] Op_match :
+ -| [ 1:0xfcc1e0] Op_store_var : O
+ -| [ 1:0xfcc2e0] Op_push_i : "==" [MALLOC|STRING|STRCUR]
+ -| [ 1:0xfcc340] Op_push_i : "==" [MALLOC|STRING|STRCUR]
+ -| [ 1:0xfcc320] Op_equal :
+ -| [ 1:0xfcc200] Op_store_var : o
+ -| [ 1:0xfcc380] Op_push : o
+ -| [ 1:0xfcc360] Op_plus_i : 0 [MALLOC|NUMCUR|NUMBER]
+ -| [ 1:0xfcc220] Op_push_lhs : o [do_reference = true]
+ -| [ 1:0xfcc300] Op_assign_plus :
+ -| [ :0xfcc2c0] Op_pop :
+ -| [ 1:0xfcc400] Op_push : O
+ -| [ 1:0xfcc420] Op_push_i : "" [MALLOC|STRING|STRCUR]
+ -| [ :0xfcc4a0] Op_no_op :
+ -| [ 1:0xfcc480] Op_push : O
+ -| [ :0xfcc4c0] Op_concat : [expr_count = 3] [concat_flag
= 0]
+ -| [ 1:0xfcc3c0] Op_store_var : x
+ -| [ 1:0xfcc440] Op_push_lhs : X [do_reference = true]
+ -| [ 1:0xfcc3a0] Op_postincrement :
+ -| [ 1:0xfcc4e0] Op_push : x
+ -| [ 1:0xfcc540] Op_push : o
+ -| [ 1:0xfcc500] Op_plus :
+ -| [ 1:0xfcc580] Op_push : o
+ -| [ 1:0xfcc560] Op_plus :
+ -| [ 1:0xfcc460] Op_leq :
+ -| [ :0xfcc5c0] Op_jmp_false : [target_jmp = 0xfcc5e0]
+ -| [ 1:0xfcc600] Op_push_i : "%c" [MALLOC|STRING|STRCUR]
+ -| [ :0xfcc660] Op_no_op :
+ -| [ 1:0xfcc520] Op_assign_concat : c
+ -| [ :0xfcc620] Op_jmp : [target_jmp = 0xfcc440]
+ ...
+ -| [ 2:0xfcc5a0] Op_K_printf : [expr_count = 17]
[redir_type = ""]
+ -| [ :0xfcc140] Op_no_op :
+ -| [ :0xfcc1c0] Op_atexit :
+ -| [ :0xfcc640] Op_stop :
+ -| [ :0xfcc180] Op_no_op :
+ -| [ :0xfcd150] Op_after_beginfile :
+ -| [ :0xfcc160] Op_no_op :
+ -| [ :0xfcc1a0] Op_after_endfile :
+ gawk>
+
+'exit'
+ Exit the debugger. See the entry for 'quit', later in this list.
+
+'help'
+'h'
+ Print a list of all of the 'gawk' debugger commands with a short
+ summary of their usage. 'help COMMAND' prints the information
+ about the command COMMAND.
+
+'list' ['-' | '+' | N | FILENAME':'N | N-M | FUNCTION]
+'l' ['-' | '+' | N | FILENAME':'N | N-M | FUNCTION]
+ Print the specified lines (default 15) from the current source file
+ or the file named FILENAME. The possible arguments to 'list' are
+ as follows:
+
+ '-' (Minus)
+ Print lines before the lines last printed.
+
+ '+'
+ Print lines after the lines last printed. 'list' without any
+ argument does the same thing.
+
+ N
+ Print lines centered around line number N.
+
+ N-M
+ Print lines from N to M.
+
+ FILENAME':'N
+ Print lines centered around line number N in source file
+ FILENAME. This command may change the current source file.
+
+ FUNCTION
+ Print lines centered around the beginning of the function
+ FUNCTION. This command may change the current source file.
+
+'quit'
+'q'
+ Exit the debugger. Debugging is great fun, but sometimes we all
+ have to tend to other obligations in life, and sometimes we find
+ the bug and are free to go on to the next one! As we saw earlier,
+ if you are running a program, the debugger warns you when you type
+ 'q' or 'quit', to make sure you really want to quit.
+
+'trace' ['on' | 'off']
+ Turn on or off continuous printing of the instructions that are
+ about to be executed, along with the 'awk' lines they implement.
+ The default is 'off'.
+
+ It is to be hoped that most of the "opcodes" in these instructions
+ are fairly self-explanatory, and using 'stepi' and 'nexti' while
+ 'trace' is on will make them into familiar friends.
+
+
+File: gawk.info, Node: Readline Support, Next: Limitations, Prev: List of
Debugger Commands, Up: Debugger
+
+14.4 Readline Support
+=====================
+
+If 'gawk' is compiled with the GNU Readline library
+(http://cnswww.cns.cwru.edu/php/chet/readline/readline.html), you can
+take advantage of that library's command completion and history
+expansion features. The following types of completion are available:
+
+Command completion
+ Command names.
+
+Source file name completion
+ Source file names. Relevant commands are 'break', 'clear', 'list',
+ 'tbreak', and 'until'.
+
+Argument completion
+ Non-numeric arguments to a command. Relevant commands are 'enable'
+ and 'info'.
+
+Variable name completion
+ Global variable names, and function arguments in the current
+ context if the program is running. Relevant commands are
+ 'display', 'print', 'set', and 'watch'.
+
+
+File: gawk.info, Node: Limitations, Next: Debugging Summary, Prev: Readline
Support, Up: Debugger
+
+14.5 Limitations
+================
+
+We hope you find the 'gawk' debugger useful and enjoyable to work with,
+but as with any program, especially in its early releases, it still has
+some limitations. A few that it's worth being aware of are:
+
+ * At this point, the debugger does not give a detailed explanation of
+ what you did wrong when you type in something it doesn't like.
+ Rather, it just responds 'syntax error'. When you do figure out
+ what your mistake was, though, you'll feel like a real guru.
+
+ * If you perused the dump of opcodes in *note Miscellaneous Debugger
+ Commands:: (or if you are already familiar with 'gawk' internals),
+ you will realize that much of the internal manipulation of data in
+ 'gawk', as in many interpreters, is done on a stack. 'Op_push',
+ 'Op_pop', and the like are the "bread and butter" of most 'gawk'
+ code.
+
+ Unfortunately, as of now, the 'gawk' debugger does not allow you to
+ examine the stack's contents. That is, the intermediate results of
+ expression evaluation are on the stack, but cannot be printed.
+ Rather, only variables that are defined in the program can be
+ printed. Of course, a workaround for this is to use more explicit
+ variables at the debugging stage and then change back to obscure,
+ perhaps more optimal code later.
+
+ * There is no way to look "inside" the process of compiling regular
+ expressions to see if you got it right. As an 'awk' programmer,
+ you are expected to know the meaning of '/[^[:alnum:][:blank:]]/'.
+
+ * The 'gawk' debugger is designed to be used by running a program
+ (with all its parameters) on the command line, as described in
+ *note Debugger Invocation::. There is no way (as of now) to attach
+ or "break into" a running program. This seems reasonable for a
+ language that is used mainly for quickly executing, short programs.
+
+ * The 'gawk' debugger only accepts source code supplied with the '-f'
+ option. If you have a shell script that provides an 'awk' program
+ as a command line parameter, and you need to use the debugger, you
+ can write the script to a temporary file, and use that as the
+ program, with the '-f' option. This might look like this:
+
+ cat << \EOF > /tmp/script.$$
+ ... Your program here
+ EOF
+ gawk -D -f /tmp/script.$$
+ rm /tmp/script.$$
+
+
+File: gawk.info, Node: Debugging Summary, Prev: Limitations, Up: Debugger
+
+14.6 Summary
+============
+
+ * Programs rarely work correctly the first time. Finding bugs is
+ called debugging, and a program that helps you find bugs is a
+ debugger. 'gawk' has a built-in debugger that works very similarly
+ to the GNU Debugger, GDB.
+
+ * Debuggers let you step through your program one statement at a
+ time, examine and change variable and array values, and do a number
+ of other things that let you understand what your program is
+ actually doing (as opposed to what it is supposed to do).
+
+ * Like most debuggers, the 'gawk' debugger works in terms of stack
+ frames, and lets you set both breakpoints (stop at a point in the
+ code) and watchpoints (stop when a data value changes).
+
+ * The debugger command set is fairly complete, providing control over
+ breakpoints, execution, viewing and changing data, working with the
+ stack, getting information, and other tasks.
+
+ * If the GNU Readline library is available when 'gawk' is compiled,
+ it is used by the debugger to provide command-line history and
+ editing.
+
+ * Usually, the debugger does not not affect the program being
+ debugged, but occasionally it can.
+
+
+File: gawk.info, Node: Namespaces, Next: Arbitrary Precision Arithmetic,
Prev: Debugger, Up: Top
+
+15 Namespaces in 'gawk'
+***********************
+
+This major node describes a feature that is specific to 'gawk'.
+
+ CAUTION: This feature described in this chapter is new. It is
+ entirely possible, and even likely, that there are dark corners (if
+ not bugs) still lurking within the implementation. If you find any
+ such, please report them (*Note Bugs::).
+
+* Menu:
+
+* Global Namespace:: The global namespace in standard
+ 'awk'.
+* Qualified Names:: How to qualify names with a namespace.
+* Default Namespace:: The default namespace.
+* Changing The Namespace:: How to change the namespace.
+* Naming Rules:: Namespace and Component Naming Rules.
+* Internal Name Management:: How names are stored internally.
+* Namespace Example:: An example of code using a namespace.
+* Namespace And Features:: Namespaces and other 'gawk' features.
+* Namespace Summary:: Summarizing namespaces.
+
+
+File: gawk.info, Node: Global Namespace, Next: Qualified Names, Up:
Namespaces
+
+15.1 Standard 'awk''s Single Namespace
+======================================
+
+In standard 'awk', there is a single, global, "namespace". This means
+that _all_ function names and global variable names must be unique. For
+example, two different 'awk' source files cannot both define a function
+named 'min()', or define the same identifier, used as a scalar in one
+and as an array in the other.
+
+ This situation is okay when programs are small, say a few hundred
+lines, or even a few thousand, but it prevents the development of
+reusable libraries of 'awk' functions, and can inadvertently cause
+independently-developed library files to accidentally step on each
+other's "private" global variables (*note Library Names::).
+
+ Most other programming languages solve this issue by providing some
+kind of namespace control: a way to say "this function is in namespace
+XXX, and that function is in namespace YYY." (Of course, there is then
+still a single namespace for the namespaces, but the hope is that there
+are much fewer namespaces in use by any given program, and thus much
+less chance for collisions.) These facilities are sometimes referred to
+as "packages" or "modules".
+
+ Starting with version 5.0, 'gawk' provides a simple mechanism to put
+functions and global variables into separate namespaces.
+
+
+File: gawk.info, Node: Qualified Names, Next: Default Namespace, Prev:
Global Namespace, Up: Namespaces
+
+15.2 Qualified Names
+====================
+
+A "qualified name" is an identifier that includes a namespace name, the
+namespace separator '::', and a "component" name. For example, one
+might have a function named 'posix::getpid()'. Here, the namespace is
+'posix' and the function name within the namespace (the component) is
+'getpid()'. The namespace and component names are separated by a
+double-colon. Only one such separator is allowed in a qualified name.
+
+ NOTE: Unlike C++, the '::' is _not_ an operator. No spaces are
+ allowed between the namespace name, the '::', and the component
+ name.
+
+ You must use qualified names from one namespace to access variables
+and functions in another. This is especially important when using
+variable names to index the special 'SYMTAB' array (*note Auto-set::),
+and when making indirect function calls (*note Indirect Calls::).
+
+
+File: gawk.info, Node: Default Namespace, Next: Changing The Namespace,
Prev: Qualified Names, Up: Namespaces
+
+15.3 The Default Namespace
+==========================
+
+The default namespace, not surprisingly, is 'awk'. All of the
+predefined 'awk' and 'gawk' variables are in this namespace, and thus
+have qualified names like 'awk::ARGC', 'awk::NF', and so on.
+
+ Furthermore, even when you have changed the namespace for your
+current source file (*note Changing The Namespace::), 'gawk' forces
+unqualified identifiers whose names are all uppercase letters to be in
+the 'awk' namespace. This makes it possible for you to easily reference
+'gawk''s global variables from different namespaces. It also keeps your
+code looking natural.
+
+
+File: gawk.info, Node: Changing The Namespace, Next: Naming Rules, Prev:
Default Namespace, Up: Namespaces
+
+15.4 Changing The Namespace
+===========================
+
+In order to set the current namespace, use an '@namespace' directive at
+the top level of your program:
+
+ @namespace "passwd"
+
+ BEGIN { ... }
+ ...
+
+ After this directive, all simple non-completely-uppercase identifiers
+are placed into the 'passwd' namespace.
+
+ You can change the namespace multiple times within a single source
+file, although this is likely to become confusing if you do it too much.
+
+ NOTE: Association of unqualified identifiers to a namespace is
+ handled while 'gawk' parses your program, _before_ it starts to
+ run. There is no concept of a "current" namespace once your
+ program starts executing. Be sure you understand this.
+
+ Each source file for '-i' and '-f' starts out with an implicit
+'@namespace "awk"'. Similarly, each chunk of command-line code supplied
+with '-e' has such an implicit initial statement (*note Options::).
+
+ Files included with '@include' (*note Include Files::) "push" and
+"pop" the current namespace. That is, each '@include' saves the current
+namespace and starts over with an implicit '@namespace "awk"' which
+remains in effect until an explicit '@namespace' directive is seen.
+When 'gawk' finishes processing the included file, the saved namespace
+is restored and processing continues where it left off in the original
+file.
+
+ The use of '@namespace' has no influence upon the order of execution
+of 'BEGIN', 'BEGINFILE', 'END', and 'ENDFILE' rules.
+
+
+File: gawk.info, Node: Naming Rules, Next: Internal Name Management, Prev:
Changing The Namespace, Up: Namespaces
+
+15.5 Namespace and Component Naming Rules
+=========================================
+
+A number of rules apply to the namespace and component names, as
+follows.
+
+ * It is a syntax error to use qualified names for function parameter
+ names.
+
+ * It is a syntax error to use any standard 'awk' reserved word (such
+ as 'if' or 'for'), or the name of any standard built-in function
+ (such as 'sin()' or 'gsub()') as either part of a qualified name.
+ Thus, the following produces a syntax error:
+
+ @namespace "example"
+
+ function gsub(str, pat, result) { ... }
+
+ * Outside the 'awk' namespace, the names of the additional 'gawk'
+ built-in functions (such as 'gensub()' or 'strftime()') _may_ be
+ used as component names. The same set of names may be used as
+ namespace names, although this has the potential to be confusing.
+
+ * The additional 'gawk' built-in functions may still be called from
+ outside the 'awk' namespace by qualifying them. For example,
+ 'awk::systime()'. Here is a somewhat silly example demonstrating
+ this rule and the previous one:
+
+ BEGIN {
+ print "in awk namespace, systime() =", systime()
+ }
+
+ @namespace "testing"
+
+ function systime()
+ {
+ print "in testing namespace, systime() =", awk::systime()
+ }
+
+ BEGIN {
+ systime()
+ }
+
+
+ When run, it produces output like this:
+
+ $ gawk -f systime.awk
+ -| in awk namespace, systime() = 1500488503
+ -| in testing namespace, systime() = 1500488503
+
+ * 'gawk' pre-defined variable names may be used: 'NF::NR' is valid,
+ if possibly not all that useful.
+
+
+File: gawk.info, Node: Internal Name Management, Next: Namespace Example,
Prev: Naming Rules, Up: Namespaces
+
+15.6 Internal Name Management
+=============================
+
+For backwards compatibility, all identifiers in the 'awk' namespace are
+stored internally as unadorned identifiers (that is, without a leading
+'awk::'). This is mainly relevant when using such identifiers as
+indices for 'SYMTAB', 'FUNCTAB', and 'PROCINFO["identifiers"]' (*note
+Auto-set::), and for use in indirect function calls (*note Indirect
+Calls::).
+
+ In program code, to refer to variables and functions in the 'awk'
+namespace from another namespace, you must still use the 'awk::' prefix.
+For example:
+
+ @namespace "awk" This is the default namespace
+
+ BEGIN {
+ Title = "My Report" Qualified name is awk::Title
+ }
+
+ @namespace "report" Now in report namespace
+
+ function compute() This is really report::compute()
+ {
+ print awk::Title But would be SYMTAB["Title"]
+ ...
+ }
+
+
+File: gawk.info, Node: Namespace Example, Next: Namespace And Features,
Prev: Internal Name Management, Up: Namespaces
+
+15.7 Namespace Example
+======================
+
+The following example is a revised version of the suite of routines
+developed in *note Passwd Functions::. See there for an explanation of
+how the code works.
+
+ The formulation here, due mainly to Andrew Schorr, is rather elegant.
+All of the implementation functions and variables are in the 'passwd'
+namespace, whereas the main interface functions are defined in the 'awk'
+namespace.
+
+ # ns_passwd.awk --- access password file information
+
+ @namespace "passwd"
+
+ BEGIN {
+ # tailor this to suit your system
+ Awklib = "/usr/local/libexec/awk/"
+ }
+
+ function Init( oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)
+ {
+ if (Inited)
+ return
+
+ oldfs = FS
+ oldrs = RS
+ olddol0 = $0
+ using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
+ using_fpat = (PROCINFO["FS"] == "FPAT")
+ FS = ":"
+ RS = "\n"
+
+ pwcat = Awklib "pwcat"
+ while ((pwcat | getline) > 0) {
+ Byname[$1] = $0
+ Byuid[$3] = $0
+ Bycount[++Total] = $0
+ }
+ close(pwcat)
+ Count = 0
+ Inited = 1
+ FS = oldfs
+ if (using_fw)
+ FIELDWIDTHS = FIELDWIDTHS
+ else if (using_fpat)
+ FPAT = FPAT
+ RS = oldrs
+ $0 = olddol0
+ }
+
+ function awk::getpwnam(name)
+ {
+ Init()
+ return Byname[name]
+ }
+
+ function awk::getpwuid(uid)
+ {
+ Init()
+ return Byuid[uid]
+ }
+
+ function awk::getpwent()
+ {
+ Init()
+ if (Count < Total)
+ return Bycount[++Count]
+ return ""
+ }
+
+ function awk::endpwent()
+ {
+ Count = 0
+ }
+
+ As you can see, this version also follows the convention mentioned in
+*note Library Names::, whereby global variable and function names start
+with a capital letter.
+
+ Here is a simple test program. Since it's in a separate file,
+unadorned identifiers are sought for in the 'awk' namespace:
+
+ BEGIN {
+ while ((p = getpwent()) != "")
+ print p
+ }
+
+
+ Here's what happens when it's run:
+
+ $ gawk -f ns_passwd.awk -f testpasswd.awk
+ -| root:x:0:0:root:/root:/bin/bash
+ -| daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
+ -| bin:x:2:2:bin:/bin:/usr/sbin/nologin
+ -| sys:x:3:3:sys:/dev:/usr/sbin/nologin
+ ...
+
+
+File: gawk.info, Node: Namespace And Features, Next: Namespace Summary,
Prev: Namespace Example, Up: Namespaces
+
+15.8 Namespaces and Other 'gawk' Features
+=========================================
+
+This minor node looks briefly at how the namespace facility interacts
+with other important 'gawk' features.
+
+ The profiler and pretty-printer (*note Profiling::) have been
+enhanced to understand namespaces and the namespace naming rules
+presented in *note Naming Rules::. In particular, the output groups
+functions in the same namespace together, and has '@namespace'
+directives in front of rules as necessary. This allows component names
+to be simple identifiers, instead of using qualified identifiers
+everywhere.
+
+ Interaction with the debugger (*note Debugging::) has not had to
+change (at least as of this writing). Some of the internal byte codes
+changed in order to accommodate namespaces, and the debugger's 'dump'
+command was adjusted to match.
+
+ The extension API (*note Dynamic Extensions::) has always allowed for
+placing functions into a different namespace, although this was not
+previously implemented. However, the symbol lookup and symbol update
+routines did not have provision for including a namespace. That has now
+been corrected (*note Symbol table by name::). *Note Extension Sample
+Inplace::, for a nice example of an extension that leverages a namespace
+shared by cooperating 'awk' and C code.
+
+
+File: gawk.info, Node: Namespace Summary, Prev: Namespace And Features, Up:
Namespaces
+
+15.9 Summary
+============
+
+ * Standard 'awk' provides a single namespace for all global
+ identifiers (scalars, arrays, and functions). This is limiting
+ when one wants to develop libraries of reusable functions or
+ function suites.
+
+ * 'gawk' provides multiple namespaces by using qualified names: names
+ consisting of a namespace name, a double colon, '::', and a
+ component name. Namespace names might still possibly conflict, but
+ this is true of any language providing namespaces, modules, or
+ packages.
+
+ * The default namespace is 'awk'. The rules for namespace and
+ component names are provided in *note Naming Rules::. The rules
+ are designed in such a way as to make namespace-aware code continue
+ to look and work naturally while still providing the necessary
+ power and flexibility.
+
+ * Other parts of 'gawk' have been extended as necessary to integrate
+ namespaces smoothly with their operation. This applies most
+ notably to the profiler / pretty-printer (*note Profiling::) and to
+ the extension facility (*note Dynamic Extensions::).
+
+ * Overall, the namespace facility was designed and implemented such
+ that backwards compatibility is paramount. Programs that don't use
+ namespaces should see absolutely no difference in behavior when run
+ by a namespace-capable version of 'gawk'.
+
+
+File: gawk.info, Node: Arbitrary Precision Arithmetic, Next: Dynamic
Extensions, Prev: Namespaces, Up: Top
+
+16 Arithmetic and Arbitrary-Precision Arithmetic with 'gawk'
+************************************************************
+
+This major node introduces some basic concepts relating to how computers
+do arithmetic and defines some important terms. It then proceeds to
+describe floating-point arithmetic, which is what 'awk' uses for all its
+computations, including a discussion of arbitrary-precision
+floating-point arithmetic, which is a feature available only in 'gawk'.
+It continues on to present arbitrary-precision integers, and concludes
+with a description of some points where 'gawk' and the POSIX standard
+are not quite in agreement.
+
+ NOTE: Most users of 'gawk' can safely skip this chapter. But if
+ you want to do scientific calculations with 'gawk', this is the
+ place to be.
+
+* Menu:
+
+* Computer Arithmetic:: A quick intro to computer math.
+* Math Definitions:: Defining terms used.
+* MPFR features:: The MPFR features in 'gawk'.
+* FP Math Caution:: Things to know.
+* Arbitrary Precision Integers:: Arbitrary Precision Integer Arithmetic with
+ 'gawk'.
+* Checking for MPFR:: How to check if MPFR is available.
+* POSIX Floating Point Problems:: Standards Versus Existing Practice.
+* Floating point summary:: Summary of floating point discussion.
+
+
+File: gawk.info, Node: Computer Arithmetic, Next: Math Definitions, Up:
Arbitrary Precision Arithmetic
+
+16.1 A General Description of Computer Arithmetic
+=================================================
+
+Until now, we have worked with data as either numbers or strings.
+Ultimately, however, computers represent everything in terms of "binary
+digits", or "bits". A decimal digit can take on any of 10 values: zero
+through nine. A binary digit can take on any of two values, zero or
+one. Using binary, computers (and computer software) can represent and
+manipulate numerical and character data. In general, the more bits you
+can use to represent a particular thing, the greater the range of
+possible values it can take on.
+
+ Modern computers support at least two, and often more, ways to do
+arithmetic. Each kind of arithmetic uses a different representation
+(organization of the bits) for the numbers. The kinds of arithmetic
+that interest us are:
+
+Decimal arithmetic
+ This is the kind of arithmetic you learned in elementary school,
+ using paper and pencil (and/or a calculator). In theory, numbers
+ can have an arbitrary number of digits on either side (or both
+ sides) of the decimal point, and the results of a computation are
+ always exact.
+
+ Some modern systems can do decimal arithmetic in hardware, but
+ usually you need a special software library to provide access to
+ these instructions. There are also libraries that do decimal
+ arithmetic entirely in software.
+
+ Despite the fact that some users expect 'gawk' to be performing
+ decimal arithmetic,(1) it does not do so.
+
+Integer arithmetic
+ In school, integer values were referred to as "whole" numbers--that
+ is, numbers without any fractional part, such as 1, 42, or -17.
+ The advantage to integer numbers is that they represent values
+ exactly. The disadvantage is that their range is limited.
+
+ In computers, integer values come in two flavors: "signed" and
+ "unsigned". Signed values may be negative or positive, whereas
+ unsigned values are always greater than or equal to zero.
+
+ In computer systems, integer arithmetic is exact, but the possible
+ range of values is limited. Integer arithmetic is generally faster
+ than floating-point arithmetic.
+
+Floating-point arithmetic
+ Floating-point numbers represent what were called in school "real"
+ numbers (i.e., those that have a fractional part, such as
+ 3.1415927). The advantage to floating-point numbers is that they
+ can represent a much larger range of values than can integers. The
+ disadvantage is that there are numbers that they cannot represent
+ exactly.
+
+ Modern systems support floating-point arithmetic in hardware, with
+ a limited range of values. There are software libraries that allow
+ the use of arbitrary-precision floating-point calculations.
+
+ POSIX 'awk' uses "double-precision" floating-point numbers, which
+ can hold more digits than "single-precision" floating-point
+ numbers. 'gawk' has facilities for performing arbitrary-precision
+ floating-point arithmetic, which we describe in more detail
+ shortly.
+
+ Computers work with integer and floating-point values of different
+ranges. Integer values are usually either 32 or 64 bits in size.
+Single-precision floating-point values occupy 32 bits, whereas
+double-precision floating-point values occupy 64 bits.
+(Quadruple-precision floating point values also exist. They occupy 128
+bits, but such numbers are not available in 'awk'.) Floating-point
+values are always signed. The possible ranges of values are shown in
+*note Table 16.1: table-numeric-ranges. and *note Table 16.2:
+table-floating-point-ranges.
+
+
+Representation Minimum value Maximum value
+---------------------------------------------------------------------------
+32-bit signed integer -2,147,483,648 2,147,483,647
+32-bit unsigned 0 4,294,967,295
+integer
+64-bit signed integer -9,223,372,036,854,775,8089,223,372,036,854,775,807
+64-bit unsigned 0 18,446,744,073,709,551,615
+integer
+
+Table 16.1: Value ranges for integer representations
+
+
+Representation Minimum Minimum finite Maximum finite
+ positive value value
+ nonzero value
+--------------------------------------------------------------------------------
+Single-precision 1.175494e-38 -3.402823e+38 3.402823e+38
+floating-point
+Double-precision 2.225074e-308 -1.797693e+308 1.797693e+308
+floating-point
+Quadruple-precision 3.362103e-4932 -1.189731e+4932 1.189731e+4932
+floating-point
+
+Table 16.2: Approximate value ranges for floating-point number
+representations
+
+ ---------- Footnotes ----------
+
+ (1) We don't know why they expect this, but they do.
+
+
+File: gawk.info, Node: Math Definitions, Next: MPFR features, Prev:
Computer Arithmetic, Up: Arbitrary Precision Arithmetic
+
+16.2 Other Stuff to Know
+========================
+
+The rest of this major node uses a number of terms. Here are some
+informal definitions that should help you work your way through the
+material here:
+
+"Accuracy"
+ A floating-point calculation's accuracy is how close it comes to
+ the real (paper and pencil) value.
+
+"Error"
+ The difference between what the result of a computation "should be"
+ and what it actually is. It is best to minimize error as much as
+ possible.
+
+"Exponent"
+ The order of magnitude of a value; some number of bits in a
+ floating-point value store the exponent.
+
+"Inf"
+ A special value representing infinity. Operations involving
+ another number and infinity produce infinity.
+
+"NaN"
+ "Not a number." A special value that results from attempting a
+ calculation that has no answer as a real number. *Note Strange
+ values::, for more information about infinity and not-a-number
+ values.
+
+"Normalized"
+ How the significand (see later in this list) is usually stored.
+ The value is adjusted so that the first bit is one, and then that
+ leading one is assumed instead of physically stored. This provides
+ one extra bit of precision.
+
+"Precision"
+ The number of bits used to represent a floating-point number. The
+ more bits, the more digits you can represent. Binary and decimal
+ precisions are related approximately, according to the formula:
+
+ PREC = 3.322 * DPS
+
+ Here, _prec_ denotes the binary precision (measured in bits) and
+ _dps_ (short for decimal places) is the decimal digits.
+
+"Rounding mode"
+ How numbers are rounded up or down when necessary. More details
+ are provided later.
+
+"Significand"
+ A floating-point value consists of the significand multiplied by 10
+ to the power of the exponent. For example, in '1.2345e67', the
+ significand is '1.2345'.
+
+"Stability"
+ From the Wikipedia article on numerical stability
+ (https://en.wikipedia.org/wiki/Numerical_stability): "Calculations
+ that can be proven not to magnify approximation errors are called
+ "numerically stable"."
+
+ See the Wikipedia article on accuracy and precision
+(https://en.wikipedia.org/wiki/Accuracy_and_precision) for more
+information on some of those terms.
+
+ On modern systems, floating-point hardware uses the representation
+and operations defined by the IEEE 754 standard. Three of the standard
+IEEE 754 types are 32-bit single precision, 64-bit double precision, and
+128-bit quadruple precision. The standard also specifies extended
+precision formats to allow greater precisions and larger exponent
+ranges. ('awk' uses only the 64-bit double-precision format.)
+
+ *note Table 16.3: table-ieee-formats. lists the precision and
+exponent field values for the basic IEEE 754 binary formats.
+
+
+Name Total bits Precision Minimum Maximum
+ exponent exponent
+---------------------------------------------------------------------------
+Single 32 24 -126 +127
+Double 64 53 -1022 +1023
+Quadruple 128 113 -16382 +16383
+
+Table 16.3: Basic IEEE format values
+
+ NOTE: The precision numbers include the implied leading one that
+ gives them one extra bit of significand.
+
+
+File: gawk.info, Node: MPFR features, Next: FP Math Caution, Prev: Math
Definitions, Up: Arbitrary Precision Arithmetic
+
+16.3 Arbitrary-Precision Arithmetic Features in 'gawk'
+======================================================
+
+By default, 'gawk' uses the double-precision floating-point values
+supplied by the hardware of the system it runs on. However, if it was
+compiled to do so, and the '-M' command-line option is supplied, 'gawk'
+uses the GNU MPFR (http://www.mpfr.org) and GNU MP (https://gmplib.org)
+(GMP) libraries for arbitrary-precision arithmetic on numbers. You can
+see if MPFR support is available like so:
+
+ $ gawk --version
+ -| GNU Awk 4.1.2, API: 1.1 (GNU MPFR 3.1.0-p3, GNU MP 5.0.2)
+ -| Copyright (C) 1989, 1991-2015 Free Software Foundation.
+ ...
+
+(You may see different version numbers than what's shown here. That's
+OK; what's important is to see that GNU MPFR and GNU MP are listed in
+the output.)
+
+ Additionally, there are a few elements available in the 'PROCINFO'
+array to provide information about the MPFR and GMP libraries (*note
+Auto-set::).
+
+ The MPFR library provides precise control over precisions and
+rounding modes, and gives correctly rounded, reproducible,
+platform-independent results. With the '-M' command-line option, all
+floating-point arithmetic operators and numeric functions can yield
+results to any desired precision level supported by MPFR.
+
+ Two predefined variables, 'PREC' and 'ROUNDMODE', provide control
+over the working precision and the rounding mode. The precision and the
+rounding mode are set globally for every operation to follow. *Note
+Setting precision:: and *note Setting the rounding mode:: for more
+information.
+
+
+File: gawk.info, Node: FP Math Caution, Next: Arbitrary Precision Integers,
Prev: MPFR features, Up: Arbitrary Precision Arithmetic
+
+16.4 Floating-Point Arithmetic: Caveat Emptor!
+==============================================
+
+ Math class is