Re: [Help-bash] Reading and handling "control" characters from a file

[Pierre Gaston]

On Fri, Apr 20, 2012 at 1:11 AM, Conrad J. Sabatier <address@hidden> wrote:

I've just started doing a little prototyping in bash for a program I'll
eventually code most likely in C, and have hit a serious stumbling block
re: the handling of characters (bytes) in the very low range of the ASCII
table.

In a nutshell, I'm trying to read and parse the ID3 tag on an MP3 file.
Barely out of the gate, I'm running smack into the second field in the 10-
byte ID3 tag header -- bytes 3 and 4, to be precise (counting from 0) --
which contain the ID3 major version and revision numbers, one byte each.

The data which starts an ID3 tag header is arranged as:

bytes 0-2: the literal string "ID3"
bytes 3-4: the ID3 major version and revision numbers

Using "read -r -n 1 id3_version", bash complains loudly upon attempting
to use the value just read (id3_version == 4):

$ id3parse.sh "Supertramp - Breakfast In America - 02 - The Logical
Song.mp3"

[no problems with the first three bytes]
IDstr=ID3

[but then, after "read -r -n 1 id3_version", the following statement,
which attempts to convert the value to a printable numeric character,
causes the following]

/home/conrads/bin/id3parse.sh: line 124:  : syntax error: operand
expected (error token is " ")

I realize that I am, in effect, trying to read a CTRL-D, otherwise known
as end-of-file, so it's not too surprising that bash would get its
panties all in a wad over this, but how to work around it?  For that
matter, how to read *any* of the lesser-valued ASCII characters and use
them without generating an error?

Here's the snippet of code where this is occurring:

function read_tag_header
#
# read the initial 10-byte id3v2 tag header
#
{
       # look for the initial ID string
       read -n 3 IDstr

       if ((IDstr != "ID3"))
       then
               echo "ID3 header not found!" >&2
               return 1
       fi

       ((DEBUG)) && echo "IDstr=${IDstr}" >&2

       # OK, now get the version number
       read -r -n 1 id3_version ||
       {
               echo "ID3 version number not found!" >&2
               return 1
       }

       id3_version=$((id3_version + 48))

It is this last _expression_ that triggers the error.  I've tried both
explicitly declaring id3_version as an integer type, and leaving its type
undefined.  The _expression_ fails the same either way.

I've wracked my brain trying to figure out a way to deal with this, and
am just plain stumped.  Any suggestions will be much appreciated!

--
Conrad J. Sabatier
address@hidden

((IDstr != "ID3"))

(( is for arithmetic evaluation, it only works for numbers not strings