Re: [Help-bash] does bash stop children processes when reading from them??

[Chet Ramey]

On 11/25/14, 8:25 AM, Greg Wooledge wrote:
> On Tue, Nov 25, 2014 at 04:09:22AM -0800, R . wrote:
>> alright, say we have this:
>>
>> while read line; do echo "$line"; done < <(func_or_process)
>>
>> when doing piping and redirection, does bash run "func_or_process" all in 
>> one go, then dump all the output to an intermediate file then makes use of 
>> them for the rest of the commands??
> 
> It depends on the platform bash is built for.  Process substitution may
> use a FIFO (named pipe), or it may use a special entry in /dev/fd/, or
> it may (I think) use a temporary file.

Only if someone has written and separately distributed a patch to use
temporary files.

> 
> On systems with /dev/fd/* (like BSD and Linux), func_or_process is
> executed as a detached background process, with /dev/fd/* reading
> to its output.
> 
> On Unix-like systems without /dev/fd/* (like HP-UX), a named pipe is
> created, and func_or_process is executed as a detached background
> process, writing to the named pipe, which is opened for reading by
> the foreground shell.
> 
> On systems with neither (like Microsoft Windows?), I think it uses
> a temp file.  The func_or_process is executed first, writing to the
> temp file.  When it terminates, the rest of the shell command is
> allowed to run, reading from the temp file.

Bash doesn't do this by default.  Some systems may emulate pipes using
temporary files, and there may be patches for Windows-like systems that
add support for using temporary files, but bash as distributed doesn't use
them.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/