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Re: Process-substitution FDs closed prematurely

From: Chet Ramey
Subject: Re: Process-substitution FDs closed prematurely
Date: Wed, 07 Nov 2012 18:05:21 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:16.0) Gecko/20121026 Thunderbird/16.0.2

On 11/6/12 7:09 AM, Zev Weiss wrote:

> Bash Version: 4.2
> Patch Level: 39
> Release Status: release
> Description:
>       It seems that when bash passes (the expanded form of) a
>       process substitution to a function and that function then uses
>       the argument in a pipeline, the file descriptor for the
>       substituted pipe gets closed prematurely.  This does not occur
>       with bash-4.1.11 on FreeBSD, but does with 4.2.39 on Linux (I
>       attempted to compile 4.1.11 from source on Linux to test it,
>       but the build failed).
> Repeat-By:
>       Shell transcript demonstrating it:
>       address@hidden: tmp]% cat test.sh
>       #!/bin/bash
>       fn() { cat | cat "$1"; }
>       fn <(:) < /dev/null
>       address@hidden: tmp]% bash ./test.sh
>       cat: /dev/fd/63: No such file or directory
>       address@hidden: tmp]% echo $?
>       1
>       Expected behavior is no output and an exit status of zero.

Thanks for the report.  The difference you observed is not the change
from bash-4.1 to bash-4.2, but Linux using /dev/fd and FreeBSD using
explicitly-created and destroyed FIFOs.

It's important to close /dev/fd file descriptors (which are essentially
pipes) in the various processes that use and inherit them.  The bash-4.2
(and previous) code closes those file descriptors after creating the first
process in a pipeline instead of, say, the last (thereby making them
unavailable to the rest of the pipeline processes).  I will look at
changing that for the next version.

``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/

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