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Re: redirection inside a process-substitution

From: Chet Ramey
Subject: Re: redirection inside a process-substitution
Date: Tue, 23 Aug 2016 11:00:50 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

On 8/22/16 3:38 PM, address@hidden wrote:
> When doing redirection inside a sub-process to a descriptor that is
> redirected to a file the output of the subshell goes into the file.

Child processes created to run subshells inherit open file descriptors
from their parent.

> Now when the same descriptor is again redirected to another descriptor for 
> this whole
> command-list, the output of the sub-process goes to the other descriptor.
> Except when the subshell is subject to process-substitution: In this
> case the outer redirection has no effect to the sub-process. Why is that?

It has to do when things are processed.  Process substitution is not a
command or a redirection: it is a word expansion that expands to a file
name.  Word expansions are performed before redirections.

When the process substitution is expanded, nothing has run that changes
file descriptor 3 away from the file (tb.err).

> rm -f tb.err
> exec 3>tb.err
> echo ------ 1 ----------
> (echo 1 1>&3) 3>&1

This is not relevant; the subshell is a command to which the redirection

> echo ........tb.err ...........
> cat tb.err
> echo ------ 2 ---------
> echo >(echo 2 1>&3) 3>&1

The process substitution is expanded before the redirection (which applies
to the `echo', since that is the command) is performed.  When the subshell
starts, fd 3 still points to tb.err.

> echo ------ 3 ---------
> echo >(echo 3 1>&3)
> echo ........tb.err ...........

This redirects file descriptor 1 away from the pipe to the  descriptor the
subshell inherits from its parent.

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

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