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Re: redirection inside a process-substitution


From: Chet Ramey
Subject: Re: redirection inside a process-substitution
Date: Wed, 24 Aug 2016 12:00:25 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

On 8/23/16 5:00 PM, Helmut Karlowski wrote:

> Now extending Pierre's example:
> 
> exec 3>tb.err
> echo 3>&1 > >(echo 1 1>&3)
> echo +++++
> cat tb.err
> echo -----
> echo 3>&1 > >(echo 11 1>&3)
> echo +++++
> cat tb.err
> echo -----
> echo > >(echo 2 1>&3) 3>&1
> echo +++++
> echo -----
> cat tb.err
> echo > >(echo 3 1>&3)
> echo +++++
> cat tb.err
> echo -----
> wait
> 
> I get in bash:

Strange.  I get

+++++
1
-----
+++++
11
-----
+++++
-----
2
+++++
2
3
-----

on Mac OS X and RHEL 6.  I'm not sure why the last redirection (the one
with just process substitution and not 3>&1) doesn't cause the file to
be truncated, or the file pointer to be adjusted, but the trace is the
same in all cases.

> 
> 1
> 
> in ksh93 (where > >() does not work anyway, at least on cygwin):

This doesn't work on any ksh93, whether /dev/fd and process substitution
are available or not.

> in zsh (sometimes, sometimes it prints nothing):

I speculate that this has to do with something that zsh does to force
appending, whether that's lseek or something else, other than the fact
that zsh doesn't seem to use /dev/fd at all (I think it just straight
uses pipes).  Bash doesn't do anything special with tb.err after opening
it, just manipulates the file descriptors with dup2 and fcntl.

Chet
-- 
``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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