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Date: Mon, 13 Jan 2003 02:37:36 -0500
From: Jon LaBadie <jcyg AT jgcomp DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Return codes and pipelines
Message-ID: <20030113073736.GA18104@butch.jgcomp.com>
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On Sun, Jan 12, 2003 at 09:22:55PM -0800, Randall R Schulz wrote:
> Jon,
> 
> Well, Cygwin does not include Bourne shell, it has BASH, ash and zsh, but 
> it does appear that the status returned by a parenthesized pipeline under 
> BASH is that of the last command in that pipeline.


As specified on the bash man page.


> Strictly speaking, that's not at odds with the first part of what I said, 
> which is that the parent - child process relationship between processes in 
> a pipeline is not specified.
> 
> So this leaves one puzzle: Why is it that Rolf is getting different results 
> for this command between Linux and Cygwin?
> 
>     (false | true) && echo true || echo false
> 
> Rolf, which shells are you using on Linux and Cygwin?


And what results?


> At 20:53 2003-01-12, Jon LaBadie wrote:
> >On Sun, Jan 12, 2003 at 02:21:45PM -0800, Randall R Schulz wrote:
> >> Rolf,
> >>
> >> One posting is enough, really.
> >>
> >> There is no guaranteed or specified parent child relationship between the
> >> processes in a pipeline. This means that you cannot predict which 
> >process's
> >> status will be the one returned as that of the pipeline as a whole.
> >
> >
> >> At 14:09 2003-01-12, Rolf Campbell wrote:
> >> >/home/rcampbell> (true | true) && echo true || echo false
> >> >true
> >> >/home/rcampbell> (true | false) && echo true || echo false
> >> >false
> >> >/home/rcampbell> (false | true) && echo true || echo false
> >> >true
> >> >/home/rcampbell> (false | false) && echo true || echo false
> >> >false
> >> >
> >> >The third test above yields different results when run on Linux.  I'm
> >> >wondering if this was the desired result or not?
> >
> >
> >There is (or had been last I looked at the source) a defined relationship
> >in the Bourne and Korn shells at least.
> >
> >The parent forks the last command in a pipeline which then becomes the
> >parent of the other commands in the pipeline.  Thus the original parent
> >only sees the last command's exit status.
> >
> >This scheme was done to implement the following behavior back around
> >System V.2 in the great Bourne shell "rewrite in C".
> >
> >>From the current Solaris man page for Bourne Shell:
> >
> >       ...  .  Each  command  is  run  as a separate process; the
> >     shell waits for the last  command  to  terminate.  The  exit
> >     status  of a pipeline is the exit status of the last command
> >     in the pipeline.
> >
> >The addition of the pipefail option to recent ksh93 versions suggests
> >this relation-ship scheme has been altered in those shells.  But I
> >do not think it appropriate to generically say there is no defined
> >relationship as if it applies to every shell out there.
> >
> >--
> >Jon H. LaBadie                  jcyg AT jgcomp DOT com
> 
> 
> --
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> 
>>> End of included message <<<

-- 
Jon H. LaBadie                  jcyg AT jgcomp DOT com
 JG Computing
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