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Re: Return codes and pipelines
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.
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?
Randall Schulz
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@jgcomp.com
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