From: Bart Schaefer <schaefer@brasslantern.com>
To: zsh-users@sunsite.dk
Subject: Re: Slightly OT: Error-Handling in a Pipeline, preferably non-zsh
Date: Sun, 15 Aug 2004 19:42:25 -0700 (PDT) [thread overview]
Message-ID: <Pine.LNX.4.61.0408151913400.935@toltec.zanshin.com> (raw)
In-Reply-To: <2FF1BBB1-EF20-11D8-9C9B-000A95EDC31A@louisville.edu>
On Sun, 15 Aug 2004, Aaron Davies wrote:
> How do I do return-value error handling in the middle of a pipeline?
The trouble is that you're not asking for return-value error handling,
you're asking for conditional execution. That is, you want a pipeline
that acts like an && conjunction. This just can't be; in order to set up
all the stdout-to-stdin connections between the processes in the pipeline,
the controlling shell must start them up without knowing whether any one
of them is (or isn't) actually going to produce (or consume) any output.
The closest you could get would be to use a named pipe:
mkfifo psgrep
{ ps aux | grep $name | grep -v grep | grep -v $0; } > psgrep &&
{ awk '{ print $2 }' | xargs $@; } < psgrep
rm psgrep
However, that's subject to deadlock if the the first pipeline produces
more output before it exits than can be buffered in the fifo, and except
for the mkfifo it looks exactly the same as the temp file solution you
already suggested.
There is no reliable way to do this without capturing the first part of
the output somewhere. Instead of using a temp file, you could capture in
a variable:
psgrep="`ps aux | grep $name | grep -v grep | grep -v $0`"
[ -n "$psgrep" ] && { echo "$psgrep" | awk '{ print $2 }' | xargs $@; }
Or you could skip the awk and xargs entirely and use a while loop:
ps aux | grep $name | grep -v grep | grep -v $0 |
while read user pid remainder
do
$@ $pid
done
As a final note, you probably want "$@" in double quotes.
next prev parent reply other threads:[~2004-08-16 2:44 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-08-16 1:04 Aaron Davies
2004-08-16 2:42 ` Bart Schaefer [this message]
2004-08-16 3:33 ` Aaron Davies
2004-08-16 7:41 ` Bart Schaefer
2004-08-16 12:40 ` Aaron Davies
2004-08-16 14:53 ` DervishD
2004-08-17 3:06 ` Bart Schaefer
2004-08-16 3:03 ` Philippe Troin
2004-08-16 3:30 ` Aaron Davies
2004-08-16 7:51 ` Bart Schaefer
2004-08-16 12:41 ` Aaron Davies
2004-08-16 16:08 ` Dan Nelson
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