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Date: Sun, 15 Aug 2004 19:42:25 -0700 (PDT)
From: Bart Schaefer <schaefer@brasslantern.com>
Reply-To: zsh-users@sunsite.dk
To: zsh-users@sunsite.dk
Subject: Re: Slightly OT: Error-Handling in a Pipeline, preferably non-zsh
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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.