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From: "Bart Schaefer" <schaefer@brasslantern.com>
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Date: Sun, 3 May 1998 18:35:49 -0700
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Comments: In reply to Sweth Chandramouli <sweth@astaroth.nit.gwu.edu>
        "Re: Re: exit value of intermediate program in pipe" (May  3,  6:15pm)
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To: Sweth Chandramouli <sweth@astaroth.nit.gwu.edu>, zsh-users@math.gatech.edu
Subject: Re: exit value of intermediate program in pipe
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On May 3,  6:15pm, Sweth Chandramouli wrote:
: Subject: Re: Re: exit value of intermediate program in pipe
:
: in ksh i would do
: 
: {
: grep -v bar |&
: print -p `/bin/blah ; exitstatus=$?`
: read -p output
: echo $output
: return $exitstatus
: }

You can do exactly that same thing, except insted of

	grep -v bar |&

You'd say

	coproc grep -v bar

Both print -p and read -p are the same in zsh as in ksh.

Of course, you probably want

	while read -p output
	do print -r $output
	done

as each read consumes only one line, not the entire stream.

: 	for zsh, would i just do
: 
: {
: grep -v bar coproc |
: >&p `/bin/blah ; exitstatus=$?`

That line doesn't do what you think.  What you'd want is just

	/bin/blah >&p
	exitstatus=$?

: <&p output ; echo $output

That's not right either.  You'd probably need

	cat <&p

and you'd probably have to start it before you started /bin/blah, if
blah could potentially produce more than a few kbytes of output.

: return $exitstatus
: }

The last problem is that grep won't exit until it sees EOF on its stdin,
but >&p dups the coproc input without actually closing it.  So the grep
won't get EOF when blah exits.  You have to shut it down some other way;
the only thing I've found is to start another coprocess.  I don't know
if this is a bug, or what.

So you get

    {
	coproc grep -v bar
	cat <&p &
	/bin/blah >&p
	exitstatus=$?
	coproc exit	# Shuts down grep by closing its input, as
			#   a side-effect of starting a new coproc
			#   which then immediately exits.  Ewww.
	wait		# Allows cat to finish, just in case.
	return $exitstatus
    }

I think { /bin/blah >>(grep -v bar) } is a lot nicer, don't you?

: 	what zsh really needs is something like the hawksbill book from
: oreilly for ksh, that gives a lot of examples and compares it to other
: shells

There's a lot of that in the FAQ, found in Etc/FAQ in the zsh dist.

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com