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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.questions
Subject: Re: C-shell idiosyncracy ??
Message-ID: <9389@mimsy.UUCP>
Date: Mon, 16-Nov-87 06:52:31 EST
Article-I.D.: mimsy.9389
Posted: Mon Nov 16 06:52:31 1987
Date-Received: Tue, 17-Nov-87 06:37:15 EST
References: <33414@sun.uucp> <9316@mimsy.UUCP> <3387@xanth.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 63

As several people have pointed out, it is possible (not necessarily
easy) to have builtin commands run as the source or sink of a
pipeline (although at most one such source or sink).  The question
in <33414@sun.uucp>, however, was `why does this happen'; that was
what I answered.

Actually, to be even more specific, csh forks the head ends of all
pipes.  The tail end of a pipe will be done directly by csh if the
command is a csh builtin.  Contrast this with sh, which forks the
tail ends of pipes first:

	a | b | c | d | e &

In csh, you might get

	% jobs
	[1]	714 Runnning		a |
		715			b |
		716			c |
		717			d |
		718			e

In sh, had it a jobs command, these might be listed as

	718 Running	a |
	717		b |
	716		c |
	715		d |
	714		e

In particular, in csh, builtins are run directly by the shell when
they are at the tail end of pipes.  This is the most straightforward
implementation, given the direction of creation shown above.  Try,
for instance,

	% jobs | cat

and

	% cat /dev/null | jobs

The former produces no output; the latter lists some jobs, then
prints

	Reset tty pgrp from xxx to yyy

---which just goes to show that there are bugs in csh (what a
revelation :-) !).  Indeed, it is necessary to do another `jobs'
command to flush the `cat | jobs' job from csh's joblist.  Also
incidentally, this particular bug would wreak havoc with tty signals
(interrupt, stop) if any of csh's builtins were long-running.

Were csh more clever, it could indeed either avoid forking the jobs
command in `jobs | more' (but at least one `jobs' in `jobs | jobs |
more' must be forked), or defer forgetting about its parent's jobs.
Csh is just not that clever.

Having once again demonstrated that I can write more exposition
on a smaller subject than anyone else :-), I now return you to
your regular netnews,
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris