Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!spool.mu.edu!agate!usenet.ins.cwru.edu!odin!chet
From: chet@odin.INS.CWRU.Edu (Chet Ramey)
Newsgroups: comp.unix.shell
Subject: Re: Bash, tar, and broken pipe
Message-ID: <1991May22.192914.22142@usenet.ins.cwru.edu>
Date: 22 May 91 19:29:14 GMT
Article-I.D.: usenet.1991May22.192914.22142
References: <16345@helios.TAMU.EDU> <heinz.674916703@cc.univie.ac.at>
Sender: news@usenet.ins.cwru.edu
Reply-To: chet@po.CWRU.Edu
Organization: Yoyodyne Propulsion Systems, Inc.
Lines: 56
Nntp-Posting-Host: odin.ins.cwru.edu

In article <heinz.674916703@cc.univie.ac.at> heinz@cc.univie.ac.at () writes:

>This does not explain the broken pipe, though. I tried the following:
>
>	cat <some_long_file> | more
>
>and killed 'more' by pressing 'q' at the first prompt (so more terminates
>first). No 'Broken Pipe'.

I guess I'll take a shot at this one.  First of all, other shells (csh and
ksh for sure) special-case the message printed when a child process dies due
to an interrupt (SIGINT) or a broken pipe (SIGPIPE).  Bash does not skip
over SIGPIPE, hence the unexpected `Broken Pipe' message.

>Then I tried:
>
>	echo Hallo | (sleep 10; more) # first process terminates first, since
>		'Hallo' should fit into the pipe's buffer
>
>No 'Broken Pipe' either.

Try

slc2$ cat /etc/termcap | sleep 1
Broken pipe

(I also get the `Broken Pipe' message when I do `cat /etc/termcap | more' and
immediately hit `q'.)

The broken pipe/SIGPIPE/EPIPE happens to the *first* process in a pipeline;
the error occurs when an attempt is made to write on a pipe when no process
has it open for reading.

The process must exit due to the SIGPIPE, by the way -- no message will be
printed if it catches the SIGPIPE and calls exit(), unless the fatal signal
handler is coded like this:

fatal(sig)
int	sig;
{
	cleanup();
	_exit(128+sig);
}

>Maybe I should take the time and hack up the source code of bash,
>but I'm not sure if it's worth the effort.

It's a several-minute job, to be sure ;-)

Chet
-- 
Chet Ramey			  Internet: chet@po.CWRU.Edu
Case Western Reserve University	  NeXT Mail: chet@macbeth.INS.CWRU.Edu

``Now,  somehow we've brought our sins back physically -- and they're pissed.''