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From: rayan@utegc.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: shell stderr in popen()
Message-ID: <8708111952.AA03137@ephemeral.ai.toronto.edu>
Date: Tue, 11-Aug-87 15:52:41 EDT
Article-I.D.: ephemera.8708111952.AA03137
Posted: Tue Aug 11 15:52:41 1987
Date-Received: Wed, 12-Aug-87 05:42:26 EDT
References: <4511@sunybcs.UUCP> <8708100615.AA06203@ephemeral.ai.toronto.edu>
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Organization: University of Toronto, AI group
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>In article <7928@mimsy.UUCP> Chris suggests:
># 	(void) sprintf(expandcmd, "sh -c '%s' 2>/dev/null", command);
># 	iop = popen(expandcmd, "r");

In article <8708100615.AA06203@ephemeral.ai.toronto.edu> I write:
>Seems too expensive. Try
>
>	popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r");

In article <7932@mimsy.UUCP> Chris replies:
# This is a good idea.  Unfortunately, it does not work; the shell has
# already set its own standard error to another descriptor, so that it
# can still print errors.

Before posting my suggestion above, I tried it out. I'm on a SunOS 3.4 box,
and using exec as shown did indeed get rid of the ": not found" message.
But Chris usually knows what he's talking about (too...), so I just tried it
on 4.3bsd. There his admonition holds true. Therefore I conclude that early
v7-based sh's (as in 4.3bsd) require something like:

1.	popen("( /usr/bin/mynewcommand -flags ) 2> /dev/null", "r");

while sysV-based sh's (as in SunOS) will work with:

2.	popen("exec 2> /dev/null ; /usr/bin/mynewcommand -flags", "r");

Note that method 1. is less overhead (one less path search and exec), and
less problematic vis-a-vis quoting, than the original suggestion at the top
of this message.

I thought of specifying the fd that the 4.3 sh dup's stderr to, instead of '2'.
Unfortunately, that is fd 11, and only single digits may be used to specify
the fd for a redirection. I'm glad this feature of juggling the fd's around
was removed in sysV (presumably).

Live and learn.

rayan
-- 
Rayan Zachariassen
AI group, University of Toronto