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From: dce@mips.UUCP (David Elliott)
Newsgroups: comp.unix.wizards
Subject: Re: shell stderr in popen()
Message-ID: <574@quacky.UUCP>
Date: Fri, 7-Aug-87 11:17:30 EDT
Article-I.D.: quacky.574
Posted: Fri Aug  7 11:17:30 1987
Date-Received: Tue, 18-Aug-87 03:02:37 EDT
References: <4511@sunybcs.UUCP>
Reply-To: dce@quacky.UUCP (David Elliott)
Distribution: na
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 44
Keywords: shell popen stderr redirection

In article <4511@sunybcs.UUCP> ugbinns@marvin.UUCP (Leonard Binns) writes:
>In article <124@foobar.UUCP> tw@foobar.UUCP (Tom Walsh) writes:
>
>>  i have a need to use popen(command, "r").  if command is not
>>  found, i would like to dispose quietly of the shell's complaint:
>>  sh: command: not found
>>  the stderr of the command itself can be handled ala:
>>  popen("/usr/bin/mynewcommand -flags 2> /dev/null", "r");
>>
>>  question:
>>   is there an easier way than:
>
>Did you ever try:
>	popen("/usr/bin/mynewcommand -flags 2>& /dev/null", "r");
>					     ^^^
>I think that should do the trick.

Why should it do the trick? It isn't even valid shell syntax!
Remember that popen() uses /bin/sh, not your SHELL (and if AT&T
changes this, I QUIT!). The >& is used in the sequence

	m>&n

which says "make file descriptor m a duplicate of file descriptor
n". The sequence 2>& /dev/null results in "/dev/null: bad file
number".

Anyway, the easiest way to do this is

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

The sequence

	( command )

runs the command in a subshell, so all redirection done outside of
the parenthesis applies to the subshell.

This is also useful in cases like:

	( cat file1 ; echo separator ; cat file2 ) | filter

-- 
David Elliott		{decvax,ucbvax,ihnp4}!decwrl!mips!dce