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From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: How can I capture STDERR from a command executed externally
Message-ID: <1991May23.055849.6100@jpl-devvax.jpl.nasa.gov>
Date: 23 May 91 05:58:49 GMT
References: <4352@inews.intel.com>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Distribution: usa
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 55

In article <4352@inews.intel.com> rpaul@sedona.intel.com writes:
: 
: I want to be able to capture STDERR from an external command and do some 
: substitution and reprint it back out.  Following is an experiment I was doing
: to get this to work.  The ls command used is the GNU ls, which with the -h
: option, prints its options to stderr:
: 
: #!/stor/gnu/bin/perl
: 
: open(STDERR, ">&STDOUT");

Fine.

: open(STDOUT);

What's this supposed to do?

: select(STDERR); $| = 1;         # make unbuffered

You mean, make STDERR command buffered.  Note that this has no effect
on the buffering of subprocesses.

: open(TMP, "ls -h >&STDERR");

This is just plain wrong.  You're trying to open a file called "ls -h >&STDERR",
which I suspect doesn't exist in your current directory.  You have to use a
vertical bar to indicate a piped command, and even if you do that, the string
STDERR will certainly not be recognized by /bin/sh, which only knows about
numeric file descriptors.

: while(<STDOUT>)

Why are you reading from STDOUT?  I'd think you'd want to read from TMP.

: {
: 	s/\+/\%/g;
: 	print $_;
: }
: 
: close(TMP);
: close(STDERR);
: close(STDOUT);
: 
: The stderr is still going to the tty and not the STDERR filehandle.  Is there
: a way to capture the STDERR output of the TMP filehandle.

You're making this all much too hard.  Just say

open(TMP, "ls -h 2>&1 |");
while (<TMP>) {
    tr/+/%/;
    print;
}

Larry