From: Tom Christiansen <tchrist@mox.perl.com>
Subject: SRC: capturing stdout and stderr differently
Date: 22 May 1998 14:03:22 GMT

To capture a command's stderr and stdout together:

    $output = `cmd 2>&1`;			# either with backticks
    $pid = open(PH, "cmd 2>&1 |");		# or with an open pipe
    while (<PH>) { }				#    plus a read

To capture a command's stdout but discard its stderr:

    $output = `cmd 2>/dev/null`;		# either with backticks
    $pid = open(PH, "cmd 2>/dev/null |");	# or with an open pipe
    while (<PH>) { }				#    plus a read

To capture a command's stderr and discard its stdout:

    $output = `cmd 2>&1 1>/dev/null`;		# either with backticks
    $pid = open(PH, "cmd 2>&1 1>/dev/null |");	# or with an open pipe
    while (<PH>) { }				#    plus a read

To exchange a command's stdout and stderr in order to capture the stderr
but leave its stdout to come out our old stderr:

    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;	# either with backticks
    $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
    while (<PH>) { }				#    plus a read

To read both a command's stdout and its stderr separately, it's easiest
and safest to redirect them separately to files, and then read from
those files when the program is done:

    system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");

--tom
-- 
"SPARC" is "CRAPS" backwards --Rob Pike