Getting STDOUT, STDERR, and response code from external *nix command in perl

Question

I want to execute an external command from within my Perl script, putting the output of both stdout and stderr into a $variable of my choice, and to get the com

Answer 1

Actually, the proper way to write this is:

#!/usr/bin/perl
$cmd = 'lsss';  
my $out=qx($cmd 2>&1);
my $r_c=$?;
print "output was $out\n";
print "return code = ", $r_c, "\n";

You will get a '0' if no error and '-1' if error.

Answer 2

This was exactly the challenge that David Golden faced when he wrote Capture::Tiny. I think it will help you do exactly what you need.

Basic example:

#!/usr/bin/env perl

use strict;
use warnings;

use Capture::Tiny 'capture';

my ($stdout, $stderr, $return) = capture {
  system( 'echo Hello' );
};

print "STDOUT: $stdout\n";
print "STDERR: $stderr\n";
print "Return: $return\n";

After rereading you might actually want capture_merged to join STDOUT and STDERR into one variable, but the example I gave is nice and general, so I will leave it.

Answer 3

STDERR is intended to be used for errors or messages that might need to be separated from the STDOUT output stream. Hence, I would not expect any STDERR from the output of a command like less.

If you want both (or either) stream and the return code, you could do:

my $out=qx($cmd 2>&1);
my $r_c=$?
print "output was $out\n";
print "return code = ", $r_c == -1 ? $r_c : $r_c>>8, "\n";

If the command isn't executable (perhaps because you meant to use less but wrote lsss instead), the return code will be -1. Otherwise, the correct exit value is the high 8-bits. See system.

Answer 4

A frequently given answer to this question is to use a command line containing shell type redirection. However, suppose you want to avoid that, and use open() with a command and argument list, so you have to worry less about how a shell might interpret the input (which might be partly made up of user-supplied values). Then without resorting to packages such as IPC::Open3, the following will read both stdout and stderr:

my ($child_pid, $child_rc);

unless ($child_pid = open(OUTPUT, '-|')) {
  open(STDERR, ">&STDOUT");
  exec('program', 'with', 'arguments');
  die "ERROR: Could not execute program: $!";
}
waitpid($child_pid, 0);
$child_rc = $? >> 8;

while (<OUTPUT>) {
  # Do something with it
}
close(OUTPUT);