How to capture both STDOUT and STDERR in two different variables using Backticks in Perl

Question

Let's say I want to run an external program from my script with backticks and at the same time I want to capture both STDOUT and STDERR but in two different variables. How can I do that? For istance if I run this script...

my $cmd = `snmpwalk -v $version -c $community $hostname $oid`;

...if there is no error everything works just fine BUT if the command raise an error this error will be printed on the command line and I don't want that to happen. I want to capture the error as well. Nothing has to be printed on the screen. Any ideas?

Answer 1

You needn't go all the way to open3 , which IIRC is only for when you need to read and write to an external command, and even then there are other methods.

For your problem I suggest using Capture::Tiny , which can capture (or even tee) the STDOUT and STDERR from anything run inside its block. For example, per your question:

#!/usr/bin/env perl

use strict;
use warnings;

use Capture::Tiny qw/capture/;

...

my ($stdout, $stderr) = capture {
  system ( "snmpwalk -v $version -c $community $hostname $oid" );
};

For another example consider this functioning code:

#!/usr/bin/env perl

use strict;
use warnings;

use Capture::Tiny qw/capture/;

my ($stdout, $stderr) = capture {
  system ( "echo 'hello'" );
  system ( "date" );
  warn "Arg1!";
};

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

which just gave me:

STDOUT:
hello
Mon Dec 19 23:59:06 CST 2011
STDERR:
Arg1! at ./test.pl line 11.

Answer 2

The only way to do this with backticks is to redirect to a file inside the shell command:

   my $cmd = `snmpwalk -v $version -c $community $hostname $oid 2>error.dat`;

If you want to capture the STDERR inside your script, you need IPC::Open3 instead of backticks

Answer 3

In the Perl FAQ you have different options depending how do you want to proceed:

http://perldoc.perl.org/perlfaq8.html#How-can-I-capture-STDERR-from-an-external-command%3f

Answer 4

IO::CaptureOutput

is a very convenient wrapper for what you want to do.