How can I read stderr when a command execution fails in Perl?

Question

Hi,

I am executing a diff command in perl.

my @lines = `/usr/local/bin/diff -udr \"$expected_file\" \"$gen_file\"`;

if ($? != 0)
{
    print ERRFILE "Diff between $expected_file and $gen_file failed\n";
    return $diff_err;
}

Here the diff might have failed because of some reason. For example: the stderr showed that /usr/local/bin/diff: test.txt: No such file or directory. I want to read this message in the program. How I can find the stderr message of the diff command (or grep or any command I execute)?

Appreciate the help in advance.

Thanks, Mathew Liju

Answer 1

This is answered in perlfaq8: How can I capture STDERR from an external command?

If I want to read STDOUT and STDERR of a process, I use IPC::Open3, which comes with Perl. That way, I don't have to merge those streams and later figure out what part of the output came from what.

I would try to avoid temporary files whenever possible (so no 2>file.txt). That's just too much work and code when I can read STDERR directly.

Good luck, :)

Answer 2

There are several CPAN modules that make this easy and let you keep STDOUT and STDERR separate. For example, IO::CaptureOutput would let you do it like this (though you'll need to split the lines yourself):

use IO::CaptureOutput 'qxx';


my ($stdout, $stderr, $ok) = 
    qxx( qq(/usr/local/bin/diff -udr "$expected_file" "$gen_file") );

if (! $ok)
{
    print ERRFILE "Diff between $expected_file and $gen_file failed\n";
    return $stderr;
}

my @lines = split /\n/, $stdout;

-- David