perlexternal-process

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

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

Solution

  • 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.