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Capturing STDERR using IO::Handle

by rvosa (Curate)
on Nov 26, 2007 at 08:57 UTC ( #652944=perlquestion: print w/ replies, xml ) Need Help??
rvosa has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

I am trying to capture the STDERR from a process I'm launching with system(). I thought I had it all figured out by doing:
use IO::Handle; my @logmessages; my $stderr = IO::Handle->new; $stderr->autoflush( 1 ); $stderr->fdopen( fileno( STDERR ), 'r' ); system( 'command', 'with', 'args' ); while( defined( my $line = $stderr->getline ) ) { push @logmessages, $line; } $stderr->close; # now do something useful with @logmessages
However, things seem to get stuck in the while loop. What am I doing wrong? What's the canonical way to capture STDERR (I know, I know, TMTOWTDI so I may not need the canonical way, just one that works).


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Re: Capturing STDERR using IO::Handle
by ysth (Canon) on Nov 26, 2007 at 09:11 UTC
    You are suffering from a little confusion. The fdopen method doesn't associate some other filehandle with your IO::Handle; it's the other way around: after the fdopen, your $stderr will be associated with whatever STDERR was before (only with the incorrect 'r' mode).

    To capture STDERR of a system call, you'd want the stderr output to go to some temporary file and then (rewind if necessary and) read from the file.

Re: Capturing STDERR using IO::Handle
by ikegami (Pope) on Nov 26, 2007 at 09:28 UTC

    IPC::Open3's open3 is one way.

    use IPC::Open3 qw( open3 ); use Symbol qw( gensym ); my $pid = open3( my $to_chld = gensym(), my $fr_chld = gensym(), my $fr_chld_err = gensym(), 'command', 'with', 'args' ); close($to_chld); close($fr_chld); my @logmessages = <$fr_chld_err>; waitpid($pid, 0);


      IO::CaptureOutput is another way.

      use IO::CaptureOutput qw/capture_exec/; my ($stdout, $stderr) = capture_exec( 'command', 'with', 'args' );


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Re: Capturing STDERR using IO::Handle
by phio (Acolyte) on Nov 26, 2007 at 11:44 UTC
    I think you can get what you need here. Also, you can find some other examples in the 'qx' section in perlop
Re: Capturing STDERR using IO::Handle
by DrHyde (Prior) on Nov 26, 2007 at 11:44 UTC
      from Tie::STDERR docs:

      The module will catch all output, including your explicit prints to STDERR... However, if you run external command (system, ``), stderr output from that process won't be caught.

      So, it is not what the OP is looking for!

Re: Capturing STDERR using IO::Handle
by educated_foo (Vicar) on Nov 26, 2007 at 14:04 UTC
    Perlfunc has an example that does exactly what you want under open.

      I don't see anything of the kind. Could you be more specific, please?

      Remember tie(*HANDLE, ...) and open(..., \$var) don't produce system file handles, so they can't be inherited by child processes.

        Here is a script that saves, redirects, and restores STDOUT and STDERR using various methods:
        #!/usr/bin/perl open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!" +; open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; select STDERR; $| = 1; # make unbuffered select STDOUT; $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; print STDOUT "stdout 2\n"; print STDERR "stderr 2\n";
        It would be straightforward to then slurp in the contents of the saved-off files. Or did I misunderstand the question?
Re: Capturing STDERR using IO::Handle
by Sidhekin (Priest) on Nov 26, 2007 at 15:48 UTC

    This can't be done with simple ties.

    It can be done with Test::Trap, though. :) (Shameless as ever ....)

    use Test::Trap qw/ :flow:stderr(systemsafe) /; # or similar trap { system 'command', 'with', 'args' }; my @logmessages = split /\n/, $trap->stderr; # chomped! or season to t +aste

    Not exclusively for use in test scripts, indeed. :)

    print "Just another Perl ${\(trickster and hacker)},"
    The Sidhekin proves Sidhe did it!

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