IO::Capture::Stderr(3) Capture all output sent to STDERR


use IO::Capture::Stderr;
$capture = IO::Capture::Stderr->new();
$capture->start(); # STDERR Output captured
print STDERR "Test Line One\n";
print STDERR "Test Line Two\n";
print STDERR "Test Line Three\n";
$capture->stop(); # STDERR output sent to wherever it was before 'start'
# In 'scalar context' returns next line
$line = $capture->read;
print "$line"; # prints "Test Line One"
$line = $capture->read;
print "$line"; # prints "Test Line Two"
# move line pointer to line 1
$line = $capture->read;
print "$line"; # prints "Test Line One"
# Find out current line number
$current_line_position = $capture->line_pointer;
# In 'List Context' return an array(list)
@all_lines = $capture->read;
# Example 1 - "Using in module tests"
# Note: If you don't want to make users install
# the IO::Capture module just for your tests,
# you can just install in the t/lib directory
# of your module and use the lib pragma in
# your tests.
use lib "t/lib";
use IO::Capture:Stderr;
use Test::More;
# Create new capture object. Showing FORCE_CAPTURE_WARN being cleared
# for example, but 0 is the default, so you don't need to specify
# unless you want to set.
my $capture = IO::Capture:Stderr->new( {FORCE_CAPTURE_WARN => 0} );
# execute with a bad parameter to make sure get
# an error.
ok( ! $test("Bad Parameter") );


The module "IO::Capture::Stderr", is derived from the abstract class "IO::Capture". See IO::Capture. The purpose of the module (as the name suggests) is to capture any output sent to "STDOUT". After the capture is stopped, the STDOUT filehandle will be reset to the previous location. E.g., If previously redirected to a file, when "IO::Capture->stop" is called, output will start going into that file again.

Note: This module won't work with the perl function, system(), or any other operation
       involving a fork().  If you want to capture the output from a system command,
       it is faster to use open() or back-ticks.

       my $output = `/usr/sbin/ls -l 2>&1`;



Creates a new capture object.
An object can be reused as needed, so will only need to do one of these.
  • Be aware, any data previously captured will be discarded if a new capture session is started.


  • Start capturing data into the "IO::Capture" Object.
  • Can not be called on an object that is already capturing.
  • Can not be called while STDERR tied to an object.
  • "undef" will be returned on an error.


  • Stop capturing data and point STDERR back to it's previous output location I.e., untie STDERR


  • In Scalar Context
  • Lines are read from the buffer at the position of the "line_pointer", and the pointer is incremented by one.

        $next_line = $capture->read;
  • In List Context
  • The array is returned. The "line_pointer" is not affected.

        @buffer = $capture->read;
  • Data lines are returned exactly as they were captured. You may want to use "chomp" on them if you don't want the end of line character(s)

        while (my $line = $capture->read) {
            chomp $line;
            $cat_line = join '', $cat_line, $line;


  • Reads or sets the "line_pointer".

        my $current_line = $capture->line_pointer;


Pass any arguments to new() in a single array reference.

   IO::Capture::Stderr->new( {FORCE_CAPTURE_WARN => 1} );


Normally, IO::Capture::Stderr will capture text from warn() function calls. This is because output from warn() is normally directed to STDERR. If you wish to force IO::Capture::Stderr to grab the text from warn(), set FORCE_CAPTURE_WARN to a 1. Then "IO::Capture::Stderr" will save the handle that $SIG{__WARN__} was set to, redirect it to itself on "start()", and then set $SIG{__WARN__} back after "stop()" is called.


Adding Features

If you would like to sub-class this module to add a feature (method) or two, here is a couple of easy steps. Also see IO::Capture::Overview.
Give your package a name

    package MyPackage;
Use this "IO::Capture::Stderr" as your base class like this:

    package MyPackage;
    use base qw/IO::Capture::Stderr/;
Add your new method like this

    package MyPackage;
    use base qw/IO::Capture::Stderr/;
    sub grep {
        my $self = shift;
        for $line (

See Also





Mark Reynolds [email protected]

Jon Morgan [email protected]


Copyright (c) 2003, Mark Reynolds. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.