IO::CaptureOutput(3) capture STDOUT and STDERR from Perl code, subprocesses or XS


version 1.1104


use IO::CaptureOutput qw(capture qxx qxy);
# STDOUT and STDERR separately
capture { noisy_sub(@args) } \$stdout, \$stderr;
# STDOUT and STDERR together
capture { noisy_sub(@args) } \$combined, \$combined;
# STDOUT and STDERR from external command
($stdout, $stderr, $success) = qxx( @cmd );
# STDOUT and STDERR together from external command
($combined, $success) = qxy( @cmd );


This module is no longer recommended by the maintainer - see Capture::Tiny instead.

This module provides routines for capturing STDOUT and STDERR from perl subroutines, forked system calls (e.g. "system()", "fork()") and from XS or C modules.



The following functions will be exported on demand.


    capture \&subroutine, \$stdout, \$stderr;

Captures everything printed to "STDOUT" and "STDERR" for the duration of &subroutine. $stdout and $stderr are optional scalars that will contain "STDOUT" and "STDERR" respectively.

"capture()" uses a code prototype so the first argument can be specified directly within brackets if desired.

    # shorthand with prototype
    capture C< print __PACKAGE__ > \$stdout, \$stderr;

Returns the return value(s) of &subroutine. The sub is called in the same context as "capture()" was called e.g.:

    @rv = capture C< wantarray > ; # returns true
    $rv = capture C< wantarray > ; # returns defined, but not true
    capture C< wantarray >;       # void, returns undef

"capture()" is able to capture output from subprocesses and C code, which traditional "tie()" methods of output capture are unable to do.

Note: "capture()" will only capture output that has been written or flushed to the filehandle.

If the two scalar references refer to the same scalar, then "STDERR" will be merged to "STDOUT" before capturing and the scalar will hold the combined output of both.

    capture \&subroutine, \$combined, \$combined;

Normally, "capture()" uses anonymous, temporary files for capturing output. If desired, specific file names may be provided instead as additional options.

    capture \&subroutine, \$stdout, \$stderr, $out_file, $err_file;

Files provided will be clobbered, overwriting any previous data, but will persist after the call to "capture()" for inspection or other manipulation.

By default, when no references are provided to hold STDOUT or STDERR, output is captured and silently discarded.

    # Capture STDOUT, discard STDERR
    capture \&subroutine, \$stdout;
    # Discard STDOUT, capture STDERR
    capture \&subroutine, undef, \$stderr;

However, even when using "undef", output can be captured to specific files.

    # Capture STDOUT to a specific file, discard STDERR
    capture \&subroutine, \$stdout, undef, $outfile;
    # Discard STDOUT, capture STDERR to a specific file
    capture \&subroutine, undef, \$stderr, undef, $err_file;
    # Discard both, capture merged output to a specific file
    capture \&subroutine, undef, undef, $mergedfile;

It is a fatal error to merge STDOUT and STDERR and request separate, specific files for capture.

    # ERROR:
    capture \&subroutine, \$stdout, \$stdout, $out_file, $err_file;
    capture \&subroutine, undef, undef, $out_file, $err_file;

If either STDOUT or STDERR should be passed through to the terminal instead of captured, provide a reference to undef --- "\undef" --- instead of a capture variable.

    # Capture STDOUT, display STDERR
    capture \&subroutine, \$stdout, \undef;
    # Display STDOUT, capture STDERR
    capture \&subroutine, \undef, \$stderr;


    ($stdout, $stderr, $success, $exit_code) = capture_exec(@args);

Captures and returns the output from "system(@args)". In scalar context, "capture_exec()" will return what was printed to "STDOUT". In list context, it returns what was printed to "STDOUT" and "STDERR" as well as a success flag and the exit value.

    $stdout = capture_exec('perl', '-e', 'print "hello world"');
    ($stdout, $stderr, $success, $exit_code) =
        capture_exec('perl', '-e', 'warn "Test"');

"capture_exec" passes its arguments to "system()" and on MSWin32 will protect arguments with shell quotes if necessary. This makes it a handy and slightly more portable alternative to backticks, piped "open()" and "IPC::Open3".

The $success flag returned will be true if the command ran successfully and false if it did not (if the command could not be run or if it ran and returned a non-zero exit value). On failure, the raw exit value of the "system()" call is available both in the $exit_code returned and in the $? variable.

  ($stdout, $stderr, $success, $exit_code) =
      capture_exec('perl', '-e', 'warn "Test" and exit 1');
  if ( ! $success ) {
      print "The exit code was " . ($exit_code >> 8) . "\n";

See perlvar for more information on interpreting a child process exit code.


    ($combined, $success, $exit_code) = capture_exec_combined(
        'perl', '-e', 'print "hello\n"', 'warn "Test\n"

This is just like "capture_exec()", except that it merges "STDERR" with "STDOUT" before capturing output.

Note: there is no guarantee that text printed to "STDOUT" and "STDERR" in the subprocess will be appear in order. The actual order will depend on how IO buffering is handled in the subprocess.


This is an alias for "capture_exec()".


This is an alias for "capture_exec_combined()".


Bugs / Feature Requests

Please report any bugs or feature requests through the issue tracker at <>. You will be notified automatically of any progress on your issue.

Source Code

This is open source software. The code repository is available for public review and contribution under the terms of the license.


  git clone




This software is copyright (c) 2015 by Simon Flack and David Golden.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.