The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


IO::CaptureOutput - capture STDOUT and STDERR from Perl code, subprocesses or XS


This documentation describes version 1.1102.


     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 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 { 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 { wantarray } ; # returns true
     $rv = capture { wantarray } ; # returns defined, but not true
     capture { 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().



  • Simon Flack <simonflk _AT_> (original author)

  • David Golden <dagolden _AT_> (co-maintainer since version 1.04)


Portions copyright 2004, 2005 Simon Flack. Portions copyright 2007, 2008 David Golden. All rights reserved.

You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.