++ed by:

38 PAUSE user(s)
22 non-PAUSE user(s).

Jeffrey Ryan Thalhammer


Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators - Discourage stuff like @files = `ls $directory`.


This Policy is part of the core Perl::Critic distribution.


Backticks are super-convenient, especially for CGI programs, but I find that they make a lot of noise by filling up STDERR with messages when they fail. I think its better to use IPC::Open3 to trap all the output and let the application decide what to do with it.

    use IPC::Open3 'open3';
    $SIG{CHLD} = 'IGNORE';

    @output = `some_command`;                      #not ok

    my ($writer, $reader, $err);
    open3($writer, $reader, $err, 'some_command'); #ok;
    @output = <$reader>;  #Output here
    @errors = <$err>;     #Errors here, instead of the console


Alternatively, if you do want to use backticks, you can restrict checks to void contexts by adding the following to your .perlcriticrc file:

    only_in_void_context = 1

The purpose of backticks is to capture the output of an external command. Use of them in a void context is likely a bug. If the output isn't actually required, system() should be used. Otherwise assign the result to a variable.

    `some_command`;                      #not ok
    $output = `some_command`;            #ok
    @output = `some_command`;            #ok


This policy also prohibits the generalized form of backticks seen as qx{}.

See perlipc for more discussion on using wait() instead of $SIG{CHLD} = 'IGNORE'.

You might consider using the capture() function from the IPC::System::Simple module for a safer way of doing what backticks do, especially on Windows. The module also has a safe wrapper around system().


Jeffrey Ryan Thalhammer <jeff@imaginative-software.com>


Copyright (c) 2005-2011 Imaginative Software Systems. All rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of this license can be found in the LICENSE file included with this module.

Hosting generously
sponsored by Bytemark