Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators3pm

Affiliation

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

Overview

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

Configuration

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

    [InputOutput::ProhibitBacktickOperators]
    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

Notes

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().