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.IX Title "Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators 3pm"
.TH Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators 3pm "2018-07-27" "perl v5.26.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators \- Discourage stuff like "@files = \`ls $directory\`".
.SH "AFFILIATION"
.IX Header "AFFILIATION"
This Policy is part of the core Perl::Critic
distribution.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Backticks are super-convenient, especially for \s-1CGI\s0 programs, but I
find that they make a lot of noise by filling up \s-1STDERR\s0 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.
.PP
.Vb 2
\&    use IPC::Open3 \*(Aqopen3\*(Aq;
\&    $SIG{CHLD} = \*(AqIGNORE\*(Aq;
\&
\&    @output = \`some_command\`;                      #not ok
\&
\&    my ($writer, $reader, $err);
\&    open3($writer, $reader, $err, \*(Aqsome_command\*(Aq); #ok;
\&    @output = <$reader>;  #Output here
\&    @errors = <$err>;     #Errors here, instead of the console
.Ve
.SH "CONFIGURATION"
.IX Header "CONFIGURATION"
Alternatively, if you do want to use backticks, you can restrict
checks to void contexts by adding the following to your
\&\fI.perlcriticrc\fR file:
.PP
.Vb 2
\&    [InputOutput::ProhibitBacktickOperators]
\&    only_in_void_context = 1
.Ve
.PP
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, \f(CW\*(C`system()\*(C'\fR should be used.  Otherwise
assign the result to a variable.
.PP
.Vb 3
\&    \`some_command\`;                      #not ok
\&    $output = \`some_command\`;            #ok
\&    @output = \`some_command\`;            #ok
.Ve
.SH "NOTES"
.IX Header "NOTES"
This policy also prohibits the generalized form of backticks seen as
\&\f(CW\*(C`qx{}\*(C'\fR.
.PP
See perlipc for more discussion on using \f(CW\*(C`wait()\*(C'\fR instead
of \f(CW\*(C`$SIG{CHLD} = \*(AqIGNORE\*(Aq\*(C'\fR.
.PP
You might consider using the \f(CW\*(C`capture()\*(C'\fR 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 \f(CW\*(C`system()\*(C'\fR.
.SH "AUTHOR"
.IX Header "AUTHOR"
Jeffrey Ryan Thalhammer <jeff@imaginative\-software.com>
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2005\-2011 Imaginative Software Systems.  All rights reserved.
.PP
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 \s-1LICENSE\s0 file included with this module.