[Discussion] POSIX, the Love of Backticks and All That Jazz Post: 302990689

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Top Forums UNIX for Advanced & Expert Users [Discussion] POSIX, the Love of Backticks and All That Jazz Post 302990689 by bakunin on Monday 30th of January 2017 06:56:04 PM

01-30-2017

Registered User

Quote:

Originally Posted by Peasant

So if you have small, efficient script with no nested subshells, are backquotes still the lowest denominator e.g will it run without magic cookie on any unix OS ever produced ?

I would never run the risk of having whatever shell executing my code. Out of pure paranoia you won't see any script without a shebang line stating the shell it is intended to run in explicitly.

You see, i once wrote shell scripts for an installed base of ~22k systems worldwide, ranging from IBM 42Ts and 43Ps (desktop systems) up to big SP/2s and everything in between. Believe me, whatever administrative horror you are able to think of - 5 systems somewhere had exactly this and then some. root not allowed to write a file in /tmp? Filesystems with no more free inodes? A user named "root" but with a non-0 UID? I have seen all this and more.

Most of my scripts are not "small" therefore. Not, because they do so much, but because they take extreme care of error handling and take nothing i can think of for granted.

Quote:

Originally Posted by Peasant

Example would be a simple mv script using basename command in subshell.

basename is IMHO a bad example. I would do that with parameter expansion as (in the interest of speed) anything else that could be done with it.

bakunin

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LEARN ABOUT CENTOS

perl::critic::policy::inputoutput::prohibitbacktickoperators

Perl::Critic::Policy::InputOutput::ProhibitBacktickOperaUser(Contributed Perl DocumPerl::Critic::Policy::InputOutput::ProhibitBacktickOperators(3)

NAME

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

AFFILIATION

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

DESCRIPTION

       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()".

AUTHOR

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

COPYRIGHT

       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.

perl v5.16.3							    2014-06-09	   Perl::Critic::Policy::InputOutput::ProhibitBacktickOperators(3)