perltutorial toolic Are you new to Perl? Is your program misbehaving? Not sure where or how to begin debugging? Well, here is a concise checklist of tips and techniques to get you started. This list is meant for debugging some of the most common Perl programming problems; it assumes no prior working experience with the Perl debugger ([doc://perldebtut]). Think of it as a First Aid kit, rather than a fully-staffed state-of-the-art operating room. These tips are meant to act as a guide to help you answer the following questions: <ul> <li>Are you sure your data is what you think it is? <li>Are you sure your code is what you think it is? <li>Are you inadvertently ignoring error and warning messages? </ul> <readmore> <ol> <li>Add the "stricture" pragmas ([id://111088])</li> <c> use strict; use warnings; use diagnostics; </c> <li>Display the contents of variables using [doc://print] or [doc://warn]</li> <c> warn "$var\n"; print "@things\n"; # array with spaces between elements </c> <li>Check for unexpected whitespace</li> <ul> <li>[doc://chomp|<c>chomp</c>], then [doc://print|<c>print</c>] with delimiters of your choice, such as colons or balanced brackets, for visibility <c> chomp $var; print ">>>$var<<<\n"; </c> </li> <li> Check for unprintable characters by converting them into their ASCII hex codes using [doc://ord|<c>ord</c>] <c> my $copy = $str; $copy =~ s/([^\x20-\x7E])/sprintf '\x{%02x}', ord $1/eg; print ":$copy:\n"; </c> </li> </ul> <li> [doc://Data::Dumper|Dump] arrays, hashes and arbitrarily complex data structures. You can get started using the core module [doc://Data::Dumper]. Should the output prove to be unsuitable to you, other alternatives can be downloaded from CPAN, such as [mod://Data::Dump], [mod://YAML], or [mod://JSON]. See also [id://481745] </li> <c> use Data::Dumper; print Dumper(\%hash); print Dumper($ref); </c> <li> If you were expecting a [doc://ref|<c>ref</c>]erence, make sure it is the right kind (ARRAY, HASH, etc.) </li> <c> print ref $ref, "\n"; </c> <li> Check to see if your code is what you thought it was: [doc://B::Deparse|<c>B::Deparse</c>] </li> <c> $ perl -MO=Deparse -p program.pl </c> <li> Check the return ([doc://perlvar#Error-Indicators|error]) status of your commands </li> <ul> <li> [doc://open|<c>open</c>] with [doc://perlvar#$!|<c>$!</c>] <c> open my $fh, '<', 'foo.txt' or die "can not open foo.txt: $!"; </c> </li> <li> [doc://system|<c>system</c>] and backticks ([doc://qx|<c>qx</c>]) with [doc://perlvar#$?|<c>$?</c>] <c> if (system $cmd) { print "Error: $? for command $cmd" } else { print "Command $cmd is OK" } $out = `$cmd`; print $? if $?; </c> </li> <li> [doc://eval|<c>eval</c>] with [doc://perlvar#$@|<c>$@</c>] <c> eval { do_something() }; warn $@ if $@; </c> </li> </ul> <li> Use [doc://Carp] to display variables with a stack trace of module names and function calls. </li> <c> use Carp qw(cluck); cluck("var is ($var)"); </c> Better yet, install and use the [mod://Carp::Always|<c>Carp::Always</c>] CPAN module to make your existing [doc://warn|<c>warn</c>]/[doc://die|<c>die</c>] complain with a stack trace: <c> $ perl -MCarp::Always program.pl </c> <li> Demystify regular expressions by installing and using the CPAN module [mod://YAPE::Regex::Explain|<c>YAPE::Regex::Explain</c>] </li> <c> # what the heck does /^\s+$/ mean? use YAPE::Regex::Explain; print YAPE::Regex::Explain->new('/^\s+$/')->explain(); </c> <li> Neaten up your code by installing and using the CPAN script [mod://perltidy]. Poor indentation can often obscure problems. </li> <li> Checklist for debugging when using CPAN modules: </li> <ul> <li> Check the Bug List by following the module's "View Bugs" link. </li> <li> Is your installed version the latest version? If not, check the change log by following the "Changes" link. Also follow the "Other Tools" link to "Diff" and "Grep" the release. </li> <li> If a module provides status methods, check them in your code as you would check return status of built-in functions: <c> use WWW::Mechanize; if ($mech->success()) { ... } </c> </ul> </ol> What's next? If you are not already doing so, use an editor that understands Perl syntax (such as vim or emacs), a GUI debugger (such as [mod://Devel::ptkdb]) or use a full-blown IDE. Lastly, use a version control system so that you can fearlessly make these temporary hacks to your code without trashing the real thing. For more relevant discussions, refer to the initial Meditation post: [id://744845] Updated: Sep 8, 2009: Added CPAN Diff/Grep tip. Updated: Jan 11, 2011: Added Carp::Always. </readmore>