The Perl Debugger

Sticking in extra print statements is one way to debug your Perl code, but a full-featured debugger can give you more information.
Customizing the Perl Debugger

The first thing to know is that the debugger is simply a Perl library that takes advantage of hooks in the Perl interpreter. You could replace the debugger completely, if you like, by copying the file somewhere and requiring the file in your code in a BEGIN loop:

cp /usr/lib/perl5/5.6.1/ ~/

And, place this line in your program:

BEGIN { require "~/" }

You might do this, for example, if you preferred the syntax and operation of the 5.6 version debugger over the 5.8 version.

You also can specify an alternative debugger with the -d command switch. Perl versions 5.6 onward include DProf, a profiler that uses debugger hooks. You can use it like this:

perl -d:DProf

You also can use the debugger hooks in your own programs. You can set a breakpoint directly in your code by setting the variable $DB::single = 1;, which is useful if you need to debug code in a BEGIN block. Otherwise, they are executed before the debugger gives you a prompt. Or, you could use the hooks to run particular code whenever any subroutine is run. To find out more about these and other hooks, check the perldebug man page.

The debugger has a set of internal variables, also described in the perldebug man page. To change these variables you can use a configuration file, .perldb in the current directory or in your home directory. This configuration file has Perl code that is run when the debugger starts. For example, you can add new commands of your own, like this:

$DB::alias{'quit'} = 's/^quit(\s*)/q/';

This allows you to quit the debugger by typing quit at the prompt. The perldebug man page describes a few similar aliases that might be useful.

A number of debugger options can be set inside the debugger with the O command. The only one I have used changes the pager:

O pager=|less

This way, any command that would print more than a screen of output can be sent through your favorite pager by using a pipe character before the command: |L.

Resources for this article:

Daniel Allen ( discovered UNIX courtesy of a 1,200-baud modem, a free local dial-up and a guest account at MIT, back when those things existed. He has been an enthusiastic Linux user since 1995. He is President of Prescient Code Solutions, a software consulting company in Kitchener, Ontario, and Ithaca, New York.


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