Saturday, March 6, 2010

perl debugger

Perl is (still) a popular scripting language in system administration but it seems to me that few administrators know it has a debugger. I believe that it is a bit sad because this debugger is quite powerful and it can ease your job a lot at writing script and enable you to do it faster. So this article is a short introduction to this useful tool.

To explain how to use the debugger, we need a sample script. Here is a very basic one :
luangsay@ramiro:/tmp$ cat
#!/usr/bin/perl -w

use strict;

my $bar = {};

sub myFunc {
$bar->{'one'} = 1;
$bar->{'two'} = 2;
print $bar

sub callFunc {


One of the feature I do like, is that the debugger is embedded in the perl interpreter (not like with python, which needs the pdb program). To activate the debug mode, just call the interpreter with the d switch :
luangsay@ramiro:/tmp$ perl -d ./

Loading DB routines from version 1.3
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(./ my $bar = {};

First thing you can do to learn the debugger is looking at the help :
DB<1> h
List/search source lines: Control script execution:
l [ln|sub] List source code T Stack trace
- or . List previous/current line s [expr] Single step [in expr]
v [line] View around line n [expr] Next, steps over subs
f filename View source in file Repeat last n or s

If you have a very complicated script, it may be useful to list the loaded perl modules :
DB<1> M
'' => '1.08 from /usr/share/perl/5.10/'
'Carp/' => '/usr/share/perl/5.10/Carp/'
'' => '/usr/lib/perl/5.10/'
'' => '/usr/lib/perl/5.10/'
'' => '5.62 from /usr/share/perl/5.10/'
'' => '1.23_01 from /usr/lib/perl/5.10/'
'IO/' => '1.27 from /usr/lib/perl/5.10/IO/'
To view some lines in the script, we can use the l command :
DB<1> l
5==> my $bar = {};
7 sub myFunc {
8: $bar->{'one'} = 1;
9: $bar->{'two'} = 2;
10: print $bar
11 }
13 sub callFunc {
14: myFunc()
We can see that the debugger is ready to execute the first instruction of the script at line 5. To execute this instruction, we type n:
DB<1> n
main::(./ callFunc()
Now, we are about to call the callFunc function. If we want the debugger to execute just one step inside this funcion, we must use the s command :
DB<4> s
main::callFunc(./ myFunc()

Let's say now that we want to know the value of the bar variable before printing it. We must therefore execute the script till line number 10 :
DB<5> c 10
main::myFunc(./ print $bar
DB<6> v
7 sub myFunc {
8: $bar->{'one'} = 1;
9: $bar->{'two'} = 2;
10==> print $bar
11 }
13 sub callFunc {
14: myFunc()
15 }
We have just entered in another function. The T command gives us a listing of all the function calls that were made to get there :
DB<9> T
. = main::myFunc() called from file `./' line 14
. = main::callFunc() called from file `./' line 17

To view the value of a variable, you may use the p command. But as $bar is a reference of a hash, it proves to be better to do a dump of the variable with the x command. This latter command is very useful for instance to know the state of a huge perl object.
DB<6> p $bar
DB<7> x $bar
0 HASH(0x82496b8)
'one' => 1
'two' => 2

The last feature I would like to explain, is the ability to run perl commands directly in the debugger. This enables you to change values on the fly or to do some tests . For instance, we can load another module to get the time or change the value of $bar->{'two'} :
DB<15> use HTTP::Date;
DB<24> p time2str($time);
Sat, 06 Mar 2010 13:08:40 GMT
DB<25> $bar->{'two'} = 3;
DB<31> x $bar
0 HASH(0x824bfd0)
'one' => 1
'two' => 3

I hope this introduction will make you want to discover a bit more the perl debugger!

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