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 foo.pl
#!/usr/bin/perl -w

use strict;

my $bar = {};

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

sub callFunc {
myFunc()
}

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 ./foo.pl

Loading DB routines from perl5db.pl version 1.3
Editor support available.

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

main::(./foo.pl:5): 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
'Carp.pm' => '1.08 from /usr/share/perl/5.10/Carp.pm'
'Carp/Heavy.pm' => '/usr/share/perl/5.10/Carp/Heavy.pm'
'Config.pm' => '/usr/lib/perl/5.10/Config.pm'
'Config_heavy.pl' => '/usr/lib/perl/5.10/Config_heavy.pl'
'Exporter.pm' => '5.62 from /usr/share/perl/5.10/Exporter.pm'
'IO.pm' => '1.23_01 from /usr/lib/perl/5.10/IO.pm'
'IO/Handle.pm' => '1.27 from /usr/lib/perl/5.10/IO/Handle.pm'
...

To view some lines in the script, we can use the l command :

DB<1> l
5==> my $bar = {};
6
7 sub myFunc {
8: $bar->{'one'} = 1;
9: $bar->{'two'} = 2;
10: print $bar
11 }
12
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::(./foo.pl:17): 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(./foo.pl:14): 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(./foo.pl:10): print $bar
DB<6> v
7 sub myFunc {
8: $bar->{'one'} = 1;
9: $bar->{'two'} = 2;
10==> print $bar
11 }
12
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 `./foo.pl' line 14
. = main::callFunc() called from file `./foo.pl' 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
HASH(0x82496b8)
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!

local usefull databases

By default, linux and unix systems have two usefull databases installed. Those databases may speed up your work as a system administrator.

First of all, there is the locate database. This database contains the list of the files in your system. Imagine you need to edit a file whose name is "password" but you can't remember its path. A quick way to do it would be :

luangsay@ramiro:/tmp$ locate -r password$
/etc/pam.d/common-password
/home/luangsay/personnel/password
/mnt/chroot/usr/share/doc/cdialog-0.9b/samples/password
/usr/share/doc/dialog/examples/password
/usr/share/pam/common-password
/var/lib/pam/password

(-r switch is for using regular expressions).
To initialize the database or update it, you can use the command updatedb. Normally, your operating system does it with a cron task.

The second database is apropos, a french word which means "about". This is a collection of the first description sentences (the whatis sentence) of all the man pages. To know all the manpages that deal with the configuration of passwords, you would type :

luangsay@ramiro:/tmp$ apropos -s 5 password
login.defs (5) - configuration de la suite des mots de passe cachés shadow password
passwd (5) - the password file
shadow (5) - encrypted password file
smbpasswd (5) - The Samba encrypted password file

Here, to create/update the database, you will use makewhatis (or mandb in Debian).

Another interesting database that is less known is perlindex. Quite usefull because perl is (still...) used by many system admins. This tool indexes all your perl modules documentation pages. So, to find all the modules installed on your system that deal with LDAP, you can type:

luangsay@ramiro:/tmp$ perlindex ldap
1 1.791 share/perl5/URI/ldap.pm
2 0.317 share/perl5/URI.pm
3 0.137 share/perl/5.10.0/pod/perlhpux.pod

Highest mark (1.791) gives you the most relevant page. If you pulse 1, you'll get the perldoc page printed. Of course, such database isn't as big as the CPAN one, but it proved to have helped me quite a few times in the past.

Finally, I would like to say a word about maybe my prefered database. I mean, the debian packages database. Debian apt-get system is a lot more faster than redhat yum software and it offers you many more packages. I believe every debian admin knows the power of the "apt-cache search/show" commands so I won't waste my time giving an example. For those who don't have a debian based distribution, you can get an idea of this database on this web page. If you have to install a new solution on your servers, and you don't know what software can handle it, the debian package can give you a precious help to get the information. Even if you don't have debian, it can be very usefull.