How to Debug Perl Library and Scripts?

 

1. Changing
@INC - where Perl loads its modules
(http://www.wellho.net/mouth/588_Changing-INC-where-Perl-loads-its-modules.html) 

The
@
INC array
is a list of directories 
Perl searches
when attempting to load modules. 

Where
does Perl load modules from in its 
use and require statements?
It loads them from directorys in a special list called 
@INC,
from files with a .pm extension in those directorys. 
 
When
Perl's installed, @INC is set to a list of directorys that includes
generic locations for its standard modules, some release specific
directories, and "." the current directory, which are
checked in order each time you do a 
use or require
 
Some
ways to modify @INC
 

  • **
    You can add to the list in @INC by using the -I command line option:

perl
-I /Users/grahamellis/jan06 i2
 
says
"run the perl program i2, additionally checking the jan06
directory for modules" 

  • **
    You can add to the list within your program by doing so in a BEGIN
    block prior to the 
    use statements:

BEGIN

push @INC,"/Users/grahamellis/jan06"; 

use
demo; 
print "hello world"; 

Rather
curiously, 
use calls
are run at compile time not at run time ... but then so are BEGIN
blocks ... so you put your manipulation of @INC into one of those to
get it to happen early enough. 

  • **
    You can add to the beginning of the list by setting the PERL5LIB
    environment variable prior to running your program:

export
PERL5LIB=/Users/grahamellis/jan06
 

and
you can use a colon separated list for that if you want to pre-pend
more than one directory.

  • **There
    is also the "use lib" pragma - this is probably the most
    common approach. And $PERL5LIB can also be called just $PERLLIB.

2.
Debugging Perl http://ralphie.perlmonk.org/mosix/debug.html

To
check the syntax of your scripts, type "
perl
-c 
scriptname.pl
"
. It is also worthwhile to add the -w switch ("perl -cw ..."),
to turn on warning mode. If you were wise enough to "use strict"
in the body of the code, the syntax check will also specify
inappropriate scalar scoping, which goes a little beyond the normal
syntactical problems that are identified. Once your script will pass
through the syntax check with a "syntax ok" message, you
will have a legal perl script from the standpoint that all of the
statements are legal perl statements. That doesn't mean that the
manner in which any given statement is expressed will return the
result that you are looking for, that the functions that are used in
the code are the right ones for the job, or that the overall
structure of the script does anything like what you think it supposed
to do. It just means that all of the perl i's are dotted and t's are
crossed. (On the other hand, this is not such a minor thing, either.)

3.
perldebug

(
http://perldoc.perl.org/perldebug.html)

Calling
the debugger

There
are several ways to call the debugger:


  • perl -d program_name

On
the given program identified by program_name .


  • perl -d -e 0

Interactively
supply an arbitrary expression using -e .


  • perl -d:Ptkdb program_name

Debug
a given program via the Devel::Ptkdb GUI.


  • perl -dt
    threaded_program_name

Debug
a given program using threads (experimental).

4. Summary
of Subprocess Operations

Table
14.1: Summary of Subprocess Operations

Operation

Standard
Input

Standard
Output

Standard
Error

Waited
for?

system()

Inherited
from program

Inherited
from program

Inherited
from program

Yes

Backquoted
string

Inherited
from program

Captured as
string value

Inherited
from program

Yes

open() command
as filehandle for output

Connected to
filehandle

Inherited
from program

Inherited
from program

Only at time
of close()

open() command
as filehandle for input

Inherited
from program

Connected to
filehandle

Inherited
from program

Only at time
of close()

fork, exec, wait, waitpid

User
selected

User
selected

User
selected

User
selected

 

Above
table is from the book
Learning
Perl
.