On Thu, Apr 16, 2009 at 11:14, Raheel Hassan <raheel.has...@gmail.com> wrote:
> Hello,
>
> I have seen some Perl scripts where some times these lines are mentioned in
> the begining of the script can any body explain these lines.
>
> *Script-1*
> use DBI; # What is been called here.
snip
The use statement[1] loads a Perl module. In this case you are loading the DBI
module. The DBI module gives you the ability to connect to Relational
Databases.
snip
> BEGIN #What operation is been
> performed by Begin
> {
> unshift �...@inc, "/home/LIBS"; #What is the role of unshift and @INC
> unshift �...@inc, "libdata/perl5/site_perl";
> }
snip
Execution of Perl programs happens in a set of phases[2]: BEGIN, UNITCHECK,
CHECK, INIT, normal code, and END. When you write normal code it executes in
the fifth phase, but you can mark code as belonging to a given phase by
using a named block like BEGIN {} or END {}. In this case the programmer
wants this code to run during the BEGIN phase.
The @INC array[3] holds a list of paths to look in for modules. In this
case the programmer has installed some modules in /home/LIBS and
libdata/perl5/site_perl and wants Perl to be able to find them, so he/she
is using unshift[4] to put them at the start of the array.
The code is in a BEGIN block because use statements are also sneaky BEGIN
blocks (that is they execute in the BEGIN phase), so if the this code were
not in a BEGIN block Perl would throw an error saying that it couldn't
find the modules before his/her code could add the paths to @INC.
1. http://perldoc.perl.org/functions/use.html
2.
http://perldoc.perl.org/perlmod.html#BEGIN%2c-UNITCHECK%2c-CHECK%2c-INIT-and-END
3. http://perldoc.perl.org/perlvar.ht...@inc
4. http://perldoc.perl.org/functions/unshift.html
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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