Perl, Tutorials

Minimal Introduction to Perl

 Welcome to Perlistan world! I have been using Perl (Perl 5.10 specifically) from last 1 year and I am really fond of this programming language. There are hell lots of features exist for Perl but I have written only few which I felt is important and interesting for beginners.

Perl (Practical Extraction & Report Language) is the Swiss Army knife for  scripting languages(rather call it programming language): powerful and adaptable. It was first developed by Larry Wall, a linguist working as a systems administrator for NASA in the late 1980s, as a way to make report processing easier.

Since then, it has moved into a large number of roles: automating system administration, acting as glue between different computer systems; and, of course, being one of the most popular languages for CGI programming on the Web.

Why did Perl become so popular when the Web came along?
There may be at least two most important  reasons:

First, most of what is being done on the Web happens with text, and is best done with a language that’s designed for text processing. More importantly, Perl was appreciably better than the alternatives at the time when people needed something to use. C is complex and can produce security problems (especially with untrusted data), Tcl can be awkward and Python didn’t really have a foothold.

Second: It also didn’t hurt that Perl is a friendly language. It plays well with your personal programming style. The Perl slogan is “There’s more than one way to do it,” The growth of Internet also complemented Perl. The initial attempt at providing dynamic content was through CGI (even now CGI is used extensively), and Perl’s remarkable text handling features made it a quick fit. CGI programming is now synonymous with Perl programming.

CPAN – Comprehensive Perl Archive Network, was set up to share Perl code. Perl supports modules and chances are that for 99% of the programming requirements, there is already a tested module in CPAN (for the remaining 1%, write modules and contribute to CPAN!). Using modules really mask the complexities of adhering to pre-defined standards and frees you to concentrate on your tasks – no point in re-inventing the wheel. Now, you have modules which handles graphics, CGI etc… You can also embed Perl code in your C/C++ programs. A very popular embedded Perl architecture is mod_perl for Apache web server.

Lets dive little bit deeper

Data manipulation
Perl can handle strings, dates, binary data, database connectivity, streams, sockets and many more. This ability to manipulate multiple data types help immensely in data conversion (and by the way, it is much faster than PL/SQL!). Perl also has provision for lists (or arrays) and for hashes (associative arrays). Perl also supports references, which are similar to the pointers in C. Lists, hashes and references together make it possible to define and manipulate powerful custom-defined data-types.

Portability Most of the Perl code will run without any change in Unix or Windows or Macintosh. Typical changes you might have to make include specifying file paths and use of low-level OS specific functions.

CGI Period. Almost all CGI programs written today are using the module from CPAN. Even before this was written, people used to use Perl extensively for CGI programming. made the process streamlined and easy, even for beginners. The graphics library GD is used extensively in producing dynamic web charts. Ever since Kernighan and Ritchie came out with C programming language, people have started learning almost any programming language with the obligatory “Hello World” program. Let us do the same!

Hello World!

Here is the basic perl program that we’ll use to get started.

 #! /usr/bin/perl<br />
	# prints Hello world.<br />
	use strict;<br />
	use warnings;<br />
	print &#39;Hello world.&#39;;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; # Print a message 

#!  is called sh-bang (she bang 😉 ). Always let it be your first line of code in your program.  Write the location of executable perl here. Always try to write use strict and use warnings in your program. It will minimize error and you will be following standard coding rules !

Comments Perl treats any thing from a hash # to the end of line as a comment. Block comments are not possible. So, if you want to have a block of comments, you must ensure that each line starts with #.

Statements Everything other than comments are Perl statements, which must end with a semicolon, like the last line above. Unlike C, you need not put a wrapping character \ for long statements. A Perl statement always ends with a semicolon.

Running Perl Write a small  program using a text editor, and save it. The first line of the program is a typical shell construct, which will make the shell start the interpreter and feed the remaining lines of the file as an input to the interpreter. After you’ve entered and saved the program make sure the file is executable by using the command

<br />
chmod u+x perlfile

at the UNIX prompt, where perlfile  is the filename of the program (of course with .pl or .pm extension mostly). Now, to run the program, just type any of the following at the prompt.

 <br />
perl perlfile

If something goes wrong then you may get error messages, or you may get nothing. You can always run the program with warnings using the command

<br />
perl -w progname

at the prompt. This will display warnings and other (hopefully) helpful messages before it tries to execute the program. To run the program with a debugger use the command

<br />
perl -d progname

When the file is executed Perl first compiles it and then executes that compiled version. Unlike many other interpreted languages, Perl scripts are compiled first, helping you to catch most of errors before program actually starts executing. In this context, the -w switch is very helpful. It will warn you about unused variables, suspicious statements etc.

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