CGI (Common Gateway Interface)

Well, the most basic tool to access system resources is CGI (Common Gateway Interface). CGI is a service provided by all web servers that allows you to 1) create an executable script that the web server may call on demand, 2) pass incoming HTTP GET or POST data to the CGI script, and 3) filter CGI generated answers back to the browser.

You can think of a CGI script as taking the place of the HTML file in the standard web server activity. However, the key difference is dynamism. Rather than containing a pre-defined set of text (like an HTML document) which can only change when the author edits it, a CGI script can dynamically generate any information it is programmed to generate.

A simple example would be a CGI-generated clock that would always show the current time when it is loaded. To do the same thing with HTML, an HTML author would have to edit the HTML document every second with the new time.

A CGI script, on the other hand, can access the time/date resources of the operating system it runs on and independently output the current time whenever it is called without the author ever having to do anything.

If you know Perl, here is the code for that CGI script:

           #!/usr/local/bin/perl
           use Time::localtime;
           print "Content-type:text/html\n\n";
           my $time = localtime;
           print "Today is: " . 
                  ($time->year() + 1900) . "/" . 
                   $time->mon() . "/" . 
                   $time->mday();

Of course, CGI is not a programming language. CGI is an "interface". It defines a way of bridging the web server to the back end resources. It does not say anything about how that bridging will be implemented. In fact, CGI applications can be written in just about any programming language in use today.

So why exactly do most web developers choose to use the programming language Perl for their CGI applications? Could one use another language like C, C++, Apple Script or Visual Basic instead?

This is a good and extremely frequently asked question. In fact, CGI applications can be written in any programming language that is able to accept and process input, and that is able to output the results of that processing.

However, for most of the CGI applications on the web, Perl has been by far the best choice for two main reasons: 1) Perl is the right tool for the job and 2) Perl is easy.

Introduction to Server-Side Processing
Introduction to the Web Application Development Environment (Tools)
Perl is the Right Tool for the Job