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HTML, HTTP, and Your CGI Program (cont'd)

Your CGI Program

What about your CGI program? What is it and how does it fit into this scenario? Well, your CGI program can be anything you can imagine. That is what makes programming so much fun. Your CGI program must be aware of the HTTP request headers coming in and its responsibility to send HTTP response headers back out. Beyond that, your CGI program can do anything and work in any manner you choose.

For the purposes of these tutorials, I concentrate on CGI programs that work on UNIX platforms, and I use the Perl programming language. I focus on the UNIX platform because that is the platform of choice on the Net at this time. The most popular WWW servers are the ncSA httpd, CERN, Apache, and Netscape servers; all these Web servers sit most comfortably on UNIX operating systems. So, for the moment, most platforms on which CGI programs are developed are UNIX servers. It just makes sense to concentrate on the operating system on which most of the CGI applications are required to run.

But why Perl? Well, wouldn't it be nice to work with a language that you didn't have to compile? No messing with painful linker commands. No compilation steps at all. Just type it in and it's ready to go. What about a language that is free? Easy to get a hold of and available on just about any machine on the Net? How about a language that works well with and even looks like C, arguably the most popular programming language in the world? And wouldn't it be nice if that language worked well with the operating system, making each of your system calls easy to implement? And what about a programming language that works on almost any operating system? That way, if you change platforms from UNIX to Windows, NT, or Mac, your programs still run. Heck, why not just ask for a language that's easy to learn and for which a ton of free technical help is available? Ask for it. You've got it! Did that sound like an advertisement? And no, I don't have any vested interest in Perl.

Perl is rapidly becoming one of the most popular scripting languages anywhere because it really does satisfy most of the needs outlined here. It's free, works on almost any platform, and runs as soon as you type it in. As long as you don't have any bugs…

Perl is an excellent choice for all these reasons and more. The more is probably what makes the language so popular. If Perl could do all those wonderful things and turned out to be hard to work with, slow, and not secure, it probably would have lost the popularity war. But Perl is easy to work with, has built-in security features, and is relatively fast.

In fact, Perl was designed originally for working with text, generating reports, and manipulating files. It does all these things fairly well and fairly easily. Larry Wall and Randal L. Schwartz of Programming perl state that "The pattern matching and textual manipulation capabilities of Perl often outperform dedicated C programs."

In addition, Perl has a lovely data structure called the associative array that you can use for database manipulation. The designers of Perl also thought of security when they built the language. It has built-in security features like data-flow tracing, which enables you to find out where data that is not secure originated. This capability often prevents nonsecure operations before they can occur.

Most of these features are not covered in these tutorials. these tutorials does take the time to show you how to use Perl to develop CGI programs, however, which you will find helpful if you have never used Perl or are new to programming. After you get the basics from these tutorials, you should be able to understand other Perl CGI programs on the Net. As an added bonus, by learning Perl, you get an introduction to UNIX and C for free. These reasons were enough to make me want to learn Perl and are the reasons why you will use Perl throughout these tutorials.

At this point, you have a good overview of CGI programming and how the different pieces fit together. As you go through the book, you will see that most of the topics in these first two sections are covered again in more detail and with specific examples. The next steps now are for you to learn more about your server, how to install CGI programs, and what makes CGI programming so different from other programming paradigms.