Perl Introduction

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Overview

Getting Started
Recipe for writing a CGI script
How does it all work?

Advanced Topics
Forms
Environment Variables
Counters
Animation
User Authorization
Imagemaps

Overview

Techtalk:

The Common Gateway Interface, or CGI, permits interactivity between a client and a host operating system through the World Wide Web via the Hyper Text Transfer Protocol (HTTP).

Between you and me:

Basically, the Common Gateway Interface allows someone visiting your Web site to run a little program on your machine that performs a specified task. It doesn't matter what language the program is written in, as long as you have the resources to run it on your machine and the program is written correctly.

Getting Started

Below is a recipe for using the Common Gateway Interface. This example and the other examples in this guide use the Perl language because of its portability and relative ease of use. C examples are also included where appropriate. Also Unix is the preferred operating system for this guide.

Ingredients:

CGI enabled HTTP server (or access to one)
Program language compiler or interpreter (i.e. Perl, C)
Willpower and patience (optional)

First, edit a file in your cgi-bin* (or equivalent) directory and name it "hello_world.pl". Place the following lines in your file:
```
       #!/usr/bin/perl
       # The above line must contain the path to your Perl interpreter.
       # If you are not sure what this is, on most unix systems, you may
       # use the command "whereis perl".
       
       print "Content-type: text/html\n\n";
       print "Hello World!!\n";
       
       exit;
     
```
Now, save the file. Next, change the attributes of the file to make it executable*. Finally, create a file in your root HTML documents directory and name it "hello_world.html". Place the following lines in your file:
```
       <HTML>
         <HEAD>
           <TITLE>Hello World CGI Script</TITLE>
         </HEAD>
         <BODY>
           <A HREF="http://insert.path.to.your.script.here/">Say hello to the world!</A>
         </BODY>
       </HTML>
     
```
That's it!! Now load the file "hello_world.html" into your browser and click on the hypertext link; and if all's well, the script should respond with "Hello World!!"

*cgi-bin

This is the directory (designated in your server's configuration files) where you store all of your CGI programs.

*executable

To make this file executable in Unix, type: chmod 755 hello_world.pl

Now, one might ask... how does it all work?

Advanced Topics

Forms: Okay, so we've seen how a CGI script can send information back to a browser, but how do we send information to a CGI script? This can be accomplished using what are referred to as forms. Forms allow for user defined information to be passed along from a Web browser to a CGI program for processing.
Environment Variables: Unlike form variables, environment variables are not user defined but are server defined. These variables are passed along everytime a CGI script is invoked.
Counters: Page counters are used by Web site adminstrators who wish to count the number of accesses to a particular page.
Animation: There are basically two types of animations. One is reffered to as "server push" animations and the other is "client pull" animations. Server push animations are exactly that... a request for an image is sent to the server and in response the server sends back a sequence of images. With client pull animations, the HTML document actually makes the request, thereby "pulling" the sequence of images from the server.
User Authorization: User Authorization allows a site admistrator to "protect" or restrict access to certain areas of a site by requiring the entry of a username and password.
Imagemaps: Imagemaps allow users to click on a particular spot in an image to retrieve another HTML document or to run a specified script.

Maintained by Nick Grant < htmlstudio@talk21.com >

Copyright © 2000 Nick Grant. All Rights Reserved.