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Gateways are programs which handle information requests and return the appropriate document or generate a document on the fly. Your server can serve information which is not in a form readable by the client (e.g. an SQL database), and act as a mediator between the two to produce something which clients can use.

Gateways can be used for a variety of purposes, the most common being the handling of FORM requests for HTTP. An HTTP server is often used as a gateway to a legacy information system; for example, an existing body of documents or an existing database application. The Common Gateway Interface is a convention between HTTP server implementors about how to integrate such gateway scripts and programs.

Gateway programs, or scripts, are executable programs which can be run by themselves. They have been made external programs in order to allow them to run under various (possibly very different) information servers interchangably.

Gateways conforming to this specification can be written in any language which produces an executable file. Some of the more popular languages to use include: C or C++, Perl, Python, TCL, shells, and many others. It doesn't matter what language the program is written in, as long as you have the permission and resources to run it on your machine and the program is written correctly.

Getting Started

Some servers allow your CGI programs to be anywhere in your web directories, so long as the file name ends in ".cgi". Others require you to put them only in the "/cgi-bin" directory. Check with your system administrator. Now, create a file called Hello.cgi:

#!/usr/bin/perl

	$t	= "Hello World!";
	print	<<EOT;
Content-type: text/html

	<Title> $t </Title>
	<H1>	$t </H1>
EOT

• The first line must contain the path to your Perl interpreter. Use the command "which perl" to check this.
• Scalar variables names in Perl start with the "$" character. "$t" contains some text to be used later.
• The print statement prints everything following until the "EOT". Text printed to "standard output" goes to the server and thence to the browser.
• The first line printed is an HTTP header to tell the browser that an HTML file is coming.
• HTTP header lines must always be separated by a blank line from actual data.
• The data sent is a valid HTML document. The Perl interpreter replaces "$t" with "Hello World!".

<a href="Hello.cgi">Hello.cgi</a>

Forms

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.

Imagemaps

Imagemaps allow users to click on a particular spot in an image to retrieve another HTML document or to run a specified script.

Input
Environment Variables Headers from the Client

Processing
CGI and Perl Tracking the User

Output
Location Headers Other Headers

Examples

1. The INPUT tag: text entry for The URL-Minder. This is the most important of the form elements; with it you can allow the user to input text or passwords and submit them to the server for CGI processing.

2. The SELECT tag: Navigation Menus for CGI or JavaScript processing.

3. The TEXTAREA tag: Comments Feedback to let your visitors send their comments to an email distribution list.

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Selena Sol's WebWare
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