CGI: The Common Gateway Interface for Server-side Processing

-Parikshit Chudasma

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). It's a standard for external gateway programs to interface with information servers, such as HTTP or Web servers. A plain HTML document that the Web server delivers is static, which means it doesn't change. A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information - perhaps a weather reading, or the latest results from a database query. CGI allows someone visiting your Web site to run a program on your machine that performs a specified task.

 

NCSA/The Common Gateway Interface 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

Here is a simple example demonstrating the Common Gateway Interface. This example uses the Perl language because of its portability and relative ease of use. When we explain operating system commands we will generally speak UNIX. Note that UNIX is CaSe-sEnSiTiVe.

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!".
Save the file and exit. Change the attributes of the file to make it executable: chmod +755 hello.cgi Create a link to it like this: 
<a href="Hello.cgi">Hello.cgi</a>
Click on Hello.cgi, and if all's well, the script should respond with "Hello World!"
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.

 
& at The

 
Selena Sol's WebWare A monthly column for the cultural anthropologist and other liberal arts hackers gone Webmaster.
CGI-Capable Web Servers ServerWatch is the ultimate guide to Internet servers and Web development tools, and has information about which Web servers are CGI-capable.
CGI Authoring Resources If you can't find what you're looking for, the WDVL's CGI Authoring Resources page will get you up to speed in no time!
Introduction to Web Programming: CGI Introduction to Web Programming is a four half-day course designed by Selena Sol.

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