Common Gateway Interface

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common gateway interface

[‚käm·ən ‚gāt‚wā ′in·tər‚fās]
(computer science)
A protocol that allows the secure data transfer to and from a server and a network user by means of a program which resides on the server and handles the transaction. For example, if an intranet user sent a request with a Web browser for database information, a CGI program would execute on the server, retrieve the information from the database, format it in HTML, and send it back to the user. Abbreviated CGI.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Common Gateway Interface

(World-Wide Web)
(CGI) A standard for running external programs from a World-Wide Web HTTP server. CGI specifies how to pass arguments to the program as part of the HTTP request. It also defines a set of environment variables that are made available to the program. The program generates output, typically HTML, which the web server processes and passes back to the browser. Alternatively, the program can request URL redirection. CGI allows the returned output to depend in any arbitrary way on the request.

The CGI program can, for example, access information in a database and format the results as HTML. The program can access any data that a normal application program can, however the facilities available to CGI programs are usually limited for security reasons.

Although CGI programs can be compiled programs, they are more often written in a (semi) interpreted language such as Perl, or as Unix shell scripts, hence the common name "CGI script".

Here is a trivial CGI script written in Perl. (It requires the "CGI" module available from CPAN).

#!/usr/bin/perl use CGI qw(:standard);

print header, start_html, h1("CGI Test"), "Your IP address is: ", remote_host(), end_html;

When run it produces an HTTP header and then a simple HTML page containing the IP address or hostname of the machine that generated the initial request. If run from a command prompt it outputs:

Content-Type: text/html; charset=ISO-8859-1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" ""> <html xmlns="" lang="en-US" xml:lang="en-US"> <head> <title>Untitled Document</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> </head> <body> <h1>CGI Test</h1>Your IP address is: localhost </body> </html>

The CGI program might be saved as the file "" in the appropriate directory on a web server, e.g. "/home/httpd/test".

Accessing the appropriate URL, e.g., would cause the program to run and a custom page produced and returned.

Early web servers required all CGI programs to be installed in one directory called cgi-bin but it is better to keep them with the HTML files to which they relate unless they are truly global to the site. Similarly, it is neither necessary nor desirable for all CGI programs to have the extension ".cgi".

Each CGI request is handled by a new process. If the process fails to terminate for some reason, or if requests are received faster than the server can respond to them, the server may become swamped with processes. In order to improve performance, Netscape devised NSAPI and Microsoft developed the ISAPI standard which allow CGI-like tasks to run as part of the main server process, thus avoiding the overhead of creating a new process to handle each CGI invocation. Other solutions include mod_perl and FastCGI.

Latest version: CGI/1.1.
This article is provided by FOLDOC - Free Online Dictionary of Computing (

CGI script

(Common Gateway Interface script) A relatively compact program written in a language such as Perl, Tcl, C or C++ that processes data on a Web server. It is commonly used to process a query from the user that was entered on an HTML page (Web page) and returned as an HTML page, in which case the CGI script functions as the glue between the query and the database. For example, it would pull the search data out of the GET request in the HTML form, look it up in the database management system (DBMS) and format the results as an HTML page that is sent back to the user.

CGI scripts were the initial mechanism used to make websites interact with databases and other applications. However, as the Web evolved, other server-side processing methods were developed that are more efficient than CGI, such as Active Server Pages (ASPs), JavaServer Pages (JSPs) and servlets. See ASP, JSP, servlet and FastCGI.

Web Server Evolution
Starting at the top and moving down, this illustration shows Web and application server processing as it evolved initially using only CGI scripts and later using Java components. The separation of logic is portrayed here, and the Web server (HTTP server) and application server may reside in the same or different computers.
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