Common Gateway Interface

In computing, Common Gateway Interface (CGI) offers a standard for web servers to execute programs that execute like Console applications (also called Command-line interface programs) running on a server that generates web pages dynamically. Such programs are known as CGI scripts or simply as CGIs. The specifics of how the script is executed by the server are determined by the server. In the common case, a CGI script essentially executes at the time a request is made and generates HTML.

In 1993 the National Center for Supercomputing Applications (NCSA) team wrote the specification for calling command line executables on the www-talk mailing list; however, NCSA no longer hosts the specification. The other Web server developers adopted it, and it has been a standard for Web servers ever since. A work group chaired by Ken Coar started in November 1997 to get the NCSA definition of CGI more formally defined. This work resulted in RFC 3875, which specified CGI Version 1.1. Specifically mentioned in the RFC are the following contributors:

Historically CGI scripts were often written using the C language. RFC 3875 "The Common Gateway Interface (CGI)" partially defines CGI using C, as in saying that environment variables "are accessed by the C library routine getenv() or variable environ".

Each web server runs HTTP server software, which responds to requests from Web browsers. Generally, the HTTP server has a directory (folder), which is designated as a document collection — files that can be sent to Web browsers connected to this server. For example, if the Web server has the domain name example.com, and its document collection is stored at /usr/local/apache/htdocs in the local file system, then the Web server will respond to a request for http://example.com/index.html by sending to the browser the (pre-written) file /usr/local/apache/htdocs/index.html.

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