The Free On-line Dictionary of Computing (30 December 2018):
Common Gateway Interface
(CGI) A standard for running external
programs from a webHTTPserver. 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 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 Unixshell scripts, hence the common name
Here is a trivial CGI script written in Perl. (It requires
the "CGI" module available from CPAN).
use CGI qw(:standard);
print header, start_html,
"Your IP address is: ", remote_host(),
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
Your IP address is: localhost
The CGI program might be saved as the file "test.pl" in the
appropriate directory on a web server,
Accessing the appropriate URL, e.g.
http://acme.com/test/test.pl, 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.