The Free On-line Dictionary of Computing (30 December 2018):

Common Gateway Interface
cgi-bin
CGI program
CGI script

    (CGI) A standard for running external
   programs from a 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

     
     
     
     
     
     
     
     
CGI Test
Your IP address is: localhost
     
     

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

   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.

   (http://hoohoo.ncsa.uiuc.edu/cgi).

   (2007-05-22)