web:cgi

About

Common Gateway Interface (CGI) is the oldest interface, and is supported by nearly every web server out of the box.

The web server will:

parse the HTTP request into environment variable
call an cgi-compatible executable:
- located by the URL path
- passing the HTTP request in environment variables
takes the output and sends the response

What is a cgi-compatible executable?

The executable is called a cgi-compatible executable if it:

takes into account the request env variables
and returns a HTTP response in its standard output

Any script (written in php, python, bash, ..) can be an a cgi-compatible executable.

Example

Php

This section shows how to use Php as a CGI script.

The web server would be configured to call the php-cgi executable.

For instance, you could:

write the below php script hello-world.php in the root directory of your web server (supporting cgi and php)
call it at http://127.0.0.1/hello-world.php

<html>
 <head>
  <title>PHP Test</title>
 </head>
 <body>
 <?php echo '<p>Hello World</p>'; ?> 
 </body>
</html>

Python

You may need to save this code with a .py or .cgi extension.
Additionally, this file may also need to be in a cgi-bin folder, for security reasons.

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

# cgitb enable debugging
# nice call trace vs “Internal Server Error”  
import cgitb
cgitb.enable()

print "Content-Type: text/plain;charset=utf-8"
print

print "Hello World!"

Specification

URL / URI

The URL takes this specific form

<scheme>://<host>:<port>/<path>?<query>

where:

path is the path of the script to call
query is available in the env variable QUERY_STRING

Request

Environment variables ¹⁾ are used to pass data about the http request from the server to the script.

AUTH_TYPE
CONTENT_LENGTH
CONTENT_TYPE
GATEWAY_INTERFACE
HTTP_*
PATH_INFO
PATH_TRANSLATED
QUERY_STRING
REMOTE_ADDR
REMOTE_HOST
REMOTE_IDENT
REMOTE_USER
REQUEST_METHOD
SCRIPT_NAME
SERVER_NAME
SERVER_PORT
SERVER_PROTOCOL
SERVER_SOFTWARE

Response

A script will return the response via the standard output file descriptor ²⁾

The output may have one of the following format:

Non-parsed Header: The script returns a original HTTP response message as specified by the HTTP specification.
Parsed Header: The script returns a CGI response message. ³⁾

Performance

Programs using CGI to communicate with their web server need to be started by the server for every request. So, every request starts a new process (for python, a new Python interpreter) – which takes some time to start up – thus making the whole interface only usable for low load situations.

FastCGI tries to solve this shortcoming by loading the code into memory and making the script wait for a second request, making the process faster.