CGI Environment Variables (Perl in a Nutshell, 2nd Edition)

9.4. CGI Environment Variables

Most of the information needed by CGI programs is made available via Unix environment variables. Programs can access this information as they would any environment variable (via the %ENV hash in Perl). The table below lists environment variables commonly available through CGI. However, since servers occasionally vary on the names of environment variables they assign, check with your own server documentation for more information.

Environment variable	Content returned
AUTH_TYPE	The authentication method used to validate a user. See REMOTE_IDENT and REMOTE_USER.
CONTENT_LENGTH	The length of the query data (in bytes or the number of characters)passed to the CGI program through standard input.
CONTENT_TYPE	The media type of the query data, such as `text/html`.
DOCUMENT_ROOT	The directory from which web documents are served.
GATEWAY_INTERFACE	The revision of the Common Gateway Interface that the server uses.
HTTP_ACCEPT	A list of the media types that the client can accept.
HTTP_COOKIE	A list of cookies defined for that URL.
HTTP_FROM	The email address of the user making the query (many browsers do not support this variable).
HTTP_REFERER	The URL of the document the client read before accessing the CGI program.
HTTP_USER_AGENT	The browser the client is using to issue the request.
PATH_INFO	Extra path information passed to a CGI program.
PATH_TRANSLATED	The translated version of the path given by the variable PATH_INFO.
QUERY_STRING	The query information passed to the program. It is appended to the URL following a question mark (`?`).
REMOTE_ADDR	The remote IP address from which the user is making the request.
REMOTE_HOST	The remote hostname from which the user is making the request.
REMOTE_IDENT	The user making the request.
REMOTE_USER	The authenticated name of the user making the query.
REQUEST_METHOD	The method with which the information request was issued (e.g., GET, POST, HEAD).
SCRIPT_NAME	The virtual path (e.g., /cgi-bin/program.pl) of the script being executed.
SERVER_NAME	The server's hostname or IP address.
SERVER_PORT	The port number of the host on which the server is running.
SERVER_PROTOCOL	The name and revision number of the server protocol.
SERVER_SOFTWARE	The name and version of the server software that is answering the client request.

Here's a simple Perl CGI script that uses environment variables to display various information about the server:

#!/usr/local/bin/perl

print << EOF
Content-type: text/html

<HTML>
<HEAD><TITLE>About this Server</TITLE></HEAD>
<BODY><H1>About this Server</H1>
<HR><PRE>
Server Name:        $ENV{'SERVER_NAME'}<BR>
Running on Port:    $ENV{'SERVER_PORT'}<BR>
Server Software:    $ENV{'SERVER_SOFTWARE'}<BR>
Server Protocol:    $ENV{'SERVER_PROTOCOL'}<BR>
CGI Revision:       $ENV{'GATEWAY_INTERFACE'}<BR>
<HR></PRE>
</BODY></HTML>

The preceding program outputs five environments as an HTML document. In Perl, you can access the environment variables with the %ENV hash. Here's typical output of the program:

<HTML>
<HEAD><TITLE>About this Server</TITLE></HEAD>
<BODY><H1>About this Server</H1>
<HR><PRE>
Server Name:          www.whatever.com
Running on Port:      80
Server Software:      NCSA/1.4.2
Server Protocol:      HTTP/1.0
CGI Revision:         CGI/1.1
<HR></PRE>
</BODY></HTML>