Environment Variables

How CGI receives parameter depends on the operating system. However, most CGI scripts use environment variable to get parameters, here is some.

GATEWAY_INTERFACE	The revision of the Common Gateway Interface that the server uses
SERVER_NAME	The server hostname or IP address.
SERVER_SOFTWARE	The name and version of the server software that is answering the client request.
SERVER_PROTOCOL	The name and revision of the information protocol the request came in with.
SERVER_PORT	The port number of the host on which the server is running.
REQUEST_METHOD	The method with which the information request was issued.
PATH_INFO	Extra path information passed to a CGI program.
PATH_TRANSLATED	The translated version of the path given by the variable PATH_INFO.
SCRIPT_NAME	The virtual path (e.g., /cgi-bin/program.pl) of the script being executed.
DOCUMENT_ROOT	The directory from which Web documents are served.
QUERY_STRING	The query information passed to the program. It is appended to the URL with a "?".
REMOTE_HOST	The remote hostname of the user making the request.
REMOTE_ADDR	The remote IP address of the user making the request.
AUTH_TYPE	The authentication method used to validate a user.
REMOTE_USER	The authenticated name of the user.
REMOTE_IDENT	The user making the request. This variable will only be set if NCSA IdentityCheck flag is enabled, and the client machine supports the RFC 931 identification scheme (ident daemon).
CONTENT_TYPE	The MIME type of the query data, such as "text/html".
CONTENT_LENGTH	The length of the data (in bytes or the number of characters) passed to the CGI program through standard input.
HTTP_FROM	The email address of the user making the request. Most browsers do not support this variable.
HTTP_ACCEPT	A list of the MIME types that the client can accept.
HTTP_USER_AGENT	The browser the client is using to issue the request.
HTTP_REFERER	The URL of the document that the client points to before accessing the CGI program.

Complementary information about the Extra Path Information :

A query is one method of passing information to a CGI program. The other method sends extra path information to the program like that : URL/cgi_script/extra_path_info

Example :

http://www.eaeeie.org/search.cgi/misc/images
The server assigns "/misc/images" to PATH_INFO. This method of passing information to the CGI program is generally used to provide file information, rather than form data.