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>