CGI Environment Variables (Webmaster in a Nutshell, 3rd Edition)

12.4. CGI Environment Variables

Much 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 (e.g., via the %ENV associative array in Perl). Table 12-1 lists the 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.

Table 12-1. CGI environment variables

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`. See Chapter 17 for a listing of commonly used content types.
DOCUMENT_ROOT	The directory from which web documents are served.
GATEWAY_INTERFACE	The revision of the Common Gateway Interface the server uses.
HTTP_ACCEPT	A list of the media types the client can accept.
HTTP_COOKIE	A list of cookies defined for that URL. (See Chapter 17 for more information.)
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 points to before accessing the CGI program.
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 IP address from which the user is making the request.
REMOTE_HOST	The 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). See Chapter 17 for more information on request methods.
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 of the information protocol the request came in with.
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 "Content-type: text/html", "\n\n";

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

exit (0);

The preceding program outputs the contents of five environment variables into an HTML document. In Perl, you can access the environment variables using the %ENV associative array. Here's a typical output of the program:

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