07-08-2005
by John Saya

Environment variables are a series of hidden values that the web server sends to every CGI program that is run. You can use these variables for many different purposes within your own CGI programs. Environment variables are stored in a hash named %ENV.

Below is an example of the typical variables that a web server may have available to you.

Variable	Value
DOCUMENT_ROOT	The root directory (folder) of your server
HTTP_COOKIE	The visitor's cookie, if one is set
HTTP_HOST	The hostname of the page being attempted
HTTP_REFERER	The URL of the page that called your program
HTTP_USER_AGENT	The browser type of the visitor
HTTPS	"on" if the program is being called through a secure server
PATH	The system path (folder) your server is running under
QUERY_STRING	The query string passed to your program
REMOTE_ADDR	The IP address of the visitor
REMOTE_HOST	The hostname of the visitor (if your server has reverse-name-lookups on; otherwise this is the IP address again)
REMOTE_PORT	The port the visitor is connected to on the web server
REMOTE_USER	The visitor's username (for .htaccess protected pages)
REQUEST_METHOD	GET or POST
REQUEST_URI	The interpreted pathname of the requested document or CGI (relative to the document root)
SCRIPT_FILENAME	The full pathname of the current CGI
SCRIPT_NAME	The interpreted pathname of the current CGI program (relative to the document root)
SERVER_ADMIN	The email address for your server's webmaster
SERVER_NAME	Your server's fully qualified domain name (e.g. www.cgiconnection.com)
SERVER_PORT	The port number your server is listening on
SERVER_SOFTWARE	The server software you're using (e.g. Apache 1.3)

Some servers set other environment variables as well. Depending on yours, you may have more or less. Notice that some environment variables give information about your server, and will never change (such as SERVER_NAME and SERVER_ADMIN), while others give information about the visitor, and will be different every time someone accesses the program.

Not all environment variables get set. For example, REMOTE_USER is only set for pages in a directory (folder) or subdirectory that is password-protected using an .htaccess file. And even then, REMOTE_USER will be the username as it appears in the .htaccess file; it's not the person's email address. There is no reliable way to get a person's email address using environment variables.

You can get the value of any variable like this:

#!/usr/bin/perl

print "Content-type: text/html\n\n";
print "My web browser is: $ENV{'HTTP_USER_AGENT'}<br />\n";
print "My IP Address is: $ENV{'REMOTE_ADDR'}<br />\n";

exit;

When you run the above script, you should see your web browser's name and version, and also your IP Address. You can see all of the environment variables on your server by running a small Perl script on your web site, shown below. It's such a simple program, but it gives a lot of valuable information.

#!/usr/bin/perl

print "Content-type: text/html\n\n";

foreach my $key (sort(keys(%ENV))) {
    print "$key = $ENV{$key}<br />\n";
}

exit;

Environment variables are used heavily by programmers. They're used to track your IP Address, see what web site you came from, to transfer information back from your web browser to the CGI script on your web site, and many other ways.

If you want a quick way to see how your environment variables determine how things run our your web site, take a look at System Sleuth.

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