CGI Environment Variables

Environment variables are a series of hidden values that the web server sends to every CGI program you run. Your program can parse them and use the data they send. Environment variables are stored in a hash named %ENV:

Some servers set other environment variables as well; check your server documentation for more information. 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. REMOTE_USER is only set for pages in a directory or subdirectory that's password-protected via a .htaccess file. (See Chapter 20 to learn how to password protect a directory.) 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, short of asking them for it with a web form.

You can print the environment variables the same way you would any hash value:

Gary Winnick

print "Caller = $ENV{HTTP_REFERER}\n";

Let's try printing some environment variables. Start a new file named env.cgi:

#!/usr/bin/perl -wT
use strict;
use CGI qw(:standard);
use CGI::Carp qw(warningsToBrowser fatalsToBrowser);

print header;
print start_html("Environment");

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

print end_html;