Making Your CGI-scripts Server Independent

If your scripts contain server-dependent code, that is, code which is specific to a particular server, you would need to rewrite code if you ever decided to move your site to a different web server. One such occasion would be if you decided to upgrade to or downgrade from a High Volume account, since these accounts reside on high volume servers. Another occasion would be if you ever decided to reuse your scripts for other accounts, or give your scripts to other people. You should always try to write code that will run correctly regardless of what web server it runs on. This makes the script extremely portable and very easy to maintain.

The most common kind of server-dependent code is code which accesses files or programs using an absolute path (such as "/www26/web/someid/somefile"). Instead of using the absolute path to your home directory ("/www26/web/someid"), you should instead use the DOCUMENT_ROOT environment variable ($ENV{DOCUMENT_ROOT} in Perl) to determine the path of your files or programs within a script.

For example, if a guestbook script reads:

# Path to your guestbook file
$guestbook = "/www23/web/examp9/data/guest.html";

You should change it to:

# Path to your guestbook file
$guestbook = "$ENV{DOCUMENT_ROOT}/data/guest.html";

If you are writing scripts for others besides yourself to use, you may wish to take advantage of the many other environment variables which allow you to write server-independent code.

Other Commonly used Server-Independent Codes include:

AUTH_TYPE
* only works if you are viewing a password protected page. If viewing a password protected web page, this will be the "authentication type".

DOCUMENT_NAME
This file's name.

DOCUMENT_URL
This document's URL (path and filename)

DATE_LOCAL
The local representation of date and time of the web server.
(Tue, 03 Mar 1998 21:15:45 EST)

DATE_GMT
The date and time of the web server in GMT.
(Wed, 04 Mar 1998 05:15:46 GMT)

GATEWAY_INTERFACE
The name and version number of the gateway software running on this server. (CGI/1.1)

HTTP_USER_AGENT
The "user agent" or web browser that the visitor is using. (Mozilla/4.0)

LAST_MODIFIED
The last modification date of the current document. (Tue, 03 Mar 1998)

PATH
The paths available to the "nobody" user of the web server.

REMOTE_USER
* only works if you are viewing a password protected page. If viewing a password protected web page, this will be the "username".

REQUEST_METHOD
The method by which the visitor's web browser is retrieving the web pages. Choices are GET and POST.

REMOTE_HOST
The translated name of your (the visitor's) machine.

REMOTE_ADDR
The IP number of your (the visitor's) machine.

SCRIPT_NAME
The name of the script running. (/www26/web/someid/somefile)

SERVER_SOFTWARE
The name of the web server software currently running.

SERVER_NAME
This machine's name, or rather, what this machine thinks its name is.

SERVER_PORT
Standard http/web port.

SERVER_PROTOCOL
The name and version number of the protocol running on this server. (HTTP/1.0)
(Note: if you are writing scripts or recipes for use with procmail, you must use the HOME environment variable in place of DOCUMENT_ROOT. CGI environment variables are not available while running under procmail.)