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An Introduction to CGI
Contributor: Mike de Sousa, Director, AbleStable®

Here's your plain and simple introduction into the world of the Common Gateway Interface (CGI or CGIs), an immensely useful computer protocol that provides the opportunity to introduce interactivity and increased functionality into web sites without programming or coding knowledge.

Common sources of information, scripts and programs
The intention of this article is to introduce CGI for those who have little or no knowledge of it. In this article we'll describe what CGI can do, and run through some fundamentals to upload a CGI program effectively. For those wishing to develop CGI scripts of their own or to gain a greater understanding of CGI in general two great places to visit are:

http://www.cgi-resources.com/
http://scriptsearch.internet.com/

If you're using a PC to upload files and set permissions for CGI scripts we'd recommend you use the FTP program Smart FTP available for free from http://www.smartftp.com.

The journey from the client to the server and back

Before we begin our journey it's best we clarify a few principles to the uninitiated. Any script can be called a CGI script as long as it's installed on the server end. Anyone wishing to utilise the advantages of CGI must remember CGI is a two way exchange of information between the 'client-side' and the 'server-side'.

The client-side in this case is the general user viewing a web page on a web browser like Microsoft's Internet Explorer. The server-side is the host computer that delivers the web pages from a remote location. It is not possible to simply cut and paste a CGI script into a web page and hope that it will work on a web site. CGI requires that the host server is CGI enabled. The script processes the requests made of it, then delivers the results of the request in some meaningful form back to the client.

Remote control
The CGI requests are directed to a specific folder on the host server where the main CGI functionality resides. Think of the client's web page (those available to browse) as carrying instructions and commands which, when sent, are received, interpreted and actioned by an automated program.

CGI scripts and programs can be written in any computer language. The most popular languages used for the creation of CGI scripts and programs are:

Perl: the most common CGI scripting language
C+: a common PC based computer programming language
AppleScript: a Macintosh based scripting language.
Unix Shell: CGI scripts written in sh, ksh, csh, or other UNIX shell.
Visual Basic: a Microsoft Windows scripting language

What CGI can do
CGI programs can automatically add a great amount of information on web pages. CGI scripts and programs are used for many tasks, a few of the most common are:

the creation of dynamic documents like search engines
the implementation of a site guestbook or bulletin board
the ability to display the current date and time
the ability to update personal information
the ability to submit forms
the ability to rotate image and text content

CGI requirements
The first task of anyone wanting to use CGI is to ensure their host server:

supports CGI
is running the appropriate operating system (usually UNIX or Linux)
has the appropriate CGI language installed (usually Perl)
has sendmail, qmail or equivalent depending on the CGI to be used
supports the appropriate Perl or equivalent module

You must also have and be able to use an FTP program for uploading files, creating directories, and changing permissions (if your hosting company provides a Control Panel for this purpose, the maximum file size that can be uploaded is often around 32k. Some CGI files may be larger so check first).

There are many FTP programs for PC and Mac that allow the directory and file permissions to be changes (referred to as CHMOD), and a good selection is available for download at http://www.download.com/

Installing a CGI program
The installation of a CGI program will depend on the care and attention the programmer has given to the installation process. The best programs will all-but self-install and contain generous help, information, documentation, and administration areas. But beware, many CGI programs are sadly lacking in this regard. Do your homework before buying a CGI program and know what's on offer and what isn't.

You'll require two tools in addition to the host server to assist with the full implementation of CGI:

an FTP program like Smart FTP available for free from http://www.smartftp.com
a basic text editor (Windows users can use Notepad, MacOS users can try Mactext or BBEdit, and UNIX users can try vi or emacs).

The FTP (File Transfer Protocol) program is an application to transfer files from a local folder to your server). The text editor allows you to configure the CGI program which is a text file ending with .pl (Perl) or .cgi.

The path to nowhere
CGI programs must be placed in a directory allowed to run CGI programs, probably a directory called 'cgi-bin' or similar. Be careful however as there is no agreed protocol on the naming of files and you may find your server's path to the CGI folder is different. Your host should provide you with this information. You are best advised to create a special folder for each CGI program within your 'cgi-bin' to keep your CGI scripts organised.

Establish the path to the Perl compiler on your web host (or other compiler depending on the language being used) by asking your host.

Upload the CGI program files into the directory you decided upon as ASCII/plain text (not binary) and set the file permissions (this process is often referred to as 'chmod files').

In addition you may be required to enter your server's mail program and CGI Bin. The following addresses are provided as a guide, however you must check with your host to ensure your server details are correctly entered (there are no rules here). For sendmail the path may be: /usr/lib/sendmail, and for the CGI-BIN the path may be: /usr/local/etc/httpd/sites/mysite.com/cgi-bin.

Setting permissions
When you first upload your CGI files you'll need to set your 'permissions'. Be aware many common web site generation programs like Macromedia Dreamweaver do not surprisingly support this FTP functionality. You may require a separate FTP like Smart FTP from http://www.smartftp.com which we've found to be the best by far.

Permission numbers translated into attributes (some FTP programs use the term 'Other' or 'Global' instead of the term 'World' as used below):

0755 Global read/execute permission

Owner: read-write-execute
Group: read-execute
World: read-execute

0766 Global read/write permission

(If your server won't write to directories with 0766, use 0777):
Owner: read-write-execute
Group: read-write
World: read-write

0777 Global read/write/execute permission

(Try 0766 first):
Owner: read-write-execute
Group: read-write-execute
World: read-write-execute

Testing
Once you have successfully uploaded your CGI program onto your server you must test it as there are many reasons why a CGI program might not to work. Follow the FAQ checklist below:

is your server CGI enabled?
has the CGI script been uploaded into the correct server folder?
is your CGI file path on the web page correctly configured?
is there a problem with the CGI source code?
have you ensured the correct permissions have been set on the remote server?
have you tested your web page in a variety of browsers and operating systems?

Conclusion
To someone who has never installed a CGI program before it may at first seem an intimidating and complex task to pull off but don't be put off. As we've mentioned, there are great CGI programs out there that will auto-install, and installing one yourself is often only a matter of entering file paths correctly. Once you've CGI enabled your web site you'll wonder how you ever lived without it as it transforms your site into an interactive and dynamic web destination.

     
       
 
Authors background
Mike de Sousa is the Director of AbleStable®. Mike has been commissioned as an artist, music composer, photographer, print and web site designer, and author.

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