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CGI - What is That? Let's unlock a little bit of the mystery about something called CGI. If it helps, CGI means Common Gateway Interface. This is a method which is used to exchange data between the server (the hardware and software that actually allows you to get to your web site) and a web client (your browser). CGI is actually a set of standards where a program or script (a series of commands) can send data back to the web server where it can be processed.

Typically, you use standard HTML tags to get data from a person, then pass that data to a CGI routine. The CGI routine then performs some action with the data.
Some of the more common uses of CGI include:

• Guestbooks - The CGI routine is responsible for accepting the data, ensuring it is valid, sending an email acknowledgement back to the writer, perhaps sending an email to the webmaster, and creating the guestbook entry itself.
• Email Forms - A simple CGI form routine just formats the data into an email and sends it back to the webmaster. More complicated routines can maintain a database, send an acknowledgement and validate data.
• Mailing List Maintenance - These routines allow visitors to subscribe and unsubscribe from a mailing list. In this case, the CGI routine maintains a database of email addresses, and the better ones send acknowledgements back to the visitor and webmaster.
  

A CGI routine can be anything which understands the CGI standard. A popular CGI language is called PERL, which is simple to understand and use (well, compared to other languages). PERL is a scripting language, which means each time a PERL routine is executed the web server must examine the PERL commands to determine what to do. In contrast, a compiled language such as C++ or Visual Basic can be directly executed, which is faster and more efficient.

Okay, in a nutshell (and greatly simplified), here's how it works:

• You (the webmaster) specify a form tag which includes the name of the CGI routine.
• You create HTML tags which retrieve data from your visitors.
  
• Each of the input tags includes a variable name. The data which is retrieved from the visitor (or directly set if the tag includes the "hidden" qualifier) is placed in the variable name.
  
• When the visitor presses the "submit" button, the CGI routine which was specified in the form tag is executed. At this time, the CGI routine "takes control", meaning the browser essentially is waiting for it to complete.
  
• This CGI routine can get data from variable names. It retrieves the data and does whatever action is required.
  
• When the CGI routine finishes, it returns control back to the web client (the browser).
  

Some important things to remember about CGI routines:

• You can install CGI routines on your own site if your host allows it. Some web hosts do not allow you to install your own routines but do provide some pre-written ones to you. If these are not sufficient for your needs, you can find a remote hosting service to provide the necessary functions.
  
• Generally, if you install your own routines they must be installed in the cgi-bin directory of your site. This is a special location which allows scripts and programs to be executed.
  
• CGI routines work best on Apache-style servers. Windows NT and Windows 2000 do support CGI, but tend to be slow and problematic.
  
• If you use a remote hosting service, you must remember that although they appear to be giving you this for free, you are actually paying a price. Usually they want to display advertisements, although some of them actually take visitors away from your site.
  
• When you write a CGI routine, you have the choice of a scripting language like PERL or a compiled language such as C++ or Visual Basic. Anything which can execute on the web server is acceptable.
  

I hope this short introduction to CGI has cleared up some of the mystery.