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CGI: What the Heck Is That?

  • Posted March 10, 2006
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  • in category CGI

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Let’s unlock a little bit of the mystery about something called
CGI. If it helps any, 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

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 forms 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

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

1) You (the webmaster) specify a form tag which includes the
name of the CGI routine.

2) You create HTML tags which retrieves data from your visitors.

3) 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.

4) 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.

5) This CGI routine can get data from variable names. It
retrieves the data and does whatever action is required.

6) 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 – Addr.Com is an example of a web host which allows
for CGI routines. 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

- CGI routines work best on Apache-style servers. Windows NT and
Windows 2000 does support CGI, but it tends to be slow and

- 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

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


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  • Posted On March 10, 2006
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