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The Most Simple Intro to CGI
How the Internet Basically Works
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This
tutorial is not intended to be an in-depth or a 100% technically
correct treatise. It is an explanation in plain English by
analogy of how HTML and CGI-BIN requests are handled. This is
the first installment of a series. Oh, I swear that no animals
were hurt in the making of this web page. |
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The illustration
above is a simple, but accurate representation of the Internet.
We have rats out looking for food, which are our
browsers. We have our garbage cans, better known as
servers providing storage and a way of doling out the food when
asked for it. So in a nutshell, the rat asks for food and the
garbage can gives it to the rat.
Now of course our garbage can in addition to
holding food, also has a bunch of little beasties living in it.
You never heard of a smart garbage can, have you? Now you have.
Let's flip the lid and see what's going on in there.
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Well, actually
there are other little beasties in the Unix world that are
referred to as daemons. Hmmm. To make a file readable and
writable to everyone, we issue the command chmod 666 filename.
Coincidence? I don't think so... Now here is how all of this is
going to make sense to you very quickly:
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Telnet: This daemon is usually called
telnetd. It's job in life is to wait for you to try and
log on to the system. If at your location window in your
browser, you enter telnet:server.com, you will get a
login prompt. This is assuming your browser has a helper
application- and that the server accepts telnet requests.
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FTP: This is the ftpd daemon. It
handles requests that start with ftp. FTP stands for File
Transfer Protocol.
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HTTP: Ah, this is the daemon httpd,
the one that we must concentrate on. The httpd handles all
requests that start with http. For the purposes of doing HTML
and CGI-BIN, this is going to be our favorite beastie!
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When you issue a
command from your browser, such as http://industrialone.com/index.htm,
the request is sent to my server and the httpd daemon finds a
document named index.htm, which is then devoured by a
school of very nasty sharks. They rip the page to shreds to find
out what they have to do. Each shark takes one instruction. For
instance:
If the page has text, three images, and a
midi music file- in all, five sharks will be called to duty.
It is the job of each shark to go and handle retrieving the
requested information and packaging it to go back to the
requesting browser.
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Let's see how our
sharks are doing. It looks like the text shark, two of the image
sharks, and the music shark are doing just fine. Uh oh,
something is wrong. One of our sharks has gone belly up on us.
Did the author name that image file properly? Did somebody
delete the file? Well, it's taps for that shark. The browser
will display its icon for a missing graphic. |
Why is CGI Different?
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First, a big
hello to KJ the Killer Whale. Unlike a simple HTML document
request, a CGI-BIN (Common Gateway Interface) request
requires something a whole lot smarter than a school of hungry,
stupid sharks. When you click on a link that looks like:
http://webstreet.com/cgi-bin/Guestbook/guestbook.cgi
You are really instructing the server to run
an actual program that handles only this one type of request. In
this case it runs the program that calls up the form that allows
you to make an entry in the guestbook. Here is what happens:
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Your browser sends the request shown above
to my server.
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My server runs the guestbook.cgi program.
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The guestbook.cgi program realizes that you
are not adding anything yet, so it makes the decision to
create and return to you, a blank data entry form.
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Your browser then displays this form as it
would any other HTML document.
Please note that the item(s) sent back to
the browser do not have to be text, it can be other types of
information such as a graphical counter image.
So you take a moment and fill in all the
fields and click on the SUBMIT button. Now a more complex
series of actions take place. The command in the browser
location window may look something like this:
/cgi-bin/Guestbook/guestbook.cgi?name=jondoe&email=info@xyz.com
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When KJ receives
the request, it executes a program called guestbook.cgi
that is located in the Guestbook directory under cgi-bin. What
is different now is that in addition to a program request,
information is being fed to KJ. Think of the items listed after
each question mark as a fish. Here is what KJ has to do:
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Load the guestbook.cgi program into memory
for running.
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It notices that it has a school of fish to
munch on and proceeds to split them up using the & as a
way of telling where one fish starts and another one ends.
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KJ then writes a name on each fish so she
can remember what is in that fish.
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Seeing that the e-mail fish has a name in
it, sends a thank you note to the person that signed the book.
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Sends a note to the webmaster that a new
entry is in the guestbook.
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Takes all of the fish, and uses their
information to create a new entry that is tagged onto the
guestbook.html document.
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Creates an HTML document saying "Thank you
for signing my guestbook!" that is returned to the browser.
KJ's job is done. Everything went great and
let's have a round of applause for KJ.
The important concept is that in order to use
CGI-BIN, you must create a real, however big or small computer
program. It is not embedded in your HTML document, but it is
installed in your cgi-bin directory located on your server. |
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