Advanced Methods | CGI and Forms
CGI and Forms
Let's now take a look at a very common use of CGI scripts, processing user
input supplied in forms. We'll consider examples of both C
and Perl programs
applied to the problem.
A Very Simple Example
A typical use of gateway scripts is for processing information
submitted on a Web page form. For instance, the following text entry area is
a very simple example of a form.
To test it, enter some text in the box and then
click on the button named "Submit Query". You should then see another page
that echoes the information you entered. (Return to this page using the
"Back" function of the browser.)
Note that we have two things working together in this example. First, text is entered in the form; second, the text is passed on to a program that "processes" this information. In the present case the processing is rather trivial: the information is simply echoed back to the client. But the CGI script can be designed to do much more than this with the information; we'll look at such an example in a little while.
The FORM Tag
The form itself is just HTML (albeit using some HTML commands that we haven't
discussed before now). The form is encompassed in a FORM tag and there are
additional commands for producing individual elements in the form (e.g., text
entry fields, buttons, etc.). Here is the
code that produced the above simple form:
<FORM METHOD="POST" ACTION= "http://csep10.phys.utk.edu/cgi-bin/post-query"> Please enter some text here: <INPUT NAME="entry"> |
The program that processes the information entered into this form, however,
is not HTML. In this case, the processing is done by a compiled
C
program named post-query
that resides in the cgi-bin subdirectory of the
directory holding the server software on a computer with IP address
csep10.phys.utk.edu.
Note that the form uses the value assigned to "ACTION=" in the tag to tell
the server what program it should use to
process the information that is being sent from the
form.
More Information on Constructing Forms
More Complex Examples
An example of a more complicated form (and
more complicated processing of form data) is an online
science quiz.
A similar example at the same site features a form that provides a
quiz in
astronomy.
If you go to the page with the science quiz and use your browser to
view the document source, you'll
note the line:
From this we see that the information entered into this form is processed by a
program called qa.pl
(on the same machine as the form, which is why you don't
see an http-type url assigned to action= in this case).
This program is a Perl
script that has been written specifically to handle the information submitted
by this one form. Rather than simply echoing entered text as
post-query does,
qa.pl examines the
submitted information, extracts the part that corresponds to the answer
to the question, compares this to the correct answer for the question (which is
stored in a database on the server), and then
presents the user with one of various possible pages depending on whether or not
the correct answer to the question was entered. This illustrates that
a gateway script can do much more than simply echo arbitrary text. If you are
interested, here is a listing of the
Perl program qa.pl
If you're interested in learning more about constructing HTML forms, you might
start by consulting this
forms tutorial, and this set of
links.
The material there will tell you more about text entry areas, and show you
how to use other
form elements (checkboxes, radio buttons, scrolling lists, etc.). The former
document includes numerous examples which you can test on the spot. Note that
as long as you are only interested in designing forms (and not in customizing
the ways in which you process the data entered in the forms) you can use these
examples as models for creating
your own working forms, and you can test your creations with the
post-query program on either csep10.phys.utk.edu
or at hoohoo.ncsa.uiuc.edu
(which processes the example forms at the NCSA site).