CGI Scripts

CGI and Forms

CGI Caveats | Language Choice | 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"> 

To submit the query, press this button: 
<INPUT TYPE="submit"
VALUE="Submit Query">. 


</FORM>

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

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).

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 use your browser to view the document source for one of the science quiz question pages (i.e., one of the pages in the quiz that asks, not answers, a question), 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.

References

For additional information about CGI, see this Programming References page.