Figure 19.1 shows the relationships between a web browser, web server, and CGI program. When you click on a link while using your browser, there is a URL associated with the link. This URL specifies a web server and a resource accessible through that server. So the browser communicates with the server, requesting the given resource. If, say, the resource is an HTML fill-out form, the web server responds by downloading the form to the browser, which then displays the form for you to fill out.
Each text-input field on the form has a name (given in the form's HTML code) and an associated value, which is whatever you type into the field. The form itself is associated (via the HTML
In this case there are two
When the web server (
in this case) receives the URL from your browser, it invokes the CGI program, passing the
The conversation between the browser and the server, and also between the server and the CGI program, follows the protocol known as HTTP. You needn't worry much about this when writing your CGI program, because CGI.pm takes care of the protocol requirements for you.
The way in which the CGI program expects to receive its arguments (and other information) from the browser via the server is governed by the Common Gateway Interface specification. Again, you don't need to worry too much about this; as you will see, CGI.pm automatically unpacks the arguments for you.
Finally, you should know that CGI programs can work with any HTML document, not just forms. For example, you could write the HTML code
Click <a href="http://www.SOMEWHERE.org/cgi-bin/fortune.cgi">here</a> to receive your fortune.
could be a program that simply invokes the
program (on UNIX systems). In this case, there wouldn't be any argument supplied to the CGI program with the URL. Or the HTML document could give two links for the user to click on - one to receive a fortune, and one to receive the current date. Both links could point to the same program, in one case with the argument
<a href="http://www.SOMEWHERE.org/cgi-bin/fortune_or_date?fortune"> <a href="http://www.SOMEWHERE.org/cgi-bin/fortune_or_date?date">
The CGI program ( fortune_or_date in this case) would then see which of the two possible arguments it received and execute either the fortune or date program accordingly.
So you see that arguments do not have to be of the
In this chapter we will primarily illustrate HTML fill-out forms. And we will assume that you understand basic HTML code already.[ 3 ]