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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Chapter 3.  The Guestbook CGI Application

CGI for Dynamic Pages

There are two classes of pages on the Web, static and dynamic. A static page is written and stored on a Web server, and the same thing is returned each time a user views the page. This is the easy way to think about Web pages. You have some information to share, so you compose a page and tinker with the HTML tags to get the information to look good. If you have a home page, it is probably in this class.

In contrast, a dynamic page is computed each time it is viewed. This is how pages that give up-to-the-minute stock prices work, for example. A dynamic page does not mean it includes animations; it just means that a program computes the page contents when a user visits the page. The advantage of this approach is that a user might see something different each time he or she visits the page. As we shall see, it is also easier to maintain information in a database of some sort and generate the HTML formatting for the data with a program.

A CGI (Common Gateway Interface) program is used to compute Web pages. The CGI standard defines how inputs are passed to the program as well as a way to identify different types of results, such as images, plain text, or HTML markup. A CGI program simply writes the contents of the document to its standard output, and the Web server takes care of delivering the document to the user's Web browser. The following is a very simple CGI script:

Example 3-1 A simple CGI script.
puts "Content-Type: text/html"
puts ""
puts "<TITLE>The Current Time</TITLE>"
puts "The time is <B>[clock format [clock seconds]]</B>"

The program computes a simple HTML page that has the current time. Each time a user visits the page they will see the current time on the server. The server that has the CGI program and the user viewing the page might be on different sides of the planet. The output of the program starts with a Content-Type line that tells your Web browser what kind of data comes next. This is followed by a blank line and then the contents of the page.

The clock command is used twice: once to get the current time in seconds, and a second time to format the time into a nice looking string. The clock command is described in detail on page 173. Fortunately, there is no conflict between the markup syntax used by HTML and the Tcl syntax for embedded commands, so we can mix the two in the argument to the puts command. Double quotes are used to group the argument to puts so that the clock commands will be executed. When run, the output of the program will look like this:

Example 3-2 Output of Example 3-1.
Content-Type: text/html

<TITLE>The Current Time</TITLE>
The time is <B>Wed Oct 16 11:23:43  1996</B>

This example is a bit sloppy in its use of HTML, but it should display properly in most Web browsers. Example 3-3 includes all the required tags for a proper HTML document.

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