Class 13: CGI Scripting

Held: Monday, 10 February 2003

Summary: Today we begin our exploration of the CGI scripts that are used to dynamically generate Web pages.

Related Pages:

• EBoard.
• Reading on CGI.
• Lab on CGI.

Due

• Lab writeup 01.

Notes:

• Warning! Many students report that today's topic is both one of the most fun topics and one of the most frustrating topics.
• Reminder: All-campus forum on strategic planning, noon ARH 102.
• If you have a string with embedded control characters (such as newlines), you can see what it should look like with (display str).
• Questions on strings or characters?
• Some of you had trouble submitting today's assignment. Make sure to save as a text file (using Save Definitions as Text from under the Save Other submenu of the File menu.
• Today's lab will probably take part of tomorrow's class, but we'll also do another short lab tomorrow.

Overview:

• What is CGI?
• Giving input to CGI programs.
• Reading input in CGI programs.
• Lab: CGI.

An Introduction to CGI

• The World Wide Web was originally created to serve so-called static pages, pages whose content did not change.
• Soon after the creation of the Web, page authors realized that they wanted to be able to dynamically generate pages.
  • A page might depend on input specified by the user.
  • A page might depend on the output of a program.
• In these cases, the server needs to run a program to generate the page.
• The standard mechanism developed to handle these issues is called CGI, or common gateway interface.
• The standard sequence of steps:
  • The user sends a request for a page which is implemented by a CGI program.
  • The server packages up any information the program might need and executes the program.
  • The program generates HTML (or an image or ...) and sends it back to the server.
  • The server sends it back to the browser.
• Since the program simply generates data, you can use almost any programming language to write CGI scripts.
• How does the server know whether a file is a static page or a CGI script?
  • Sometimes, by its suffix (.cgi represents CGI scripts).
  • Sometimes, by its URL (things in cgi-bin are CGI scripts).
  • Sometimes, by custom server configuration.
• In MathLAN, anything that ends in .cgi is a CGI script.

CGI in Scheme

• Unfortunately, getting the server to call your program is sometimes a little bit harder than you'd like.
  • I've set up some helper files to make life easier.
• Your Scheme program must include a (page) procedure that returns a string that represents an HTML page.
  • You may have written a similar procedure in the strings lab.
  • You have no options on the name. It must be called (page).
• Your CGI script should have the following form

  #!/bin/bash
  /home/rebelsky/bin/schemeweb your-scheme-file
  

• /home/rebelsky/bin/schemeweb is the helper program I've set up to run Scheme-based CGI scripts.

Sending Values to a CGI Program

• How does a CGI program get values from the user?
  • Using a special kind of HTML called a form.
• A form typically has the form

  <form method="get" action="cgi-script">
  <input type="text" name="variable-name">
  <input type="submit" value="text-on-button">
  </form>
  

Getting Values in a CGI Program

• You can use the get-cgi-variable procedure to extract the value associated with a variable name from a form.
• For example

  (define fave-color
    (get-cgi-variable 'color "Blue"))
  

• In order to use get-cgi-variable, you need to load my Web utilities with

  (load "/home/rebelsky/Web/Scheme/webutils.ss")