Class 11: CGI Scripting

Held Tuesday, September 17, 2002

Summary

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

Assignments

• Read Randomness and Simulation.
• Read More CGI Scripting.

Notes:

• As you've requested, I've put last Friday's examples online.
• Any remaining questions on homework 2?
• If you need to reach me ...
  • I'll be available after class
  • I'll be in my office from about 1:30 to 2:15 today and in the MathLAN from about 2:15 to 3:05.
  • I'll check my email tonight (probably some time between 8 and 10 pm)
• The More CGI reading is not yet available. Check after 5pm. If it's still not available, assume that I'm not going to write it.

Overview

• Notes on CGI.
  • 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.
• 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 extract-binding/single procedure to extract the value associated with a variable name from a form.
• For example

  (define fave-color
    (extract-binding/single 'color (get-bindings)))