2.1 Introduction

When a CGI program is called, the information that is made available to it can be roughly broken into three groups:

• Information about the client, server, and user

• Form data that the user supplied

• Additional pathname information

Most information about the client, server, or user is placed in CGI environment variables. Form data is either incorporated into an environment variable, or is included in the "body" of the request. And extra path information is placed in environment variables.

See a trend here? Obviously, CGI environment variables are crucial to accessing input to a CGI program. In this chapter, we will first look at a number of simple CGI programs under UNIX that display and manipulate input. We will show some examples that use environment variables to perform some useful functions, followed by examples that show how to process HTML form input. Then we will focus our attention on processing this information on different platforms.