The Hypertext Transport Protocol (HTTP) is the common language that web browsers and web servers use to communicate with each other on the Internet. CGI is built on top of HTTP, so to understand CGI fully, it certainly helps to understand HTTP. One of the reasons CGI is so powerful is because it allows you to manipulate the metadata exchanged between the web browser and server and thus perform many useful tricks, including:
Serve content of varying type, language, or other encoding according to the client’s needs.
Check the user’s previous location.
Check the browser type and version and adapt your response to it.
Specify how long the client can cache a page before it is considered outdated and should be reloaded.
We won’t cover all of the details of HTTP, just what is important for our understanding of CGI. Specifically, we’ll focus on the request and response process: how browsers ask for and receive web pages.
If you are interested in understanding more about HTTP than we provide here, visit the World Wide Web Consortium’s web site at http://www.w3.org/Protocols/. On the other hand, if you are eager to get started writing CGI scripts, you may be tempted to skip this chapter. We encourage you not to. Although you can certainly learn to write CGI scripts without learning HTTP, without the bigger picture you may end up memorizing what to do instead of understanding why. This is certainly the most challenging chapter, however, because we ...