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Chapter 3. The Common Gateway Interface

Now that we have explored HTTP in general, we can return to our discussion of CGI and see how our scripts interact with HTTP servers to produce dynamic content. After you have read this chapter, you'll understand how to write basic CGI scripts and fully understand all of our previous examples. Let's get started by looking at a script now.

This script displays some basic information, including CGI and HTTP revisions used for this transaction and the name of the server software:

#!/usr/bin/perl -wT

print <<END_OF_HTML;
Content-type: text/html

    <TITLE>About this Server</TITLE>
<H1>About this Server</H1>
  Server Name:       $ENV{SERVER_NAME}
  Listening on Port: $ENV{SERVER_PORT}
  Server Software:   $ENV{SERVER_SOFTWARE}
  Server Protocol:   $ENV{SERVER_PROTOCOL}

When you request the URL for this CGI script, it produces the output shown in Figure 3-1.

Figure 3-1

Figure 3-1. Output from server_info.cgi

This simple example demonstrates the basics about how scripts work with CGI:

These details define what we will call the CGI environment . Let's explore this environment in more detail.

3.1. The CGI Environment

CGI establishes a particular environment in which CGI scripts operate. This environment includes such things as what current working directory the script starts in, what variables are preset for it, where the standard file handles are directed, and so on. In return, CGI requires that scripts be responsible for defining the content of the HTTP response and at least a minimal set of HTTP headers.

When CGI scripts are executed, their current working directory is typically the directory in which they reside on the web server; at least this is the recommended behavior according to the CGI standard, though it is not supported by all web servers (e.g., Microsoft's IIS). CGI scripts are generally executed with limited permissions. On Unix systems, CGI scripts execute with the same permission as the web server which is generally a special user such as nobody, web, or www. On other operating systems, the web server itself may need to be configured to set the permissions that CGI scripts have. In any event, CGI scripts should not be able to read and write to all areas of the file system. You may think this is a problem, but it is actually a good thing as you will learn in our security discussion in Chapter 8, "Security".

3.1.1. File Handles

Perl scripts generally start with three standard file handles predefined: STDIN, STDOUT, and STDERR. CGI Perl scripts are no different. These file handles have particular meaning within a CGI script, however.

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