Execution of CGI Programs

The Perl file that contains the CGI program should be placed in your Web server's cgi-bin directory.

Then, the URL for this program will be something like http://localhost/cgi-bin/test.pl (change localhost to correspond to your Web server's hostname).

Enter this URL into your Web browser and it should display a Web page saying "This is a test."

Note You may wonder how the Web server knows that a CGI program should be executed instead of being displayed. This is an excellent question. It can be best answered by referring to the documentation that came with your particular server.

When the Web server executes your CGI program, it automatically opens the STDIN, STDOUT, and STDERR file handles for you.

• STDIN-The standard input of your CGI program might contain information that was generated by an HTML form. Otherwise, you shouldn't use STDIN. See "Inputs to Your CGI Program" later in this chapter for more information.
• STDOUT-The standard output of your CGI program is linked to the STDIN of the Web browser. This means that when you print information using the print() function, you are essentially writing directly to the Web browser's window. This link will be discussed further in the "HTTP Headers" section later in this chapter.
• STDERR-The standard output of your CGI program is linked to the Web server's log file. This is very useful when you are debugging your program. Any output from the die() or warn() function will be placed into the server's log file. The STDERR file handle is discussed further in the "Debugging CGI Programs" section later in this chapter.

The Web server will also make some information available to your CGI program through environment variables.