Apache Guide: Dynamic Content with CGI

With this column Rich Bowen begins his weekly look at basic tasks that every Apache Webmaster must face. Rich is the author of Apache Server Unleashed.

The CGI is the simplest, and most common, way to put dynamic content on your web site. This week's column will be an introduction to setting up CGI on your Apache Web server and getting started writing CGI programs.

The CGI (Common Gateway Interface) is the simplest, and most common, way to put dynamic content on your web site. This week's column will be an introduction to setting up CGI on your Apache Web server and getting started writing CGI programs.

Configuring Apache to Permit CGI

In order to get your CGI programs to work properly, you'll need to have Apache configured to permit CGI execution. There are several ways to do this.

ScriptAlias

The ScriptAlias directive tells Apache that a particular directory is set aside for CGI programs. Apache will assume that every file in this directory is a CGI program and will attempt to execute it, when that particular resource is requested by a client.

The ScriptAlias direcive looks like:

        ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/

The example shown is from your default httpd.conf configuration file, if you installed Apache in the default location. The ScriptAlias directive is much like the Alias directive, which defines a URL prefix that is to mapped to a particular directory. Alias and ScriptAlias are usually used for directories that are outside of the DocumentRoot directory. The difference between Alias and ScriptAlias is that ScriptAlias has the added meaning that everything under that URL prefix will be considered a CGI program. So, the example above tells Apache that any request for a resource beginning with /cgi-bin/ should be served from the directory /usr/local/apache/cgi-bin/, and should be treated as a CGI program.

For example, if the URL http://dev.rcbowen.com/cgi-bin/test.pl is requested, Apache will attempt to execute the file /usr/local/apache/cgi-bin/test.pl and return the output. Of course, the file will have to exist, and be executable, and return output in a particular way, or Apache will return an error message.

CGI Outside of ScriptAlias Directories

Occasionally you will want to have CGI programs outside of ScriptAlias'ed directories. Usually, this will be for the purpose of letting users have web content in their home directories with the UserDir directive. If they want to have their own CGI programs, but don't have access to the main cgi-bin directory, they will need to be able to run CGI programs elsewhere.

.htaccess Files

A .htaccess file is a way to set configuration directives on a per-directory basis. When Apache serves a resource, it looks in the directory from which it is serving a file for a file called .htaccess, and, if it finds it, it will apply directives found therein. .htaccess files can be permitted with the AllowOverride directive, which specifies what types of directives can appear in these files, or if they are not allowed at all. To permit the directive we will need for this purpose, the following configuration will be needed:

        AllowOverride Options


In the .htaccess file, you'll need the following directive:
        Options +ExecCGI


which tells Apache that execution of CGI programs is permitted in this
directory.
Explicitly using Options to Permit CGI Execution