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 (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

You could explicitly use the Options directive, inside your main server configuration file, to specify that CGI execution was permitted in a particular directory:

        <Directory /usr/local/apache/htdocs/somedir>
                Options +ExecCGI
        </Directory>

You might do this if, for some reason, you really wanted to serve CGI out of a document directory. The ScriptAlias directive is a combination of an Alias directive and an Options +ExecCGI directive, so this is just a ScriptAlias directive without the Alias part.

Writing a CGI Program

There are two main differences between regular programming, and CGI programming.

First, all output from your CGI program must be preceeded by a MIME-type header. This is HTTP header that tells the client what sort of content it is receiving. Most of the time, this will look like:

        Content-type: text/html

Secondly, your output needs to be in HTML, or some other format that a browser will be able to display. Most of the time, this will be HTML, but occasionally you might write a CGI program that outputs a gif image, or other non-HTML content.

Apart from those two things, writing a CGI program will look a lot like any other program that you might write.

Your First CGI Program

The following is an example CGI program that prints one line to your browser. Type in the following, save it to a file called first.pl, and put it in your cgi-bin directory.

        #!/usr/bin/perl
        print "Content-type: text/html\r\n\r\n";
        print "Hello, World.";

Even if you are not familiar with Perl, you should be able to see what is happening here. The first line tells Apache (or whatever shell you happen to be running under) that this program can be executed by feeding the file to the interpreter found at the location /usr/bin/perl. The second line prints the content-type declaration we talked about, followed by two carriage-return newline pairs. This puts a blank line after the header, to indicate the end of the HTTP headers, and the beginning of the body. The third line prints the string ``Hello, World.'' And that's the end of it.

If you open your favorite browser and tell it to get the address

        http://your.server.com/cgi-bin/first.pl

or wherever you put your file, you will see the one line Hello, World. appear in your browser window. It's not very exciting, but once you get that working, you'll have a good chance of getting just about anything working.

But it's Still Not Working!

If your program still is not working, here are some of the things that you need to look for in order to resolve your problem.

File Permissions

Remember that the server does not run as you. That is, when the server starts up, it is running with the permissions of an unpriveleged user--usually nobody or www--and so it will need extra permissions to execute files that are owned by you. Usually, the way to give a file sufficient permissions to be executed by nobody is to give everyone execute permission on the file:

        chmod a+x first.pl

Also, if your program reads from, or writes to, any other files, those files will need to have the correct permissions to permit this.

Path Information

When you run a program from your command line, you have certain information that is passed to the shell without you thinking about it. For example, you have a path, which tells the shell where it can look for files that you reference.

When a program runs through the web server as a CGI program, it does not have that path. Any programs that you invoke in your CGI program (like sendmail, for example) will need to be specified by a full path, so that the shell can find them when it attempts to execute your CGI program.

Syntax Errors

Most of the time when a CGI program fails, it's because of a problem with the program itself. This is particularly true once you get the hang of this CGI stuff, and no longer make the above two mistakes. Always attempt to run your program from the command line before you test if via a browser. This will elimate most of your problems, and will prevent having to look through cryptic error logs.

For More Information

There are a large number of CGI resources on the Web. You can discuss CGI problems with other users on the Usenet group comp.infosystems.www.authoring.cgi. And the -servers mailing list from the HTML Writers Guild is a great source of answers to your questions. You can find out more at http://www.hwg.org/lists/hwg-servers/.

When you post a question about a CGI problem that you're having, whether to a mailing list, or to a newsgroup, make sure you provide enough information about what happened, what you expected to happen, and how what actually happened was different, what server you're running, what language your CGI program was in, and, if possible, the offending code. This will make finding your problem much simpler.

Rich Bowen is the Director of Web Application Development at The Creative Group and the author of Apache Server Unleashed.