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 directive looks like:

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/apache2/cgi-bin/, and should be treated as a CGI program.

For example, if the URL http://www.example.com/cgi-bin/test.pl is requested, Apache will attempt to execute the file /usr/local/apache2/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 programs are often restricted to ScriptAlias'ed directories for security reasons. In this way, administrators can tightly control who is allowed to use CGI programs. However, if the proper security precautions are taken, there is no reason why CGI programs cannot be run from arbitrary directories. For example, you may wish to let 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.

There are two steps to allowing CGI execution in an arbitrary directory. First, the cgi-script handler must be activated using the AddHandler or SetHandler directive. Second, ExecCGI must be specified in the Options directive.

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

The above directive tells Apache to permit the execution of CGI files. You will also need to tell the server what files are CGI files. The following AddHandler directive tells the server to treat all files with the cgi or pl extension as CGI programs:

The .htaccess tutorial shows how to activate CGI programs if you do not have access to httpd.conf.

To allow CGI program execution for any file ending in .cgi in users' directories, you can use the following configuration.

If you wish designate a cgi-bin subdirectory of a user's directory where everything will be treated as a CGI program, you can use the following.

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.

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

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.

Remember that the server does not run as you. That is, when the server starts up, it is running with the permissions of an unprivileged 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:

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

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 may not have the same 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.

A common manifestation of this is the path to the script interpreter (often perl) indicated in the first line of your CGI program, which will look something like:

Make sure that this is in fact the path to the interpreter.

If your CGI program depends on non-standard environment variables, you will need to assure that those variables are passed by Apache.

When you miss HTTP headers from the environment, make sure they are formatted according to RFC 2616, section 4.2: Header names must start with a letter, followed only by letters, numbers or hyphen. Any header violating this rule will be dropped silently.

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. The first thing to do is to make sure that your program runs from the command line before testing it via the web server. For example, try:

(Do not call the perl interpreter. The shell and Apache should find the interpreter using the path information on the first line of the script.)

The first thing you see written by your program should be a set of HTTP headers, including the Content-Type, followed by a blank line. If you see anything else, Apache will return the Premature end of script headers error if you try to run it through the server. See Writing a CGI program above for more details.

The error logs are your friend. Anything that goes wrong generates message in the error log. You should always look there first. If the place where you are hosting your web site does not permit you access to the error log, you should probably host your site somewhere else. Learn to read the error logs, and you'll find that almost all of your problems are quickly identified, and quickly solved.

The suexec support program allows CGI programs to be run under different user permissions, depending on which virtual host or user home directory they are located in. Suexec has very strict permission checking, and any failure in that checking will result in your CGI programs failing with Premature end of script headers.

To check if you are using suexec, run apachectl -V and check for the location of SUEXEC_BIN. If Apache finds an suexec binary there on startup, suexec will be activated.

Unless you fully understand suexec, you should not be using it. To disable suexec, simply remove (or rename) the suexec binary pointed to by SUEXEC_BIN and then restart the server. If, after reading about suexec, you still wish to use it, then run suexec -V to find the location of the suexec log file, and use that log file to find what policy you are violating.

Environment variables are values that float around you as you use your computer. They are useful things like your path (where the computer searches for the actual file implementing a command when you type it), your username, your terminal type, and so on. For a full list of your normal, every day environment variables, type env at a command prompt.

During the CGI transaction, the server and the browser also set environment variables, so that they can communicate with one another. These are things like the browser type (Netscape, IE, Lynx), the server type (Apache, IIS, WebSite), the name of the CGI program that is being run, and so on.

These variables are available to the CGI programmer, and are half of the story of the client-server communication. The complete list of required variables is at Common Gateway Interface RFC.

This simple Perl CGI program will display all of the environment variables that are being passed around. Two similar programs are included in the cgi-bin directory of the Apache distribution. Note that some variables are required, while others are optional, so you may see some variables listed that were not in the official list. In addition, Apache provides many different ways for you to add your own environment variables to the basic ones provided by default.

Other communication between the server and the client happens over standard input (STDIN) and standard output (STDOUT). In normal everyday context, STDIN means the keyboard, or a file that a program is given to act on, and STDOUT usually means the console or screen.

When you POST a web form to a CGI program, the data in that form is bundled up into a special format and gets delivered to your CGI program over STDIN. The program then can process that data as though it was coming in from the keyboard, or from a file

The "special format" is very simple. A field name and its value are joined together with an equals (=) sign, and pairs of values are joined together with an ampersand (&). Inconvenient characters like spaces, ampersands, and equals signs, are converted into their hex equivalent so that they don't gum up the works. The whole data string might look something like:

You'll sometimes also see this type of string appended to a URL. When that is done, the server puts that string into the environment variable called QUERY_STRING. That's called a GET request. Your HTML form specifies whether a GET or a POST is used to deliver the data, by setting the METHOD attribute in the FORM tag.

Your program is then responsible for splitting that string up into useful information. Fortunately, there are libraries and modules available to help you process this data, as well as handle other of the aspects of your CGI program.