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<title>Apache Tutorial: Dynamic Content with CGI</title>

<section id="intro">

<title>Introduction</title>

<related>

<modulelist>

<module>mod_alias</module>

<module>mod_cgi</module>

</modulelist>

<directivelist>

<directive module="mod_mime">AddHandler</directive>

<directive module="core">Options</directive>

<directive module="mod_alias">ScriptAlias</directive>

</directivelist>

</related>

<p>The CGI (Common Gateway Interface) defines a way for a web

server to interact with external content-generating programs,

which are often referred to as CGI programs or CGI scripts. It

is the simplest, and most common, way to put dynamic content on

your web site. This document will be an introduction to setting

up CGI on your Apache web server, and getting started writing

CGI programs.</p>

</section>

<section id="configuring">

<title>Configuring Apache to permit CGI</title>

<p>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.</p>

<section id="scriptalias">

<title>ScriptAlias</title>

<p>The

<directive module="mod_alias">ScriptAlias</directive>

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.</p>

<p>The

<directive module="mod_alias">ScriptAlias</directive>

directive looks like:</p>

<example>ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/</example>

<p>The example shown is from your default

<code>httpd.conf</code>

configuration file, if you installed Apache in the default

location. The <code>ScriptAlias</code> directive is much like the

<code>Alias</code> directive, which defines a URL prefix that

is to mapped to a particular directory. <code>Alias</code>

and <code>ScriptAlias</code> are usually used for directories

that are outside of the <code>DocumentRoot</code> directory.

The difference between <code>Alias</code> and

<code>ScriptAlias</code> is that <code>ScriptAlias</code>

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

<code>/cgi-bin/</code> should be served from the directory

<code>/usr/local/apache/cgi-bin/</code>, and should be treated

as a CGI program.</p>

<p>For example, if the URL

<code>http://www.example.com/cgi-bin/test.pl</code>

is requested, Apache will attempt to execute the file

<code>/usr/local/apache/cgi-bin/test.pl</code>

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.</p>

</section>

<section id="nonscriptalias">

<title>CGI outside of ScriptAlias directories</title>

<p>CGI programs are often restricted to

<code>ScriptAlias</code>'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

<code>UserDir</code> directive. If they want to have their own

CGI programs, but don't have access to the main

<code>cgi-bin</code> directory, they will need to be able to

run CGI programs elsewhere.</p>

</section>

<section id="options">

<title>Explicitly using Options to permit CGI execution</title>

<p>You could explicitly use the <code>Options</code>

directive, inside your main server configuration file, to

specify that CGI execution was permitted in a particular

directory:</p>

<example>&lt;Directory /usr/local/apache/htdocs/somedir&gt; <br />

Options +ExecCGI<br />

&lt;/Directory&gt;</example>

<p>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

<code>AddHandler</code>

directive tells the server to treat all files with the

<code>cgi</code> or <code>pl</code> extension as CGI programs:</p>

<example>AddHandler cgi-script cgi pl</example>

</section>

<section id="htaccess">

<title>.htaccess files</title>

<p>A <code>.htaccess</code> 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 <code>.htaccess</code>, and, if it

finds it, it will apply directives found therein.

<code>.htaccess</code> files can be permitted with the

<code>AllowOverride</code> 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 in your main server

configuration:</p>

<example>AllowOverride Options</example>

<p>In the <code>.htaccess</code> file, you'll need the

following directive:</p>

<example>Options +ExecCGI</example>

<p>which tells Apache that execution of CGI programs is

permitted in this directory.</p>

</section>

</section>

<section id="writing">

<title>Writing a CGI program</title>

<p>There are two main differences between ``regular''

programming, and CGI programming.</p>

<p>First, all output from your CGI program must be preceded 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:</p>

<example>Content-type: text/html</example>

<p>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.</p>

<p>Apart from those two things, writing a CGI program will look

a lot like any other program that you might write.</p>

<section id="firstcgi">

<title>Your first CGI program</title>

<p>The following is an example CGI program that prints one

line to your browser. Type in the following, save it to a

file called <code>first.pl</code>, and put it in your

<code>cgi-bin</code> directory.</p>

<example>#!/usr/bin/perl<br />

print "Content-type: text/html\n\n";<br />

print "Hello, World.";

</example>

<p>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 <code>/usr/bin/perl</code>.

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.</p>

<p>If you open your favorite browser and tell it to get the

address</p>

<example>http://www.example.com/cgi-bin/first.pl</example>

<p>or wherever you put your file, you will see the one line

<code>Hello, World.</code>

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.</p>

</section>

</section>

<section id="troubleshoot">

<title>But it's still not working!</title>

<p>There are four basic things that you may see in your browser

when you try to access your CGI program from the web:</p>

<dl>

<dt>The output of your CGI program</dt>

<dd>Great! That means everything worked fine.

</dd>

<dt>The source code of your CGI program or a "POST Method Not

Allowed" message</dt>

<dd>That means that you have not properly configured Apache

to process your CGI program. Reread the section on

<a href="#configuringapachetopermitcgi">configuring

Apache</a>

and try to find what you missed.

</dd>

<dt>A message starting with "Forbidden"</dt>

<dd>That means that there is a permissions problem. Check the

<a href="#errorlogs">Apache error log</a>

and the section below on

<a href="#permissions">file permissions</a>.

<br />

</dd>

<dt>A message saying "Internal Server Error"</dt>

<dd>If you check the

<a href="#errorlogs">Apache error log</a>, you will probably

find that it says "Premature end of

script headers", possibly along with an error message

generated by your CGI program. In this case, you will want to

check each of the below sections to see what might be

preventing your CGI program from emitting the proper HTTP

headers.</dd>

</dl>

<section id="permissions">

<title>File permissions</title>

<p>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:</p>

<example>chmod a+x first.pl</example>

<p>Also, if your program reads from, or writes to, any other

files, those files will need to have the correct permissions

to permit this.</p>

<p>The exception to this is when the server is configured to

use <a href="/suexec.html">suexec</a>.

This 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 an "Internal Server

Error". In this case, you will need to check the suexec log

file to see what specific security check is failing.</p>

</section>

<section id="pathinformation">

<title>Path information</title>

<p>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.</p>

<p>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.</p>

<p>A common manifestation of this is the path to the script

interpreter (often <code>perl</code>) indicated in the first

line of your CGI program, which will look something like:</p>

<example>#!/usr/bin/perl</example>

<p>Make sure that this is in fact the path to the

interpreter.</p>

</section>

<section id="syntaxerrors">

<title>Syntax errors</title>

<p>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 eliminate most of your problems.</p>

</section>

<section id="errorlogs">

<title>Error logs</title>

<p>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.</p>

</section>

</section>

<section id="behindscenes">

<title>What's going on behind the scenes?</title>

<p>As you become more advanced in CGI programming, it will

become useful to understand more about what's happening behind

the scenes. Specifically, how the browser and server

communicate with one another. Because although it's all very

well to write a program that prints ``Hello, World.'', it's not

particularly useful.</p>

<section id="env">

<title>Environment variables</title>

<p>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 a 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

<code>env</code> at a command prompt.</p>

<p>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.</p>

<p>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

<a href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">

http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a>

</p>

<p>This simple Perl CGI program will display all of the

environment variables that are being passed around. Two

similar programs are included in the

<code>cgi-bin</code>

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

<a href="/env.html">add your own environment variables</a>

to the basic ones provided by default.</p>

<example>

#!/usr/bin/perl<br />

print "Content-type: text/html\n\n";<br />

foreach $key (keys %ENV) {<br />

print "$key --&gt; $ENV{$key}&lt;br&gt;";<br />

}</example>

</section>

<section id="stdin">

<title>STDIN and STDOUT</title>

<p>Other communication between the server and the client

happens over standard input (<code>STDIN</code>) and standard

output (<code>STDOUT</code>). In normal everyday context,

<code>STDIN</code> means the keyboard, or a file that a

program is given to act on, and <code>STDOUT</code>

usually means the console or screen.</p>

<p>When you <code>POST</code> 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 <code>STDIN</code>.

The program then can process that data as though it was

coming in from the keyboard, or from a file</p>

<p>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

(&amp;). 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:</p>

<example>

name=Rich%20Bowen&amp;city=Lexington&amp;state=KY&amp;sidekick=Squirrel%20Monkey

</example>

<p>You'll sometimes also see this type of string appended to

the a URL. When that is done, the server puts that string

into the environment variable called

<code>QUERY_STRING</code>. That's called a <code>GET</code>

request. Your HTML form specifies whether a <code>GET</code>

or a <code>POST</code> is used to deliver the data, by setting the

<code>METHOD</code> attribute in the <code>FORM</code> tag.</p>

<p>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.</p>

</section>

</section>

<section id="libraries">

<title>CGI modules/libraries</title>

<p>When you write CGI programs, you should consider using a

code library, or module, to do most of the grunt work for you.

This leads to fewer errors, and faster development.</p>

<p>If you're writing CGI programs in Perl, modules are

available on <a href="http://www.cpan.org/">CPAN</a>. The most

popular module for this purpose is CGI.pm. You might

also consider CGI::Lite, which implements a minimal set of

functionality, which is all you need in most programs.</p>

<p>If you're writing CGI programs in C, there are a variety of

options. One of these is the CGIC library, from

<a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>

</p>

</section>

<section id="moreinfo">

<title>For more information</title>

<p>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

<a href="http://www.hwg.org/lists/hwg-servers/">

http://www.hwg.org/lists/hwg-servers/</a>

</p>

<p>And, of course, you should probably read the CGI

specification, which has all the details on the operation of

CGI programs. You can find the original version at the

<a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">

NCSA</a>

and there is an updated draft at the

<a href="http://web.golux.com/coar/cgi/">Common Gateway

Interface RFC project</a>.</p>

<p>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.</p>

<p>Note that questions about CGI problems should <strong>never</strong>

be posted to the Apache bug database unless you are sure you

have found a problem in the Apache source code.</p>

</section>

</manualpage>