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<h3>Apache HTTP Server Version 1.3</h3>
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<h1 align="CENTER">Dynamic Content with CGI</h1>
<a id="__index__" name="__index__"></a> <!-- INDEX BEGIN -->
<ul>
<li><a href="#dynamiccontentwithcgi">Dynamic Content with
CGI</a></li>
<li>
<a href="#configuringapachetopermitcgi">Configuring Apache
to permit CGI</a>
<ul>
<li><a href="#scriptalias">ScriptAlias</a></li>
<li>
<a href="#cgioutsideofscriptaliasdirectories">CGI
outside of ScriptAlias directories</a>
<ul>
<li><a
href="#explicitlyusingoptionstopermitcgiexecution">Explicitly
using Options to permit CGI execution</a></li>
<li><a href="#htaccessfiles">.htaccess files</a></li>
</ul>
</li>
</ul>
</li>
<li>
<a href="#writingacgiprogram">Writing a CGI program</a>
<ul>
<li><a href="#yourfirstcgiprogram">Your first CGI
program</a></li>
</ul>
</li>
<li>
<a href="#butitsstillnotworking">But it's still not
working!</a>
<ul>
<li><a href="#filepermissions">File permissions</a></li>
<li><a href="#pathinformation">Path information</a></li>
<li><a href="#syntaxerrors">Syntax errors</a></li>
<li><a href="#errorlogs">Error logs</a></li>
</ul>
</li>
<li>
<a href="#whatsgoingonbehindthescenes">What's going on
behind the scenes?</a>
<ul>
<li><a href="#environmentvariables">Environment
variables</a></li>
<li><a href="#stdinandstdout">STDIN and STDOUT</a></li>
</ul>
</li>
<li><a href="#cgimoduleslibraries">CGI
modules/libraries</a></li>
<li><a href="#formoreinformation">For more
information</a></li>
</ul>
<!-- INDEX END -->
<hr />
<h2><a id="dynamiccontentwithcgi"
name="dynamiccontentwithcgi">Dynamic Content with CGI</a></h2>
<table border="1">
<tr>
<td valign="top"><strong>Related Modules</strong><br />
<br />
<a href="../mod/mod_alias.html">mod_alias</a><br />
<a href="../mod/mod_cgi.html">mod_cgi</a><br />
</td>
<td valign="top"><strong>Related Directives</strong><br />
<br />
<a
href="../mod/mod_mime.html#addhandler">AddHandler</a><br />
<a href="../mod/core.html#options">Options</a><br />
<a
href="../mod/mod_alias.html#scriptalias">ScriptAlias</a><br />
</td>
</tr>
</table>
<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>
<hr />
<h2><a id="configuringapachetopermitcgi"
name="configuringapachetopermitcgi">Configuring Apache to
permit CGI</a></h2>
<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>
<h3><a id="scriptalias" name="scriptalias">ScriptAlias</a></h3>
<p>The <code>ScriptAlias</code> 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 <code>ScriptAlias</code> directive looks like:</p>
<pre>
ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/
</pre>
<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>
<h3><a id="cgioutsideofscriptaliasdirectories"
name="cgioutsideofscriptaliasdirectories">CGI outside of
ScriptAlias directories</a></h3>
<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>
<h3><a id="explicitlyusingoptionstopermitcgiexecution"
name="explicitlyusingoptionstopermitcgiexecution">Explicitly
using Options to permit CGI execution</a></h3>
<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>
<pre>
<Directory /usr/local/apache/htdocs/somedir>
Options +ExecCGI
</Directory>
</pre>
<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>
<pre>
AddHandler cgi-script cgi pl
</pre>
<h3><a id="htaccessfiles" name="htaccessfiles">.htaccess
files</a></h3>
<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>
<pre>
AllowOverride Options
</pre>
<p>In the <code>.htaccess</code> file, you'll need the
following directive:</p>
<pre>
Options +ExecCGI
</pre>
<p>which tells Apache that execution of CGI programs is
permitted in this directory.</p>
<hr />
<h2><a id="writingacgiprogram"
name="writingacgiprogram">Writing a CGI program</a></h2>
<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>
<pre>
Content-type: text/html
</pre>
<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>
<h3><a id="yourfirstcgiprogram" name="yourfirstcgiprogram">Your
first CGI program</a></h3>
<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>
<pre>
#!/usr/bin/perl
print "Content-type: text/html\r\n\r\n";
print "Hello, World.";
</pre>
<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>
<pre>
http://www.example.com/cgi-bin/first.pl
</pre>
<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>
<hr />
<h2><a id="butitsstillnotworking"
name="butitsstillnotworking">But it's still not
working!</a></h2>
<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.<br />
<br />
</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.<br />
<br />
</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="#filepermissions">file
permissions</a>.<br />
<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>
<h3><a id="filepermissions" name="filepermissions">File
permissions</a></h3>
<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>
<pre>
chmod a+x first.pl
</pre>
<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>
<h3><a id="pathinformation" name="pathinformation">Path
information</a></h3>
<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>
<pre>
#!/usr/bin/perl
</pre>
<p>Make sure that this is in fact the path to the
interpreter.</p>
<h3><a id="syntaxerrors" name="syntaxerrors">Syntax
errors</a></h3>
<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>
<h3><a id="errorlogs" name="errorlogs">Error logs</a></h3>
<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>
<hr />
<h2><a id="whatsgoingonbehindthescenes"
name="whatsgoingonbehindthescenes">What's going on behind the
scenes?</a></h2>
<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>
<h3><a id="environmentvariables"
name="environmentvariables">Environment variables</a></h3>
<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>
<pre>
#!/usr/bin/perl
print "Content-type: text/html\n\n";
foreach $key (keys %ENV) {
print "$key --> $ENV{$key}<br>";
}
</pre>
<h3><a id="stdinandstdout" name="stdinandstdout">STDIN and
STDOUT</a></h3>
<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 (&).
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>
<pre>
name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
</pre>
<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>
<hr />
<h2><a id="cgimoduleslibraries" name="cgimoduleslibraries">CGI
modules/libraries</a></h2>
<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>
<hr />
<h2><a id="formoreinformation" name="formoreinformation">For
more information</a></h2>
<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>
<hr />
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