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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head>
<title>Apache Tutorial: Dynamic Content with CGI</title>
<link rev="made" href="mailto:rbowen@rcbowen.com">
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#000080"
alink="#FF0000">
<!--#include virtual="header.html" -->
<h1 align="CENTER">Dynamic Content with CGI</h1>
<a 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 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 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 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://dev.rcbowen.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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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>
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