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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">How-To / Tutorials</a></div><div id="page-content"><div id="preamble"><h1>Apache Tutorial: Dynamic Content with CGI</h1>
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<p><span>Available Languages: </span><a href="../en/howto/cgi.html" title="English"> en </a> |
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<a href="../ja/howto/cgi.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="../ko/howto/cgi.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#intro">Introduction</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#configuring">Configuring Apache to permit CGI</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#writing">Writing a CGI program</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#troubleshoot">But it's still not working!</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#behindscenes">What's going on behind the scenes?</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#libraries">CGI modules/libraries</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#moreinfo">For more information</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="intro" id="intro">Introduction</a></h2>
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<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code></li><li><code class="module"><a href="../mod/mod_cgi.html">mod_cgi</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code></li><li><code class="directive"><a href="../mod/core.html#options">Options</a></code></li><li><code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code></li></ul></td></tr></table>
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<p>The CGI (Common Gateway Interface) defines a way for a web
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server to interact with external content-generating programs,
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which are often referred to as CGI programs or CGI scripts. It
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is the simplest, and most common, way to put dynamic content on
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your web site. This document will be an introduction to setting
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up CGI on your Apache web server, and getting started writing
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="configuring" id="configuring">Configuring Apache to permit CGI</a></h2>
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<p>In order to get your CGI programs to work properly, you'll
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need to have Apache configured to permit CGI execution. There
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are several ways to do this.</p>
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<h3><a name="scriptalias" id="scriptalias">ScriptAlias</a></h3>
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<code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
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directive tells Apache that a particular directory is set
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aside for CGI programs. Apache will assume that every file in
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this directory is a CGI program, and will attempt to execute
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it, when that particular resource is requested by a
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<p>The <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
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directive looks like:</p>
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<div class="example"><p><code>
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ScriptAlias /cgi-bin/ /usr/local/apache2/cgi-bin/
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<p>The example shown is from your default <code>httpd.conf</code>
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configuration file, if you installed Apache in the default
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location. The <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
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directive is much like the <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> directive, which defines a URL prefix that
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is to mapped to a particular directory. <code class="directive">Alias</code>
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and <code class="directive">ScriptAlias</code> are usually used for
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directories that are outside of the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> directory. The difference between
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<code class="directive">Alias</code> and <code class="directive">ScriptAlias</code>
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is that <code class="directive">ScriptAlias</code> has the added meaning
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that everything under that URL prefix will be considered a CGI
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program. So, the example above tells Apache that any request for a
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resource beginning with <code>/cgi-bin/</code> should be served from
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the directory <code>/usr/local/apache2/cgi-bin/</code>, and should be
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treated as a CGI program.</p>
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<p>For example, if the URL
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<code>http://www.example.com/cgi-bin/test.pl</code>
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is requested, Apache will attempt to execute the file
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<code>/usr/local/apache2/cgi-bin/test.pl</code>
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and return the output. Of course, the file will have to
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exist, and be executable, and return output in a particular
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way, or Apache will return an error message.</p>
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<h3><a name="nonscriptalias" id="nonscriptalias">CGI outside of ScriptAlias directories</a></h3>
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<p>CGI programs are often restricted to <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>'ed directories for security reasons.
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In this way, administrators can tightly control who is allowed to
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use CGI programs. However, if the proper security precautions are
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taken, there is no reason why CGI programs cannot be run from
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arbitrary directories. For example, you may wish to let users
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have web content in their home directories with the
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<code class="directive"><a href="../mod/mod_userdir.html#userdir">UserDir</a></code> directive.
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If they want to have their own CGI programs, but don't have access to
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the main <code>cgi-bin</code> directory, they will need to be able to
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run CGI programs elsewhere.</p>
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<p>There are two steps to allowing CGI execution in an arbitrary
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directory. First, the <code>cgi-script</code> handler must be
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activated using the <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> or <code class="directive"><a href="../mod/core.html#sethandler">SetHandler</a></code> directive. Second,
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<code>ExecCGI</code> must be specified in the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive.</p>
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<h3><a name="options" id="options">Explicitly using Options to permit CGI execution</a></h3>
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<p>You could explicitly use the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive, inside your main server configuration
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file, to specify that CGI execution was permitted in a particular
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<div class="example"><p><code>
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<Directory /usr/local/apache2/htdocs/somedir><br />
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<span class="indent">
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Options +ExecCGI<br />
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<p>The above directive tells Apache to permit the execution
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of CGI files. You will also need to tell the server what
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files are CGI files. The following <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> directive tells the server to treat all
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files with the <code>cgi</code> or <code>pl</code> extension as CGI
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<div class="example"><p><code>
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AddHandler cgi-script .cgi .pl
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<h3><a name="htaccess" id="htaccess">.htaccess files</a></h3>
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<p>The <a href="htaccess.html"><code>.htaccess</code> tutorial</a>
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shows how to activate CGI programs if you do not have
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access to <code>httpd.conf</code>.</p>
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<h3><a name="userdir" id="userdir">User Directories</a></h3>
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<p>To allow CGI program execution for any file ending in
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<code>.cgi</code> in users' directories, you can use the
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following configuration.</p>
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<div class="example"><p><code>
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<Directory /home/*/public_html><br />
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<span class="indent">
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Options +ExecCGI<br />
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AddHandler cgi-script .cgi<br />
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<p>If you wish designate a <code>cgi-bin</code> subdirectory of
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a user's directory where everything will be treated as a CGI
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program, you can use the following.</p>
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<div class="example"><p><code>
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<Directory /home/*/public_html/cgi-bin><br />
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<span class="indent">
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Options ExecCGI<br />
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SetHandler cgi-script<br />
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="writing" id="writing">Writing a CGI program</a></h2>
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<p>There are two main differences between ``regular''
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programming, and CGI programming.</p>
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<p>First, all output from your CGI program must be preceded by
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a <a class="glossarylink" href="../glossary.html#mime-type" title="see glossary">MIME-type</a> header. This is HTTP header that tells the client
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what sort of content it is receiving. Most of the time, this
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<div class="example"><p><code>
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Content-type: text/html
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<p>Secondly, your output needs to be in HTML, or some other
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format that a browser will be able to display. Most of the
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time, this will be HTML, but occasionally you might write a CGI
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program that outputs a gif image, or other non-HTML
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<p>Apart from those two things, writing a CGI program will look
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a lot like any other program that you might write.</p>
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<h3><a name="firstcgi" id="firstcgi">Your first CGI program</a></h3>
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<p>The following is an example CGI program that prints one
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line to your browser. Type in the following, save it to a
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file called <code>first.pl</code>, and put it in your
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<code>cgi-bin</code> directory.</p>
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<div class="example"><p><code>
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#!/usr/bin/perl<br />
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print "Content-type: text/html\n\n";<br />
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print "Hello, World.";
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<p>Even if you are not familiar with Perl, you should be able
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to see what is happening here. The first line tells Apache
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(or whatever shell you happen to be running under) that this
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program can be executed by feeding the file to the
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interpreter found at the location <code>/usr/bin/perl</code>.
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The second line prints the content-type declaration we
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talked about, followed by two carriage-return newline pairs.
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This puts a blank line after the header, to indicate the end
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of the HTTP headers, and the beginning of the body. The third
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line prints the string "Hello, World.". And that's the end
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<p>If you open your favorite browser and tell it to get the
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<div class="example"><p><code>
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http://www.example.com/cgi-bin/first.pl
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<p>or wherever you put your file, you will see the one line
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<code>Hello, World.</code> appear in your browser window.
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It's not very exciting, but once you get that working, you'll
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have a good chance of getting just about anything working.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="troubleshoot" id="troubleshoot">But it's still not working!</a></h2>
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<p>There are four basic things that you may see in your browser
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when you try to access your CGI program from the web:</p>
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<dt>The output of your CGI program</dt>
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<dd>Great! That means everything worked fine. If the output is correct,
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but the browser is not processing it correctly, make sure you have the
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correct <code>Content-Type</code> set in your CGI program.</dd>
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<dt>The source code of your CGI program or a "POST Method Not
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Allowed" message</dt>
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<dd>That means that you have not properly configured Apache
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to process your CGI program. Reread the section on
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<a href="#configuring">configuring
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Apache</a> and try to find what you missed.</dd>
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<dt>A message starting with "Forbidden"</dt>
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<dd>That means that there is a permissions problem. Check the
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<a href="#errorlogs">Apache error log</a> and the section below on
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<a href="#permissions">file permissions</a>.</dd>
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<dt>A message saying "Internal Server Error"</dt>
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<a href="#errorlogs">Apache error log</a>, you will probably
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find that it says "Premature end of
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script headers", possibly along with an error message
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generated by your CGI program. In this case, you will want to
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check each of the below sections to see what might be
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preventing your CGI program from emitting the proper HTTP
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<h3><a name="permissions" id="permissions">File permissions</a></h3>
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<p>Remember that the server does not run as you. That is,
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when the server starts up, it is running with the permissions
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of an unprivileged user - usually <code>nobody</code>, or
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<code>www</code> - and so it will need extra permissions to
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execute files that are owned by you. Usually, the way to give
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a file sufficient permissions to be executed by <code>nobody</code>
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is to give everyone execute permission on the file:</p>
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<div class="example"><p><code>
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<p>Also, if your program reads from, or writes to, any other
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files, those files will need to have the correct permissions
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<h3><a name="pathinformation" id="pathinformation">Path information and environment</a></h3>
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<p>When you run a program from your command line, you have
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certain information that is passed to the shell without you
312
thinking about it. For example, you have a <code>PATH</code>,
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which tells the shell where it can look for files that you
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<p>When a program runs through the web server as a CGI program,
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it may not have the same <code>PATH</code>. Any programs that you
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invoke in your CGI program (like <code>sendmail</code>, for
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example) will need to be specified by a full path, so that the
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shell can find them when it attempts to execute your CGI
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<p>A common manifestation of this is the path to the script
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interpreter (often <code>perl</code>) indicated in the first
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line of your CGI program, which will look something like:</p>
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<div class="example"><p><code>
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<p>Make sure that this is in fact the path to the
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<p>In addition, if your CGI program depends on other <a href="#env">environment variables</a>, you will need to
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assure that those variables are passed by Apache.</p>
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<h3><a name="syntaxerrors" id="syntaxerrors">Program errors</a></h3>
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<p>Most of the time when a CGI program fails, it's because of
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a problem with the program itself. This is particularly true
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once you get the hang of this CGI stuff, and no longer make
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the above two mistakes. The first thing to do is to make
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sure that your program runs from the command line before
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testing it via the web server. For example, try:</p>
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<div class="example"><p><code>
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cd /usr/local/apache2/cgi-bin<br />
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<p>(Do not call the <code>perl</code> interpreter. The shell
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and Apache should find the interpreter using the <a href="#pathinformation">path information</a> on the first line of
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<p>The first thing you see written by your program should be
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a set of HTTP headers, including the <code>Content-Type</code>,
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followed by a blank line. If you see anything else, Apache will
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return the <code>Premature end of script headers</code> error if
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you try to run it through the server. See <a href="#writing">Writing a CGI program</a> above for more
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<h3><a name="errorlogs" id="errorlogs">Error logs</a></h3>
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<p>The error logs are your friend. Anything that goes wrong
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generates message in the error log. You should always look
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there first. If the place where you are hosting your web site
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does not permit you access to the error log, you should
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probably host your site somewhere else. Learn to read the
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error logs, and you'll find that almost all of your problems
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are quickly identified, and quickly solved.</p>
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<h3><a name="suexec" id="suexec">Suexec</a></h3>
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<p>The <a href="../suexec.html">suexec</a> support program
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allows CGI programs to be run under different user permissions,
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depending on which virtual host or user home directory they are
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located in. Suexec has very strict permission checking, and any
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failure in that checking will result in your CGI programs
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failing with <code>Premature end of script headers</code>.</p>
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<p>To check if you are using suexec, run <code>apachectl
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-V</code> and check for the location of <code>SUEXEC_BIN</code>.
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If Apache finds an <code class="program"><a href="../programs/suexec.html">suexec</a></code> binary there on startup,
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suexec will be activated.</p>
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<p>Unless you fully understand suexec, you should not be using it.
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To disable suexec, simply remove (or rename) the <code class="program"><a href="../programs/suexec.html">suexec</a></code>
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binary pointed to by <code>SUEXEC_BIN</code> and then restart the
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server. If, after reading about <a href="../suexec.html">suexec</a>,
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you still wish to use it, then run <code>suexec -V</code> to find
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the location of the suexec log file, and use that log file to
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find what policy you are violating.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="behindscenes" id="behindscenes">What's going on behind the scenes?</a></h2>
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<p>As you become more advanced in CGI programming, it will
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become useful to understand more about what's happening behind
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the scenes. Specifically, how the browser and server
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communicate with one another. Because although it's all very
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well to write a program that prints "Hello, World.", it's not
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particularly useful.</p>
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<h3><a name="env" id="env">Environment variables</a></h3>
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<p>Environment variables are values that float around you as
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you use your computer. They are useful things like your path
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(where the computer searches for the actual file
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implementing a command when you type it), your username, your
420
terminal type, and so on. For a full list of your normal,
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every day environment variables, type
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<code>env</code> at a command prompt.</p>
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<p>During the CGI transaction, the server and the browser
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also set environment variables, so that they can communicate
426
with one another. These are things like the browser type
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(Netscape, IE, Lynx), the server type (Apache, IIS, WebSite),
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the name of the CGI program that is being run, and so on.</p>
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<p>These variables are available to the CGI programmer, and
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are half of the story of the client-server communication. The
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complete list of required variables is at
433
<a href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a>.</p>
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<p>This simple Perl CGI program will display all of the
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environment variables that are being passed around. Two
437
similar programs are included in the
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directory of the Apache distribution. Note that some
441
variables are required, while others are optional, so you may
442
see some variables listed that were not in the official list.
443
In addition, Apache provides many different ways for you to
444
<a href="../env.html">add your own environment variables</a>
445
to the basic ones provided by default.</p>
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<div class="example"><p><code>
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#!/usr/bin/perl<br />
449
print "Content-type: text/html\n\n";<br />
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foreach $key (keys %ENV) {<br />
451
<span class="indent">
452
print "$key --> $ENV{$key}<br>";<br />
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<h3><a name="stdin" id="stdin">STDIN and STDOUT</a></h3>
461
<p>Other communication between the server and the client
462
happens over standard input (<code>STDIN</code>) and standard
463
output (<code>STDOUT</code>). In normal everyday context,
464
<code>STDIN</code> means the keyboard, or a file that a
465
program is given to act on, and <code>STDOUT</code>
466
usually means the console or screen.</p>
468
<p>When you <code>POST</code> a web form to a CGI program,
469
the data in that form is bundled up into a special format
470
and gets delivered to your CGI program over <code>STDIN</code>.
471
The program then can process that data as though it was
472
coming in from the keyboard, or from a file</p>
474
<p>The "special format" is very simple. A field name and
475
its value are joined together with an equals (=) sign, and
476
pairs of values are joined together with an ampersand
477
(&). Inconvenient characters like spaces, ampersands, and
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equals signs, are converted into their hex equivalent so that
479
they don't gum up the works. The whole data string might look
482
<div class="example"><p><code>
483
name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
486
<p>You'll sometimes also see this type of string appended to
487
a URL. When that is done, the server puts that string
488
into the environment variable called
489
<code>QUERY_STRING</code>. That's called a <code>GET</code>
490
request. Your HTML form specifies whether a <code>GET</code>
491
or a <code>POST</code> is used to deliver the data, by setting the
492
<code>METHOD</code> attribute in the <code>FORM</code> tag.</p>
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<p>Your program is then responsible for splitting that string
495
up into useful information. Fortunately, there are libraries
496
and modules available to help you process this data, as well
497
as handle other of the aspects of your CGI program.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="libraries" id="libraries">CGI modules/libraries</a></h2>
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<p>When you write CGI programs, you should consider using a
505
code library, or module, to do most of the grunt work for you.
506
This leads to fewer errors, and faster development.</p>
508
<p>If you're writing CGI programs in Perl, modules are
509
available on <a href="http://www.cpan.org/">CPAN</a>. The most
510
popular module for this purpose is <code>CGI.pm</code>. You might
511
also consider <code>CGI::Lite</code>, which implements a minimal
512
set of functionality, which is all you need in most programs.</p>
514
<p>If you're writing CGI programs in C, there are a variety of
515
options. One of these is the <code>CGIC</code> library, from
516
<a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>.</p>
517
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="moreinfo" id="moreinfo">For more information</a></h2>
522
<p>There are a large number of CGI resources on the web. You
523
can discuss CGI problems with other users on the Usenet group
524
<a href="news:comp.infosystems.www.authoring.cgi">comp.infosystems.www.authoring.cgi</a>. And the -servers mailing
525
list from the HTML Writers Guild is a great source of answers
526
to your questions. You can find out more at
527
<a href="http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a>.</p>
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<p>And, of course, you should probably read the CGI
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specification, which has all the details on the operation of
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CGI programs. You can find the original version at the
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<a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a> and there is an updated draft at the
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<a href="http://web.golux.com/coar/cgi/">Common Gateway
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Interface RFC project</a>.</p>
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<p>When you post a question about a CGI problem that you're
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having, whether to a mailing list, or to a newsgroup, make sure
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you provide enough information about what happened, what you
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expected to happen, and how what actually happened was
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different, what server you're running, what language your CGI
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program was in, and, if possible, the offending code. This will
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make finding your problem much simpler.</p>
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<p>Note that questions about CGI problems should <strong>never</strong>
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be posted to the Apache bug database unless you are sure you
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have found a problem in the Apache source code.</p>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/howto/cgi.html" title="English"> en </a> |
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<a href="../ja/howto/cgi.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="../ko/howto/cgi.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
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