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			588 lines
		
	
	
		
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			XML
		
	
	
	
	
	
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
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              This file is generated from xml source: DO NOT EDIT
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<title>Apache Tutorial: Dynamic Content with CGI - Apache HTTP Server</title>
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<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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</script>
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<link href="../images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/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 2.5</p>
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<img alt="" src="../images/feather.gif" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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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 2.5</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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<div class="toplang">
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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="../fr/howto/cgi.html" hreflang="fr" rel="alternate" title="Fran<61>ais"> fr </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>
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</div>
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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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						||
</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<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="intro" id="intro">Introduction</a></h2>
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						||
    
 | 
						||
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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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    CGI programs.</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="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
 | 
						||
    are several ways to do this.</p>
 | 
						||
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    <div class="warning">Note: If Apache has been built with shared module
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						||
    support you need to ensure that the module is loaded; in your
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						||
    <code>httpd.conf</code> you need to make sure the
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    <code class="directive"><a href="../mod/mod_so.html#loadmodule">LoadModule</a></code>
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						||
    directive has not been commented out.  A correctly configured directive
 | 
						||
    may look like this:
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						||
 | 
						||
    <pre class="prettyprint lang-config">LoadModule cgi_module modules/mod_cgi.so</pre>
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						||
</div>
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						||
 | 
						||
    <h3><a name="scriptalias" id="scriptalias">ScriptAlias</a></h3>
 | 
						||
      
 | 
						||
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      <p>The
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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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      client.</p>
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						||
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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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      <pre class="prettyprint lang-config">ScriptAlias /cgi-bin/ /usr/local/apache2/cgi-bin/</pre>
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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>
 | 
						||
      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
 | 
						||
      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
 | 
						||
      <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
 | 
						||
      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>
 | 
						||
      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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 | 
						||
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    <h3><a name="nonscriptalias" id="nonscriptalias">CGI outside of ScriptAlias directories</a></h3>
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 | 
						||
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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
 | 
						||
      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
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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.
 | 
						||
      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>
 | 
						||
 | 
						||
      <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>
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="options" id="options">Explicitly using Options to permit CGI execution</a></h3>
 | 
						||
      
 | 
						||
 | 
						||
      <p>You could explicitly use the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive, inside your main server configuration
 | 
						||
      file, to specify that CGI execution was permitted in a particular
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      directory:</p>
 | 
						||
 | 
						||
      <pre class="prettyprint lang-config"><Directory /usr/local/apache2/htdocs/somedir>
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    Options +ExecCGI
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						||
</Directory></pre>
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 | 
						||
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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
 | 
						||
      files with the <code>cgi</code> or <code>pl</code> extension as CGI
 | 
						||
      programs:</p>
 | 
						||
 | 
						||
      <pre class="prettyprint lang-config">AddHandler cgi-script .cgi .pl</pre>
 | 
						||
 | 
						||
    
 | 
						||
 | 
						||
    <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>
 | 
						||
      shows how to activate CGI programs if you do not have
 | 
						||
      access to <code>httpd.conf</code>.</p>
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="userdir" id="userdir">User Directories</a></h3>
 | 
						||
      
 | 
						||
 | 
						||
      <p>To allow CGI program execution for any file ending in
 | 
						||
      <code>.cgi</code> in users' directories, you can use the
 | 
						||
      following configuration.</p>
 | 
						||
 | 
						||
      <pre class="prettyprint lang-config"><Directory /home/*/public_html>
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						||
    Options +ExecCGI
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						||
    AddHandler cgi-script .cgi
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						||
</Directory></pre>
 | 
						||
 | 
						||
 | 
						||
      <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
 | 
						||
      program, you can use the following.</p>
 | 
						||
 | 
						||
      <pre class="prettyprint lang-config"><Directory /home/*/public_html/cgi-bin>
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    Options ExecCGI
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    SetHandler cgi-script
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						||
</Directory></pre>
 | 
						||
 | 
						||
 | 
						||
    
 | 
						||
 | 
						||
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 | 
						||
<div class="section">
 | 
						||
<h2><a name="writing" id="writing">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 <a class="glossarylink" href="../glossary.html#mime-type" title="see glossary">MIME-type</a> 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>
 | 
						||
 | 
						||
    <div class="example"><p><code>
 | 
						||
      Content-type: text/html
 | 
						||
    </code></p></div>
 | 
						||
 | 
						||
    <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
 | 
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    program that outputs a gif image, or other non-HTML
 | 
						||
    content.</p>
 | 
						||
 | 
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    <p>Apart from those two things, writing a CGI program will look
 | 
						||
    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>
 | 
						||
      
 | 
						||
 | 
						||
      <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
 | 
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      <code>cgi-bin</code> directory.</p>
 | 
						||
 | 
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      <pre class="prettyprint lang-perl">#!/usr/bin/perl
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print "Content-type: text/html\n\n";
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print "Hello, World.";</pre>
 | 
						||
 | 
						||
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      <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
 | 
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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
 | 
						||
      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
 | 
						||
      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>
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						||
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						||
      <p>If you open your favorite browser and tell it to get the
 | 
						||
      address</p>
 | 
						||
 | 
						||
      <div class="example"><p><code>
 | 
						||
        http://www.example.com/cgi-bin/first.pl
 | 
						||
      </code></p></div>
 | 
						||
 | 
						||
      <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>
 | 
						||
    
 | 
						||
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 | 
						||
<div class="section">
 | 
						||
<h2><a name="troubleshoot" id="troubleshoot">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.  If the output is correct,
 | 
						||
      but the browser is not processing it correctly, make sure you have the
 | 
						||
      correct <code>Content-Type</code> set in your CGI program.</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="#configuring">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>.</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="permissions" id="permissions">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 <code>nobody</code>, or
 | 
						||
      <code>www</code> - 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 <code>nobody</code>
 | 
						||
      is to give everyone execute permission on the file:</p>
 | 
						||
 | 
						||
      <div class="example"><p><code>
 | 
						||
        chmod a+x first.pl
 | 
						||
      </code></p></div>
 | 
						||
 | 
						||
      <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>
 | 
						||
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="pathinformation" id="pathinformation">Path information and environment</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 <code>PATH</code>,
 | 
						||
      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 may not have the same <code>PATH</code>. Any programs that you
 | 
						||
      invoke in your CGI program (like <code>sendmail</code>, 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 class="prettyprint lang-perl">#!/usr/bin/perl</pre>
 | 
						||
 | 
						||
 | 
						||
      <p>Make sure that this is in fact the path to the
 | 
						||
      interpreter.</p>
 | 
						||
      <div class="warning">
 | 
						||
      When editing CGI scripts on Windows, end-of-line characters may be
 | 
						||
      appended to the interpreter path. Ensure that files are then
 | 
						||
      transferred to the server in ASCII mode. Failure to do so may
 | 
						||
      result in "Command not found" warnings from the OS, due to the
 | 
						||
      unrecognized end-of-line character being interpreted as a part of
 | 
						||
      the interpreter filename.
 | 
						||
      </div>
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="missingenv" id="missingenv">Missing environment variables</a></h3>
 | 
						||
      
 | 
						||
 | 
						||
      <p>If your CGI program depends on non-standard <a href="#env">environment variables</a>, you will need to
 | 
						||
      assure that those variables are passed by Apache.</p>
 | 
						||
 | 
						||
      <p>When you miss HTTP headers from the environment, make
 | 
						||
      sure they are formatted according to
 | 
						||
      <a href="http://tools.ietf.org/html/rfc2616">RFC 2616</a>,
 | 
						||
      section 4.2: Header names must start with a letter,
 | 
						||
      followed only by letters, numbers or hyphen. Any header
 | 
						||
      violating this rule will be dropped silently.</p>
 | 
						||
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="syntaxerrors" id="syntaxerrors">Program 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.  The first thing to do is to make
 | 
						||
      sure that your program runs from the command line before
 | 
						||
      testing it via the web server.  For example, try:</p>
 | 
						||
 | 
						||
      <div class="example"><p><code>
 | 
						||
      cd /usr/local/apache2/cgi-bin<br />
 | 
						||
      ./first.pl
 | 
						||
      </code></p></div>
 | 
						||
 | 
						||
      <p>(Do not call the <code>perl</code> interpreter.  The shell
 | 
						||
      and Apache should find the interpreter using the <a href="#pathinformation">path information</a> on the first line of
 | 
						||
      the script.)</p>
 | 
						||
 | 
						||
      <p>The first thing you see written by your program should be
 | 
						||
      a set of HTTP headers, including the <code>Content-Type</code>,
 | 
						||
      followed by a blank line.  If you see anything else, Apache will
 | 
						||
      return the <code>Premature end of script headers</code> error if
 | 
						||
      you try to run it through the server. See <a href="#writing">Writing a CGI program</a> above for more
 | 
						||
      details.</p>
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="errorlogs" id="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>
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="suexec" id="suexec">Suexec</a></h3>
 | 
						||
      
 | 
						||
 | 
						||
      <p>The <a href="../suexec.html">suexec</a> support program
 | 
						||
      allows CGI programs to be run under different user permissions,
 | 
						||
      depending on which virtual host or user home directory they are
 | 
						||
      located in. Suexec has very strict permission checking, and any
 | 
						||
      failure in that checking will result in your CGI programs
 | 
						||
      failing with <code>Premature end of script headers</code>.</p>
 | 
						||
 | 
						||
      <p>To check if you are using suexec, run <code>apachectl
 | 
						||
      -V</code> and check for the location of <code>SUEXEC_BIN</code>.
 | 
						||
      If Apache finds an <code class="program"><a href="../programs/suexec.html">suexec</a></code> binary there on startup,
 | 
						||
      suexec will be activated.</p>
 | 
						||
 | 
						||
      <p>Unless you fully understand suexec, you should not be using it.
 | 
						||
      To disable suexec, simply remove (or rename) the <code class="program"><a href="../programs/suexec.html">suexec</a></code>
 | 
						||
      binary pointed to by <code>SUEXEC_BIN</code> and then restart the
 | 
						||
      server.  If, after reading about <a href="../suexec.html">suexec</a>,
 | 
						||
      you still wish to use it, then run <code>suexec -V</code> to find
 | 
						||
      the location of the suexec log file, and use that log file to
 | 
						||
      find what policy you are violating.</p>
 | 
						||
    
 | 
						||
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 | 
						||
<div class="section">
 | 
						||
<h2><a name="behindscenes" id="behindscenes">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="env" id="env">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 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://www.ietf.org/rfc/rfc3875">Common Gateway
 | 
						||
      Interface RFC</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 class="prettyprint lang-perl">#!/usr/bin/perl
 | 
						||
use strict;
 | 
						||
use warnings;
 | 
						||
 | 
						||
print "Content-type: text/html\n\n";
 | 
						||
foreach my $key (keys %ENV) {
 | 
						||
    print "$key --> $ENV{$key}<br>";
 | 
						||
}</pre>
 | 
						||
 | 
						||
    
 | 
						||
 | 
						||
    <h3><a name="stdin" id="stdin">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>
 | 
						||
 | 
						||
      <div class="example"><p><code>
 | 
						||
        name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
 | 
						||
      </code></p></div>
 | 
						||
 | 
						||
      <p>You'll sometimes also see this type of string appended to
 | 
						||
      a URL. When that is done, the server puts that string
 | 
						||
      into the environment variable called
 | 
						||
      <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>
 | 
						||
    
 | 
						||
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 | 
						||
<div class="section">
 | 
						||
<h2><a name="libraries" id="libraries">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 <code>CGI.pm</code>. You might
 | 
						||
    also consider <code>CGI::Lite</code>, 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 <code>CGIC</code> library, from
 | 
						||
    <a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>.</p>
 | 
						||
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 | 
						||
<div class="section">
 | 
						||
<h2><a name="moreinfo" id="moreinfo">For more information</a></h2>
 | 
						||
    
 | 
						||
 | 
						||
    <p>The current CGI specification is available in the
 | 
						||
    <a href="http://www.ietf.org/rfc/rfc3875">Common Gateway
 | 
						||
    Interface RFC</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>
 | 
						||
  </div></div>
 | 
						||
<div class="bottomlang">
 | 
						||
<p><span>Available Languages: </span><a href="../en/howto/cgi.html" title="English"> en </a> |
 | 
						||
<a href="../fr/howto/cgi.html" hreflang="fr" rel="alternate" title="Fran<61>ais"> fr </a> |
 | 
						||
<a href="../ja/howto/cgi.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
 | 
						||
<a href="../ko/howto/cgi.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
 | 
						||
</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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