<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta name="generator" content="HTML Tidy, see www.w3.org" />

    <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 id="__index__" name="__index__"></a> <!-- INDEX BEGIN -->
     

    <ul>
      <li><a href="#dynamiccontentwithcgi">Dynamic Content with
      CGI</a></li>

      <li>
        <a href="#configuringapachetopermitcgi">Configuring Apache
        to permit CGI</a> 

        <ul>
          <li><a href="#scriptalias">ScriptAlias</a></li>

          <li>
            <a href="#cgioutsideofscriptaliasdirectories">CGI
            outside of ScriptAlias directories</a> 

            <ul>
              <li><a
              href="#explicitlyusingoptionstopermitcgiexecution">Explicitly
              using Options to permit CGI execution</a></li>

              <li><a href="#htaccessfiles">.htaccess files</a></li>
            </ul>
          </li>
        </ul>
      </li>

      <li>
        <a href="#writingacgiprogram">Writing a CGI program</a> 

        <ul>
          <li><a href="#yourfirstcgiprogram">Your first CGI
          program</a></li>
        </ul>
      </li>

      <li>
        <a href="#butitsstillnotworking">But it's still not
        working!</a> 

        <ul>
          <li><a href="#filepermissions">File permissions</a></li>

          <li><a href="#pathinformation">Path information</a></li>

          <li><a href="#syntaxerrors">Syntax errors</a></li>

          <li><a href="#errorlogs">Error logs</a></li>
        </ul>
      </li>

      <li>
        <a href="#whatsgoingonbehindthescenes">What's going on
        behind the scenes?</a> 

        <ul>
          <li><a href="#environmentvariables">Environment
          variables</a></li>

          <li><a href="#stdinandstdout">STDIN and STDOUT</a></li>
        </ul>
      </li>

      <li><a href="#cgimoduleslibraries">CGI
      modules/libraries</a></li>

      <li><a href="#formoreinformation">For more
      information</a></li>
    </ul>
    <!-- INDEX END -->
    <hr />

    <h2><a id="dynamiccontentwithcgi"
    name="dynamiccontentwithcgi">Dynamic Content with CGI</a></h2>

    <table border="1">
      <tr>
        <td valign="top"><strong>Related Modules</strong><br />
         <br />
         <a href="../mod/mod_alias.html">mod_alias</a><br />
         <a href="../mod/mod_cgi.html">mod_cgi</a><br />
         </td>

        <td valign="top"><strong>Related Directives</strong><br />
         <br />
         <a
        href="../mod/mod_mime.html#addhandler">AddHandler</a><br />
         <a href="../mod/core.html#options">Options</a><br />
         <a
        href="../mod/mod_alias.html#scriptalias">ScriptAlias</a><br />
         </td>
      </tr>
    </table>

    <p>The CGI (Common Gateway Interface) defines a way for a web
    server to interact with external content-generating programs,
    which are often referred to as CGI programs or CGI scripts. It
    is the simplest, and most common, way to put dynamic content on
    your web site. This document will be an introduction to setting
    up CGI on your Apache web server, and getting started writing
    CGI programs.</p>
    <hr />

    <h2><a id="configuringapachetopermitcgi"
    name="configuringapachetopermitcgi">Configuring Apache to
    permit CGI</a></h2>

    <p>In order to get your CGI programs to work properly, you'll
    need to have Apache configured to permit CGI execution. There
    are several ways to do this.</p>

    <h3><a id="scriptalias" name="scriptalias">ScriptAlias</a></h3>

    <p>The <code>ScriptAlias</code> directive tells Apache that a
    particular directory is set aside for CGI programs. Apache will
    assume that every file in this directory is a CGI program, and
    will attempt to execute it, when that particular resource is
    requested by a client.</p>

    <p>The <code>ScriptAlias</code> directive looks like:</p>
<pre>
        ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/
</pre>

    <p>The example shown is from your default
    <code>httpd.conf</code> configuration file, if you installed
    Apache in the default location. The <code>ScriptAlias</code>
    directive is much like the <code>Alias</code> directive, which
    defines a URL prefix that is to mapped to a particular
    directory. <code>Alias</code> and <code>ScriptAlias</code> are
    usually used for directories that are outside of the
    <code>DocumentRoot</code> directory. The difference between
    <code>Alias</code> and <code>ScriptAlias</code> is that
    <code>ScriptAlias</code> has the added meaning that everything
    under that URL prefix will be considered a CGI program. So, the
    example above tells Apache that any request for a resource
    beginning with <code>/cgi-bin/</code> should be served from the
    directory <code>/usr/local/apache/cgi-bin/</code>, and should
    be treated as a CGI program.</p>

    <p>For example, if the URL
    <code>http://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 id="cgioutsideofscriptaliasdirectories"
    name="cgioutsideofscriptaliasdirectories">CGI outside of
    ScriptAlias directories</a></h3>

    <p>CGI programs are often restricted to
    <code>ScriptAlias</code>'ed directories for security reasons.
    In this way, administrators can tightly control who is allowed
    to use CGI programs. However, if the proper security
    precautions are taken, there is no reason why CGI programs
    cannot be run from arbitrary directories. For example, you may
    wish to let users have web content in their home directories
    with the <code>UserDir</code> directive. If they want to have
    their own CGI programs, but don't have access to the main
    <code>cgi-bin</code> directory, they will need to be able to
    run CGI programs elsewhere.</p>

    <h3><a id="explicitlyusingoptionstopermitcgiexecution"
    name="explicitlyusingoptionstopermitcgiexecution">Explicitly
    using Options to permit CGI execution</a></h3>

    <p>You could explicitly use the <code>Options</code> directive,
    inside your main server configuration file, to specify that CGI
    execution was permitted in a particular directory:</p>
<pre>
        &lt;Directory /usr/local/apache/htdocs/somedir&gt;
                Options +ExecCGI
        &lt;/Directory&gt;
</pre>

    <p>The above directive tells Apache to permit the execution of
    CGI files. You will also need to tell the server what files are
    CGI files. The following <code>AddHandler</code> directive
    tells the server to treat all files with the <code>cgi</code>
    or <code>pl</code> extension as CGI programs:</p>
<pre>
     AddHandler cgi-script cgi pl
</pre>

    <h3><a id="htaccessfiles" name="htaccessfiles">.htaccess
    files</a></h3>

    <p>A <code>.htaccess</code> file is a way to set configuration
    directives on a per-directory basis. When Apache serves a
    resource, it looks in the directory from which it is serving a
    file for a file called <code>.htaccess</code>, and, if it finds
    it, it will apply directives found therein.
    <code>.htaccess</code> files can be permitted with the
    <code>AllowOverride</code> directive, which specifies what
    types of directives can appear in these files, or if they are
    not allowed at all. To permit the directive we will need for
    this purpose, the following configuration will be needed in
    your main server configuration:</p>
<pre>
        AllowOverride Options
</pre>

    <p>In the <code>.htaccess</code> file, you'll need the
    following directive:</p>
<pre>
        Options +ExecCGI
</pre>

    <p>which tells Apache that execution of CGI programs is
    permitted in this directory.</p>
    <hr />

    <h2><a id="writingacgiprogram"
    name="writingacgiprogram">Writing a CGI program</a></h2>

    <p>There are two main differences between ``regular''
    programming, and CGI programming.</p>

    <p>First, all output from your CGI program must be preceded by
    a MIME-type header. This is HTTP header that tells the client
    what sort of content it is receiving. Most of the time, this
    will look like:</p>
<pre>
        Content-type: text/html
</pre>

    <p>Secondly, your output needs to be in HTML, or some other
    format that a browser will be able to display. Most of the
    time, this will be HTML, but occasionally you might write a CGI
    program that outputs a gif image, or other non-HTML
    content.</p>

    <p>Apart from those two things, writing a CGI program will look
    a lot like any other program that you might write.</p>

    <h3><a id="yourfirstcgiprogram" name="yourfirstcgiprogram">Your
    first CGI program</a></h3>

    <p>The following is an example CGI program that prints one line
    to your browser. Type in the following, save it to a file
    called <code>first.pl</code>, and put it in your
    <code>cgi-bin</code> directory.</p>
<pre>
        #!/usr/bin/perl
        print "Content-type: text/html\r\n\r\n";
        print "Hello, World.";
</pre>

    <p>Even if you are not familiar with Perl, you should be able
    to see what is happening here. The first line tells Apache (or
    whatever shell you happen to be running under) that this
    program can be executed by feeding the file to the interpreter
    found at the location <code>/usr/bin/perl</code>. The second
    line prints the content-type declaration we talked about,
    followed by two carriage-return newline pairs. This puts a
    blank line after the header, to indicate the end of the HTTP
    headers, and the beginning of the body. The third line prints
    the string ``Hello, World.'' And that's the end of it.</p>

    <p>If you open your favorite browser and tell it to get the
    address</p>
<pre>
        http://www.example.com/cgi-bin/first.pl
</pre>

    <p>or wherever you put your file, you will see the one line
    <code>Hello, World.</code> appear in your browser window. It's
    not very exciting, but once you get that working, you'll have a
    good chance of getting just about anything working.</p>
    <hr />

    <h2><a id="butitsstillnotworking"
    name="butitsstillnotworking">But it's still not
    working!</a></h2>

    <p>There are four basic things that you may see in your browser
    when you try to access your CGI program from the web:</p>

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

      <dd>Great! That means everything worked fine.<br />
      <br />
      </dd>

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

      <dd>That means that you have not properly configured Apache
      to process your CGI program. Reread the section on <a
      href="#configuringapachetopermitcgi">configuring Apache</a>
      and try to find what you missed.<br />
      <br />
      </dd>

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

      <dd>That means that there is a permissions problem. Check the
      <a href="#errorlogs">Apache error log</a> and the section
      below on <a href="#filepermissions">file
      permissions</a>.<br />
      <br />
      </dd>

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

      <dd>If you check the <a href="#errorlogs">Apache error
      log</a>, you will probably find that it says "Premature end
      of script headers", possibly along with an error message
      generated by your CGI program. In this case, you will want to
      check each of the below sections to see what might be
      preventing your CGI program from emitting the proper HTTP
      headers.</dd>
    </dl>

    <h3><a id="filepermissions" name="filepermissions">File
    permissions</a></h3>

    <p>Remember that the server does not run as you. That is, when
    the server starts up, it is running with the permissions of an
    unprivileged user - usually ``nobody'', or ``www'' - and so it
    will need extra permissions to execute files that are owned by
    you. Usually, the way to give a file sufficient permissions to
    be executed by ``nobody'' is to give everyone execute
    permission on the file:</p>
<pre>
        chmod a+x first.pl
</pre>

    <p>Also, if your program reads from, or writes to, any other
    files, those files will need to have the correct permissions to
    permit this.</p>

    <p>The exception to this is when the server is configured to
    use <a href="../suexec.html">suexec</a>. This program allows
    CGI programs to be run under different user permissions,
    depending on which virtual host or user home directory they are
    located in. Suexec has very strict permission checking, and any
    failure in that checking will result in your CGI programs
    failing with an "Internal Server Error". In this case, you will
    need to check the suexec log file to see what specific security
    check is failing.</p>

    <h3><a id="pathinformation" name="pathinformation">Path
    information</a></h3>

    <p>When you run a program from your command line, you have
    certain information that is passed to the shell without you
    thinking about it. For example, you have a path, which tells
    the shell where it can look for files that you reference.</p>

    <p>When a program runs through the web server as a CGI program,
    it does not have that path. Any programs that you invoke in
    your CGI program (like 'sendmail', for example) will need to be
    specified by a full path, so that the shell can find them when
    it attempts to execute your CGI program.</p>

    <p>A common manifestation of this is the path to the script
    interpreter (often <code>perl</code>) indicated in the first
    line of your CGI program, which will look something like:</p>
<pre>
     #!/usr/bin/perl
</pre>

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

    <h3><a id="syntaxerrors" name="syntaxerrors">Syntax
    errors</a></h3>

    <p>Most of the time when a CGI program fails, it's because of a
    problem with the program itself. This is particularly true once
    you get the hang of this CGI stuff, and no longer make the
    above two mistakes. Always attempt to run your program from the
    command line before you test if via a browser. This will
    eliminate most of your problems.</p>

    <h3><a id="errorlogs" name="errorlogs">Error logs</a></h3>

    <p>The error logs are your friend. Anything that goes wrong
    generates message in the error log. You should always look
    there first. If the place where you are hosting your web site
    does not permit you access to the error log, you should
    probably host your site somewhere else. Learn to read the error
    logs, and you'll find that almost all of your problems are
    quickly identified, and quickly solved.</p>
    <hr />

    <h2><a id="whatsgoingonbehindthescenes"
    name="whatsgoingonbehindthescenes">What's going on behind the
    scenes?</a></h2>

    <p>As you become more advanced in CGI programming, it will
    become useful to understand more about what's happening behind
    the scenes. Specifically, how the browser and server
    communicate with one another. Because although it's all very
    well to write a program that prints ``Hello, World.'', it's not
    particularly useful.</p>

    <h3><a id="environmentvariables"
    name="environmentvariables">Environment variables</a></h3>

    <p>Environment variables are values that float around you as
    you use your computer. They are useful things like your path
    (where the computer searches for a the actual file implementing
    a command when you type it), your username, your terminal type,
    and so on. For a full list of your normal, every day
    environment variables, type <code>env</code> at a command
    prompt.</p>

    <p>During the CGI transaction, the server and the browser also
    set environment variables, so that they can communicate with
    one another. These are things like the browser type (Netscape,
    IE, Lynx), the server type (Apache, IIS, WebSite), the name of
    the CGI program that is being run, and so on.</p>

    <p>These variables are available to the CGI programmer, and are
    half of the story of the client-server communication. The
    complete list of required variables is at <a
    href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a></p>

    <p>This simple Perl CGI program will display all of the
    environment variables that are being passed around. Two similar
    programs are included in the <code>cgi-bin</code> directory of
    the Apache distribution. Note that some variables are required,
    while others are optional, so you may see some variables listed
    that were not in the official list. In addition, Apache
    provides many different ways for you to <a
    href="../env.html">add your own environment variables</a> to
    the basic ones provided by default.</p>
<pre>
     #!/usr/bin/perl
     print "Content-type: text/html\n\n";
     foreach $key (keys %ENV) {
          print "$key --&gt; $ENV{$key}&lt;br&gt;";
     }
</pre>

    <h3><a id="stdinandstdout" name="stdinandstdout">STDIN and
    STDOUT</a></h3>

    <p>Other communication between the server and the client
    happens over standard input (<code>STDIN</code>) and standard
    output (<code>STDOUT</code>). In normal everyday context,
    <code>STDIN</code> means the keyboard, or a file that a program
    is given to act on, and <code>STDOUT</code> usually means the
    console or screen.</p>

    <p>When you <code>POST</code> a web form to a CGI program, the
    data in that form is bundled up into a special format and gets
    delivered to your CGI program over <code>STDIN</code>. The
    program then can process that data as though it was coming in
    from the keyboard, or from a file</p>

    <p>The ``special format'' is very simple. A field name and its
    value are joined together with an equals (=) sign, and pairs of
    values are joined together with an ampersand (&amp;).
    Inconvenient characters like spaces, ampersands, and equals
    signs, are converted into their hex equivalent so that they
    don't gum up the works. The whole data string might look
    something like:</p>
<pre>
     name=Rich%20Bowen&amp;city=Lexington&amp;state=KY&amp;sidekick=Squirrel%20Monkey
</pre>

    <p>You'll sometimes also see this type of string appended to
    the a URL. When that is done, the server puts that string into
    the environment variable called <code>QUERY_STRING</code>.
    That's called a <code>GET</code> request. Your HTML form
    specifies whether a <code>GET</code> or a <code>POST</code> is
    used to deliver the data, by setting the <code>METHOD</code>
    attribute in the <code>FORM</code> tag.</p>

    <p>Your program is then responsible for splitting that string
    up into useful information. Fortunately, there are libraries
    and modules available to help you process this data, as well as
    handle other of the aspects of your CGI program.</p>
    <hr />

    <h2><a id="cgimoduleslibraries" name="cgimoduleslibraries">CGI
    modules/libraries</a></h2>

    <p>When you write CGI programs, you should consider using a
    code library, or module, to do most of the grunt work for you.
    This leads to fewer errors, and faster development.</p>

    <p>If you're writing CGI programs in Perl, modules are
    available on <a href="http://www.cpan.org/">CPAN</a>. The most
    popular module for this purpose is CGI.pm. You might also
    consider CGI::Lite, which implements a minimal set of
    functionality, which is all you need in most programs.</p>

    <p>If you're writing CGI programs in C, there are a variety of
    options. One of these is the CGIC library, from <a
    href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a></p>
    <hr />

    <h2><a id="formoreinformation" name="formoreinformation">For
    more information</a></h2>

    <p>There are a large number of CGI resources on the web. You
    can discuss CGI problems with other users on the Usenet group
    comp.infosystems.www.authoring.cgi. And the -servers mailing
    list from the HTML Writers Guild is a great source of answers
    to your questions. You can find out more at <a
    href="http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a></p>

    <p>And, of course, you should probably read the CGI
    specification, which has all the details on the operation of
    CGI programs. You can find the original version at the <a
    href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a>
    and there is an updated draft at the <a
    href="http://web.golux.com/coar/cgi/">Common Gateway Interface
    RFC project</a>.</p>

    <p>When you post a question about a CGI problem that you're
    having, whether to a mailing list, or to a newsgroup, make sure
    you provide enough information about what happened, what you
    expected to happen, and how what actually happened was
    different, what server you're running, what language your CGI
    program was in, and, if possible, the offending code. This will
    make finding your problem much simpler.</p>

    <p>Note that questions about CGI problems should
    <strong>never</strong> be posted to the Apache bug database
    unless you are sure you have found a problem in the Apache
    source code.</p>
    <!--#include virtual="footer.html" -->
  </body>
</html>