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        &lt;!--#config timefmt="%D" --&gt;<br />
        This file last modified &lt;!--#echo var="LAST_MODIFIED" --&gt;
</code></p></div>

    <p>For more details on the <code>timefmt</code> format, go to
    your favorite search site and look for <code>strftime</code>. The
    syntax is the same.</p>


<h3><a name="standard-footer" id="standard-footer">Including a standard footer</a></h3>


    <p>If you are managing any site that is more than a few pages,
    you may find that making changes to all those pages can be a
    real pain, particularly if you are trying to maintain some kind
    of standard look across all those pages.</p>

    <p>Using an include file for a header and/or a footer can
    reduce the burden of these updates. You just have to make one
    footer file, and then include it into each page with the
    <code>include</code> SSI command. The <code>include</code>
    element can determine what file to include with either the
    <code>file</code> attribute, or the <code>virtual</code>
    attribute. The <code>file</code> attribute is a file path,
    <em>relative to the current directory</em>. That means that it
    cannot be an absolute file path (starting with /), nor can it
    contain ../ as part of that path. The <code>virtual</code>
    attribute is probably more useful, and should specify a URL
    relative to the document being served. It can start with a /,
    but must be on the same server as the file being served.</p>
<div class="example"><p><code>
        &lt;!--#include virtual="/footer.html" --&gt;
</code></p></div>

    <p>I'll frequently combine the last two things, putting a
    <code>LAST_MODIFIED</code> directive inside a footer file to be
    included. SSI directives can be contained in the included file,
    and includes can be nested - that is, the included file can
    include another file, and so on.</p>


</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="config" id="config">What else can I config?</a></h2>


    <p>In addition to being able to <code>config</code> the time
    format, you can also <code>config</code> two other things.</p>

    <p>Usually, when something goes wrong with your SSI directive,
    you get the message</p>
<div class="example"><p><code>
        [an error occurred while processing this directive]
</code></p></div>

    <p>If you want to change that message to something else, you
    can do so with the <code>errmsg</code> attribute to the
    <code>config</code> element:</p>
<div class="example"><p><code>
        &lt;!--#config errmsg="[It appears that you don't know how to use SSI]" --&gt;
</code></p></div>

    <p>Hopefully, end users will never see this message, because
    you will have resolved all the problems with your SSI
    directives before your site goes live. (Right?)</p>

    <p>And you can <code>config</code> the format in which file
    sizes are returned with the <code>sizefmt</code> attribute. You
    can specify <code>bytes</code> for a full count in bytes, or
    <code>abbrev</code> for an abbreviated number in Kb or Mb, as
    appropriate.</p>
    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="exec" id="exec">Executing commands</a></h2>
    

    <p>I expect that I'll have an article some time in the coming
    months about using SSI with small CGI programs. For now, here's
    something else that you can do with the <code>exec</code>
    element. You can actually have SSI execute a command using the
    shell (<code>/bin/sh</code>, to be precise - or the DOS shell,
    if you're on Win32). The following, for example, will give you
    a directory listing.</p>
<div class="example"><p><code>
        &lt;pre&gt;<br />
        &lt;!--#exec cmd="ls" --&gt;<br />
        &lt;/pre&gt;
</code></p></div>

    <p>or, on Windows</p>
<div class="example"><p><code>
        &lt;pre&gt;<br />
        &lt;!--#exec cmd="dir" --&gt;<br />
        &lt;/pre&gt;
</code></p></div>

    <p>You might notice some strange formatting with this directive
    on Windows, because the output from <code>dir</code> contains
    the string ``&lt;<code>dir</code>&gt;'' in it, which confuses
    browsers.</p>

    <p>Note that this feature is exceedingly dangerous, as it will
    execute whatever code happens to be embedded in the
    <code>exec</code> tag. If you have any situation where users
    can edit content on your web pages, such as with a
    ``guestbook'', for example, make sure that you have this
    feature disabled. You can allow SSI, but not the
    <code>exec</code> feature, with the <code>IncludesNOEXEC</code>
    argument to the <code>Options</code> directive.</p>
    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="advanced" id="advanced">Advanced SSI techniques</a></h2>


    <p>In addition to spitting out content, Apache SSI gives you
    the option of setting variables, and using those variables in
    comparisons and conditionals.</p>

<h3><a name="caveat" id="caveat">Caveat</a></h3>

    <p>Most of the features discussed in this article are only
    available to you if you are running Apache 1.2 or later. Of
    course, if you are not running Apache 1.2 or later, you need to
    upgrade immediately, if not sooner. Go on. Do it now. We'll
    wait.</p>


<h3><a name="variables" id="variables">Setting variables</a></h3>

    <p>Using the <code>set</code> directive, you can set variables
    for later use. We'll need this later in the discussion, so
    we'll talk about it here. The syntax of this is as follows:</p>
<div class="example"><p><code>
        &lt;!--#set var="name" value="Rich" --&gt;
</code></p></div>

    <p>In addition to merely setting values literally like that, you
    can use any other variable, including <a href="../env.html">environment variables</a> or the variables
    discussed above (like <code>LAST_MODIFIED</code>, for example) to
    give values to your variables. You will specify that something is
    a variable, rather than a literal string, by using the dollar sign
    ($) before the name of the variable.</p> 

    <div class="example"><p><code> &lt;!--#set var="modified" value="$LAST_MODIFIED" --&gt;
    </code></p></div>

    <p>To put a literal dollar sign into the value of your
    variable, you need to escape the dollar sign with a
    backslash.</p>
<div class="example"><p><code>
        &lt;!--#set var="cost" value="\$100" --&gt;
</code></p></div>

    <p>Finally, if you want to put a variable in the midst of a
    longer string, and there's a chance that the name of the
    variable will run up against some other characters, and thus be
    confused with those characters, you can place the name of the
    variable in braces, to remove this confusion. (It's hard to
    come up with a really good example of this, but hopefully
    you'll get the point.)</p>
<div class="example"><p><code>
        &lt;!--#set var="date" value="${DATE_LOCAL}_${DATE_GMT}" --&gt;
</code></p></div>


<h3><a name="conditional" id="conditional">Conditional expressions</a></h3>


    <p>Now that we have variables, and are able to set and compare
    their values, we can use them to express conditionals. This
    lets SSI be a tiny programming language of sorts.
    <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> provides an <code>if</code>,
    <code>elif</code>, <code>else</code>, <code>endif</code>
    structure for building conditional statements. This allows you
    to effectively generate multiple logical pages out of one
    actual page.</p>

    <p>The structure of this conditional construct is:</p>
<div class="example"><p><code>
    &lt;!--#if expr="test_condition" --&gt;<br />
    &lt;!--#elif expr="test_condition" --&gt;<br />
    &lt;!--#else --&gt;<br />
    &lt;!--#endif --&gt;
</code></p></div>

    <p>A <em>test_condition</em> can be any sort of logical
    comparison - either comparing values to one another, or testing
    the ``truth'' of a particular value. (A given string is true if
    it is nonempty.) For a full list of the comparison operators
    available to you, see the <code class="module"><a href="../mod/mod_include.html">mod_include</a></code>
    documentation. Here are some examples of how one might use this
    construct.</p>

    <p>In your configuration file, you could put the following
    line:</p>
<div class="example"><p><code>
        BrowserMatchNoCase macintosh Mac<br />
        BrowserMatchNoCase MSIE InternetExplorer
</code></p></div>

    <p>This will set environment variables ``Mac'' and
    ``InternetExplorer'' to true, if the client is running Internet
    Explorer on a Macintosh.</p>

    <p>Then, in your SSI-enabled document, you might do the
    following:</p>
<div class="example"><p><code>
        &lt;!--#if expr="${Mac} &amp;&amp; ${InternetExplorer}" --&gt;<br />
        Apologetic text goes here<br />
        &lt;!--#else --&gt;<br />
        Cool JavaScript code goes here<br />
        &lt;!--#endif --&gt;
</code></p></div>

    <p>Not that I have anything against IE on Macs - I just
    struggled for a few hours last week trying to get some
    JavaScript working on IE on a Mac, when it was working
    everywhere else. The above was the interim workaround.</p>

    <p>Any other variable (either ones that you define, or normal
    environment variables) can be used in conditional statements.
    With Apache's ability to set environment variables with the
    <code>SetEnvIf</code> directives, and other related directives,
    this functionality can let you do some pretty involved dynamic
    stuff without ever resorting to CGI.</p>

</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="conclusion" id="conclusion">Conclusion</a></h2>

    <p>SSI is certainly not a replacement for CGI, or other
    technologies used for generating dynamic web pages. But it is a
    great way to add small amounts of dynamic content to pages,
    without doing a lot of extra work.</p>
</div></div>
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