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 <title>Apache Tutorial: Introduction to Server Side Includes</title>
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 <!--#include virtual="header.html" -->
 <h1 align="CENTER">Apache Tutorial: Introduction to Server Side
 Includes</h1>

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


 <ul>
 <li><a href=
 "#apachetutorial:introductiontoserversideincludes">Apache
 Tutorial: Introduction to Server Side Includes</a></li>

 <li><a href="#whataressi">What are SSI?</a></li>

 <li><a href="#configuringyourservertopermitssi">Configuring your
 server to permit SSI</a></li>

 <li><a href="#basicssidirectives">Basic SSI directives</a>

 <ul>
   <li><a href="#today'sdate">Today's date</a></li>

   <li><a href="#modificationdateofthefile">Modification date of the
 file</a></li>

   <li><a href="#includingtheresultsofacgiprogram">Including the
 results of a CGI program</a></li>
 </ul>
 </li>

 <li><a href="#additionalexamples">Additional examples</a>

 <ul>
 <li><a href="#whenwasthisdocumentmodified">When was this document
 modified?</a></li>

 <li><a href="#includingastandardfooter">Including a standard
 footer</a></li>

 <li><a href="#whatelsecaniconfig">What else can I config?</a></li>

 <li><a href="#executingcommands">Executing commands</a></li>
 </ul>
 </li>

 <li><a href="#advancedssitechniques">Advanced SSI techniques</a>

 <ul>
 <li><a href="#settingvariables">Setting variables</a></li>

 <li><a href="#conditionalexpressions">Conditional expressions</a></li>
 </ul>
 </li>

 <li><a href="#conclusion">Conclusion</a></li>
 </ul>

 <!-- INDEX END -->
 <hr>
 <h2><a name=
 "apachetutorial:introductiontoserversideincludes">Apache
 Tutorial: Introduction to Server Side Includes</a></h2>

 <table border="1">
 <tr>
 <td valign="top"><strong>Related Modules</strong><br>
 <br>
  <a href="../mod/mod_include.html">mod_include</a><br>
 <a href="../mod/mod_cgi.html">mod_cgi</a><br>
 <a href="../mod/mod_expires.html">mod_expires</a><br>
  </td>
 <td valign="top"><strong>Related Directives</strong><br>
 <br>
  <a href="../mod/core.html#options">Options</a><br>
 <a href="../mod/mod_include.html#xbithack">XBitHack</a><br>
 <a href="../mod/mod_mime.html#addtype">AddType</a><br>
 <a href="../mod/mod_mime.html#addhandler">AddHandler</a><br>
 <a href=
 "../mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br>

  </td>
 </tr>
 </table>

 <p>This HOWTO first appeared in Apache Today
 (http://www.apachetoday.com/) as a series of three articles. They
 appear here by arrangement with ApacheToday and Internet.com.</p>

 <p>This article deals with Server Side Includes, usually called simply
 SSI. In this article, I'll talk about configuring your server to permit
 SSI, and introduce some basic SSI techniques for adding dynamic content
 to your existing HTML pages.</p>

 <p>In the latter part of the article, we'll talk about some of the
 somewhat more advanced things that can be done with SSI, such as
 conditional statements in your SSI directives.</p>

 <hr>
 <h2><a name="whataressi">What are SSI?</a></h2>

 <p>SSI (Server Side Includes) are directives that are placed in HTML
 pages, and evaluated on the server while the pages are being served.
 They let you add dynamically generated content to an existing HTML
 page, without having to serve the entire page via a CGI program, or
 other dynamic technology.</p>

 <p>The decision of when to use SSI, and when to have your page entirely
 generated by some program, is usually a matter of how much of the page
 is static, and how much needs to be recalculated every time the page is
 served. SSI is a great way to add small pieces of information, such as
 the current time. But if a majority of your page is being generated at
 the time that it is served, you need to look for some other
 solution.</p>

 <hr>
 <h2><a name="configuringyourservertopermitssi">Configuring your
 server to permit SSI</a></h2>

 <p>To permit SSI on your server, you must have the following directive
 either in your <code>httpd.conf</code> file, or in a
 <code>.htaccess</code> file:</p>

 <pre>
         Options +Includes
 </pre>

 <p>This tells Apache that you want to permit files to be parsed for SSI
 directives.</p>

 <p>Not just any file is parsed for SSI directives. You have to tell
 Apache which files should be parsed. There are two ways to do this. You
 can tell Apache to parse any file with a particular file extension,
 such as <code>.shtml</code>, with the following directives:</p>

 <pre>
         AddType text/html .shtml
         AddHandler server-parsed .shtml
 </pre>

 <p>One disadvantage to this approach is that if you wanted to add SSI
 directives to an existing page, you would have to change the name of
 that page, and all links to that page, in order to give it a
 <code>.shtml</code> extension, so that those directives would be
 executed.</p>

 <p>The other method is to use the <code>XBitHack</code> directive:</p>

 <pre>
         XBitHack on
 </pre>

 <p><code>XBitHack</code> tells Apache to parse files for SSI directives
 if they have the execute bit set. So, to add SSI directives to an
 existing page, rather than having to change the file name, you would
 just need to make the file executable using <code>chmod</code>.</p>

 <pre>
         chmod +x pagename.html
 </pre>

 <p>A brief comment about what not to do. You'll occasionally see people
 recommending that you just tell Apache to parse all <code>.html</code>
 files for SSI, so that you don't have to mess with <code>.shtml</code>
 file names. These folks have perhaps not heard about
 <code>XBitHack</code>. The thing to keep in mind is that, by doing
 this, you're requiring that Apache read through every single file that
 it sends out to clients, even if they don't contain any SSI directives.
 This can slow things down quite a bit, and is not a good idea.</p>

 <p>Of course, on Windows, there is no such thing as an execute bit to
 set, so that limits your options a little.</p>

 <p>In its default configuration, Apache does not send the last modified
 date or content length HTTP headers on SSI pages, because these values are
 difficult to calculate for dynamic content.  This can prevent your
 document from being cached, and result in slower perceived client
 performance.  There are two ways to solve this:</p>

 <ol>

 <li>Use the <code>XBitHack Full</code> configuration.  This tells
 Apache to determine the last modified date by looking only at the date
 of the originally requested file, ignoring the modification date of
 any included files. </li>

 <li>Use the directives provided by <a
 href="../mod/mod_expires.html">mod_expires</a> to set an explicit
 expiration time on your files, thereby letting browsers and proxies
 know that it is acceptable to cache them. </li>

 </ol>


 <hr>
 <h2><a name="basicssidirectives">Basic SSI directives</a></h2>

 <p>SSI directives have the following syntax:</p>

 <pre>
         &lt;!--#element attribute=value attribute=value ... --&gt;
 </pre>

 <p>It is formatted like an HTML comment, so if you don't have SSI
 correctly enabled, the browser will ignore it, but it will still be
 visible in the HTML source. If you have SSI correctly configured, the
 directive will be replaced with its results.</p>

 <p>The element can be one of a number of things, and we'll talk some
 more about most of these in the next installment of this series. For
 now, here are some examples of what you can do with SSI</p>

 <h3><a name="today'sdate">Today's date</a></h3>

 <pre>
         &lt;!--#echo var=DATE_LOCAL --&gt;
 </pre>

 <p>The <code>echo</code> element just spits out the value of a
 variable. There are a number of standard variables, which include the
 whole set of environment variables that are available to CGI programs.
 Also, you can define your own variables with the <code>set</code>
 element.</p>

 <p>If you don't like the format in which the date gets printed, you can
 use the <code>config</code> element, with a <code>timefmt</code>
 attribute, to modify that formatting.</p>

 <pre>
         &lt;!--#config timefmt="%A %B %d, %Y" --&gt;
         Today is &lt;!--#echo var=DATE_LOCAL --&gt;
 </pre>

 <h3><a name="modificationdateofthefile">Modification date of the
 file</a></h3>

 <pre>
         This document last modified &lt;!--#flastmod file="index.html" --&gt;
 </pre>

 <p>This element is also subject to <code>timefmt</code> format
 configurations.</p>

 <h3><a name="includingtheresultsofacgiprogram">Including the
 results of a CGI program</a></h3>

 <p>This is one of the more common uses of SSI - to output the results
 of a CGI program, such as everybody's favorite, a ``hit counter.''</p>

 <pre>
         &lt;!--#include virtual="/cgi-bin/counter.pl" --&gt;
 </pre>

 <hr>
 <h2><a name="additionalexamples">Additional examples</a></h2>

 <p>Following are some specific examples of things you can do in your
 HTML documents with SSI.</p>

 <hr>
 <h2><a name="whenwasthisdocumentmodified">When was this document
 modified?</a></h2>

 <p>Earlier, we mentioned that you could use SSI to inform the user when
 the document was most recently modified. However, the actual method for
 doing that was left somewhat in question. The following code, placed in
 your HTML document, will put such a time stamp on your page. Of course,
 you will have to have SSI correctly enabled, as discussed above.</p>

 <pre>
         &lt;!--#config timefmt="%A %B %d, %Y" --&gt;
         This file last modified &lt;!--#flastmod file="ssi.shtml" --&gt;
 </pre>

 <p>Of course, you will need to replace the <code>ssi.shtml</code> with
 the actual name of the file that you're referring to. This can be
 inconvenient if you're just looking for a generic piece of code that
 you can paste into any file, so you probably want to use the
 <code>LAST_MODIFIED</code> variable instead:</p>

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

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

 <hr>
 <h2><a name="includingastandardfooter">Including a standard
 footer</a></h2>

 <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>

 <pre>
         &lt;!--#include virtual="/footer.html" --&gt;
 </pre>

 <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>

 <hr>
 <h2><a name="whatelsecaniconfig">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>

 <pre>
         [an error occurred while processing this directive]
 </pre>

 <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>

 <pre>
         &lt;!--#config errmsg="[It appears that you don't know how to use SSI]" --&gt;
 </pre>

 <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>

 <hr>
 <h2><a name="executingcommands">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>

 <pre>
         &lt;pre&gt;
         &lt;!--#exec cmd="ls" --&gt;
         &lt;/pre&gt;
 </pre>

 <p>or, on Windows</p>

 <pre>
         &lt;pre&gt;
         &lt;!--#exec cmd="dir" --&gt;
         &lt;/pre&gt;
 </pre>

 <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>

 <hr>
 <h2><a name="advancedssitechniques">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">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>

 <hr>
 <h2><a name="settingvariables">Setting variables</a></h2>

 <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>

 <pre>
         &lt;!--#set var="name" value="Rich" --&gt;
 </pre>

 <p>In addition to merely setting values literally like that, you can
 use any other variable, including, for example, environment variables,
 or some of the variables we discussed in the last article (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>

 <pre>
         &lt;!--#set var="modified" value="$LAST_MODIFIED" --&gt;
 </pre>

 <p>To put a literal dollar sign into the value of your variable, you
 need to escape the dollar sign with a backslash.</p>

 <pre>
         &lt;!--#set var="cost" value="\$100" --&gt;
 </pre>

 <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>

 <pre>
         &lt;!--#set var="date" value="${DATE_LOCAL}_${DATE_GMT}" --&gt;
 </pre>

 <hr>
 <h2><a name="conditionalexpressions">Conditional expressions</a></h2>

 <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>mod_include</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>

 <pre>
         &lt;!--#if expr="test_condition" --&gt;
     &lt;!--#elif expr="test_condition" --&gt;
     &lt;!--#else --&gt;
     &lt;!--#endif --&gt;
 </pre>

 <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>mod_include</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>

 <pre>
         BrowserMatchNoCase macintosh Mac
         BrowserMatchNoCase MSIE InternetExplorer
 </pre>

 <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>

 <pre>
         &lt;!--#if expr="${Mac} &amp;&amp; ${InternetExplorer}" --&gt;
         Apologetic text goes here
         &lt;!--#else --&gt;
         Cool JavaScript code goes here
         &lt;!--#endif --&gt;
 </pre>

 <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>

 <hr>
 <h2><a name="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>
 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
	<html>
	<head>
	<title>Apache Tutorial: Introduction to Server Side Includes</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">Apache Tutorial: Introduction to Server Side
	Includes</h1>

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


	<ul>
	<li><a href=
	"#apachetutorial:introductiontoserversideincludes">Apache
	Tutorial: Introduction to Server Side Includes</a></li>

	<li><a href="#whataressi">What are SSI?</a></li>

	<li><a href="#configuringyourservertopermitssi">Configuring your
	server to permit SSI</a></li>

	<li><a href="#basicssidirectives">Basic SSI directives</a>

	<ul>
	<li><a href="#today'sdate">Today's date</a></li>

	<li><a href="#modificationdateofthefile">Modification date of the
	file</a></li>

	<li><a href="#includingtheresultsofacgiprogram">Including the
	results of a CGI program</a></li>
	</ul>
	</li>

	<li><a href="#additionalexamples">Additional examples</a>

	<ul>
	<li><a href="#whenwasthisdocumentmodified">When was this document
	modified?</a></li>

	<li><a href="#includingastandardfooter">Including a standard
	footer</a></li>

	<li><a href="#whatelsecaniconfig">What else can I config?</a></li>

	<li><a href="#executingcommands">Executing commands</a></li>
	</ul>
	</li>

	<li><a href="#advancedssitechniques">Advanced SSI techniques</a>

	<ul>
	<li><a href="#settingvariables">Setting variables</a></li>

	<li><a href="#conditionalexpressions">Conditional expressions</a></li>
	</ul>
	</li>

	<li><a href="#conclusion">Conclusion</a></li>
	</ul>

	<!-- INDEX END -->
	<hr>
	<h2><a name=
	"apachetutorial:introductiontoserversideincludes">Apache
	Tutorial: Introduction to Server Side Includes</a></h2>

	<table border="1">
	<tr>
	<td valign="top"><strong>Related Modules</strong><br>
	<br>
	<a href="../mod/mod_include.html">mod_include</a><br>
	<a href="../mod/mod_cgi.html">mod_cgi</a><br>
	<a href="../mod/mod_expires.html">mod_expires</a><br>
	</td>
	<td valign="top"><strong>Related Directives</strong><br>
	<br>
	<a href="../mod/core.html#options">Options</a><br>
	<a href="../mod/mod_include.html#xbithack">XBitHack</a><br>
	<a href="../mod/mod_mime.html#addtype">AddType</a><br>
	<a href="../mod/mod_mime.html#addhandler">AddHandler</a><br>
	<a href=
	"../mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br>

	</td>
	</tr>
	</table>

	<p>This HOWTO first appeared in Apache Today
	(http://www.apachetoday.com/) as a series of three articles. They
	appear here by arrangement with ApacheToday and Internet.com.</p>

	<p>This article deals with Server Side Includes, usually called simply
	SSI. In this article, I'll talk about configuring your server to permit
	SSI, and introduce some basic SSI techniques for adding dynamic content
	to your existing HTML pages.</p>

	<p>In the latter part of the article, we'll talk about some of the
	somewhat more advanced things that can be done with SSI, such as
	conditional statements in your SSI directives.</p>

	<hr>
	<h2><a name="whataressi">What are SSI?</a></h2>

	<p>SSI (Server Side Includes) are directives that are placed in HTML
	pages, and evaluated on the server while the pages are being served.
	They let you add dynamically generated content to an existing HTML
	page, without having to serve the entire page via a CGI program, or
	other dynamic technology.</p>

	<p>The decision of when to use SSI, and when to have your page entirely
	generated by some program, is usually a matter of how much of the page
	is static, and how much needs to be recalculated every time the page is
	served. SSI is a great way to add small pieces of information, such as
	the current time. But if a majority of your page is being generated at
	the time that it is served, you need to look for some other
	solution.</p>

	<hr>
	<h2><a name="configuringyourservertopermitssi">Configuring your
	server to permit SSI</a></h2>

	<p>To permit SSI on your server, you must have the following directive
	either in your <code>httpd.conf</code> file, or in a
	<code>.htaccess</code> file:</p>

	<pre>
	Options +Includes
	</pre>

	<p>This tells Apache that you want to permit files to be parsed for SSI
	directives.</p>

	<p>Not just any file is parsed for SSI directives. You have to tell
	Apache which files should be parsed. There are two ways to do this. You
	can tell Apache to parse any file with a particular file extension,
	such as <code>.shtml</code>, with the following directives:</p>

	<pre>
	AddType text/html .shtml
	AddHandler server-parsed .shtml
	</pre>

	<p>One disadvantage to this approach is that if you wanted to add SSI
	directives to an existing page, you would have to change the name of
	that page, and all links to that page, in order to give it a
	<code>.shtml</code> extension, so that those directives would be
	executed.</p>

	<p>The other method is to use the <code>XBitHack</code> directive:</p>

	<pre>
	XBitHack on
	</pre>

	<p><code>XBitHack</code> tells Apache to parse files for SSI directives
	if they have the execute bit set. So, to add SSI directives to an
	existing page, rather than having to change the file name, you would
	just need to make the file executable using <code>chmod</code>.</p>

	<pre>
	chmod +x pagename.html
	</pre>

	<p>A brief comment about what not to do. You'll occasionally see people
	recommending that you just tell Apache to parse all <code>.html</code>
	files for SSI, so that you don't have to mess with <code>.shtml</code>
	file names. These folks have perhaps not heard about
	<code>XBitHack</code>. The thing to keep in mind is that, by doing
	this, you're requiring that Apache read through every single file that
	it sends out to clients, even if they don't contain any SSI directives.
	This can slow things down quite a bit, and is not a good idea.</p>

	<p>Of course, on Windows, there is no such thing as an execute bit to
	set, so that limits your options a little.</p>

	<p>In its default configuration, Apache does not send the last modified
	date or content length HTTP headers on SSI pages, because these values are
	difficult to calculate for dynamic content. This can prevent your
	document from being cached, and result in slower perceived client
	performance. There are two ways to solve this:</p>

	<ol>

	<li>Use the <code>XBitHack Full</code> configuration. This tells
	Apache to determine the last modified date by looking only at the date
	of the originally requested file, ignoring the modification date of
	any included files. </li>

	<li>Use the directives provided by <a
	href="../mod/mod_expires.html">mod_expires</a> to set an explicit
	expiration time on your files, thereby letting browsers and proxies
	know that it is acceptable to cache them. </li>

	</ol>


	<hr>
	<h2><a name="basicssidirectives">Basic SSI directives</a></h2>

	<p>SSI directives have the following syntax:</p>

	<pre>
	<!--#element attribute=value attribute=value ... -->
	</pre>

	<p>It is formatted like an HTML comment, so if you don't have SSI
	correctly enabled, the browser will ignore it, but it will still be
	visible in the HTML source. If you have SSI correctly configured, the
	directive will be replaced with its results.</p>

	<p>The element can be one of a number of things, and we'll talk some
	more about most of these in the next installment of this series. For
	now, here are some examples of what you can do with SSI</p>

	<h3><a name="today'sdate">Today's date</a></h3>

	<pre>
	<!--#echo var=DATE_LOCAL -->
	</pre>

	<p>The <code>echo</code> element just spits out the value of a
	variable. There are a number of standard variables, which include the
	whole set of environment variables that are available to CGI programs.
	Also, you can define your own variables with the <code>set</code>
	element.</p>

	<p>If you don't like the format in which the date gets printed, you can
	use the <code>config</code> element, with a <code>timefmt</code>
	attribute, to modify that formatting.</p>

	<pre>
	<!--#config timefmt="%A %B %d, %Y" -->
	Today is <!--#echo var=DATE_LOCAL -->
	</pre>

	<h3><a name="modificationdateofthefile">Modification date of the
	file</a></h3>

	<pre>
	This document last modified <!--#flastmod file="index.html" -->
	</pre>

	<p>This element is also subject to <code>timefmt</code> format
	configurations.</p>

	<h3><a name="includingtheresultsofacgiprogram">Including the
	results of a CGI program</a></h3>

	<p>This is one of the more common uses of SSI - to output the results
	of a CGI program, such as everybody's favorite, a ``hit counter.''</p>

	<pre>
	<!--#include virtual="/cgi-bin/counter.pl" -->
	</pre>

	<hr>
	<h2><a name="additionalexamples">Additional examples</a></h2>

	<p>Following are some specific examples of things you can do in your
	HTML documents with SSI.</p>

	<hr>
	<h2><a name="whenwasthisdocumentmodified">When was this document
	modified?</a></h2>

	<p>Earlier, we mentioned that you could use SSI to inform the user when
	the document was most recently modified. However, the actual method for
	doing that was left somewhat in question. The following code, placed in
	your HTML document, will put such a time stamp on your page. Of course,
	you will have to have SSI correctly enabled, as discussed above.</p>

	<pre>
	<!--#config timefmt="%A %B %d, %Y" -->
	This file last modified <!--#flastmod file="ssi.shtml" -->
	</pre>

	<p>Of course, you will need to replace the <code>ssi.shtml</code> with
	the actual name of the file that you're referring to. This can be
	inconvenient if you're just looking for a generic piece of code that
	you can paste into any file, so you probably want to use the
	<code>LAST_MODIFIED</code> variable instead:</p>

	<pre>
	<!--#config timefmt="%D" -->
	This file last modified <!--#echo var="LAST_MODIFIED" -->
	</pre>

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

	<hr>
	<h2><a name="includingastandardfooter">Including a standard
	footer</a></h2>

	<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>

	<pre>
	<!--#include virtual="/footer.html" -->
	</pre>

	<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>

	<hr>
	<h2><a name="whatelsecaniconfig">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>

	<pre>
	[an error occurred while processing this directive]
	</pre>

	<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>

	<pre>
	<!--#config errmsg="[It appears that you don't know how to use SSI]" -->
	</pre>

	<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>

	<hr>
	<h2><a name="executingcommands">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>

	<pre>
	<pre>
	<!--#exec cmd="ls" -->
	</pre>
	</pre>

	<p>or, on Windows</p>

	<pre>
	<pre>
	<!--#exec cmd="dir" -->
	</pre>
	</pre>

	<p>You might notice some strange formatting with this directive on
	Windows, because the output from <code>dir</code> contains the string
	``<<code>dir</code>>'' 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>

	<hr>
	<h2><a name="advancedssitechniques">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">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>

	<hr>
	<h2><a name="settingvariables">Setting variables</a></h2>

	<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>

	<pre>
	<!--#set var="name" value="Rich" -->
	</pre>

	<p>In addition to merely setting values literally like that, you can
	use any other variable, including, for example, environment variables,
	or some of the variables we discussed in the last article (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>

	<pre>
	<!--#set var="modified" value="$LAST_MODIFIED" -->
	</pre>

	<p>To put a literal dollar sign into the value of your variable, you
	need to escape the dollar sign with a backslash.</p>

	<pre>
	<!--#set var="cost" value="\$100" -->
	</pre>

	<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>

	<pre>
	<!--#set var="date" value="${DATE_LOCAL}_${DATE_GMT}" -->
	</pre>

	<hr>
	<h2><a name="conditionalexpressions">Conditional expressions</a></h2>

	<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>mod_include</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>

	<pre>
	<!--#if expr="test_condition" -->
	<!--#elif expr="test_condition" -->
	<!--#else -->
	<!--#endif -->
	</pre>

	<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>mod_include</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>

	<pre>
	BrowserMatchNoCase macintosh Mac
	BrowserMatchNoCase MSIE InternetExplorer
	</pre>

	<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>

	<pre>
	<!--#if expr="${Mac} && ${InternetExplorer}" -->
	Apologetic text goes here
	<!--#else -->
	Cool JavaScript code goes here
	<!--#endif -->
	</pre>

	<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>

	<hr>
	<h2><a name="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>
	</body>
	</html>