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 <manualpage metafile="ssi.xml.meta">
 <parentdocument href="./">How-To / Tutorials</parentdocument>

 <title>Apache Tutorial: Introduction to Server Side Includes</title>

 <summary>
 <p>Server-side includes provide a means to add dynamic content to
 existing HTML documents.</p>
 </summary>

 <section id="related"><title>Introduction</title>
  <related>
     <modulelist>
     <module>mod_include</module>
     <module>mod_cgi</module>
     <module>mod_expires</module>
     </modulelist>

     <directivelist>
     <directive module="core">Options</directive>
     <directive module="mod_include">XBitHack</directive>
     <directive module="mod_mime">AddType</directive>
     <directive module="core">SetOutputFilter</directive>
     <directive module="mod_setenvif">BrowserMatchNoCase</directive>
     </directivelist>
 </related>

     <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>

 </section>

 <section id="what"><title>What are SSI?</title>

     <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>
 </section>

 <section id="configuring">
 <title>Configuring your server to permit SSI</title>

     <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>
 <example>
         Options +Includes
 </example>

     <p>This tells Apache that you want to permit files to be parsed
     for SSI directives.  Note that most configurations contain
     multiple <directive module="core">Options</directive> directives
     that can override each other.  You will probably need to apply the
     <code>Options</code> to the specific directory where you want SSI
     enabled in order to assure that it gets evaluated last.</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>
 <example>
         AddType text/html .shtml<br />
 	AddOutputFilter INCLUDES .shtml
 </example>

     <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 <directive
     module="mod_include">XBitHack</directive> directive:</p>
 <example>
         XBitHack on
 </example>

     <p><directive module="mod_include">XBitHack</directive>
     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>
 <example>
         chmod +x pagename.html
 </example>

     <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 <directive
     module="mod_include">XBitHack</directive>. 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
       <module>mod_expires</module> to set an explicit expiration
       time on your files, thereby letting browsers and proxies
       know that it is acceptable to cache them.</li>
     </ol>
 </section>

 <section id="basic"><title>Basic SSI directives</title>

     <p>SSI directives have the following syntax:</p>
 <example>
         &lt;!--#element attribute=value attribute=value ... --&gt;
 </example>

     <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>

 <section id="todaysdate"><title>Today's date</title>

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

     <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>

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

 <section id="lastmodified"><title>Modification date of the file</title>

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

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

 <section id="cgi"><title>Including the results of a CGI program</title>

     <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>

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

 </section>
 </section>

 <section id="additionalexamples">
 <title>Additional examples</title>

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

 <section id="docmodified"><title>When was this document
 modified?</title>

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

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

     <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>
 </section>

 <section id="standard-footer">
 <title>Including a standard footer</title>

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

     <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>
 </section>

 </section>

 <section id="config">
 <title>What else can I config?</title>

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

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

     <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>
     </section>

 <section id="exec">
     <title>Executing commands</title>

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

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

     <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>
     </section>

 <section id="advanced">
 <title>Advanced SSI techniques</title>

     <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>

 <section id="caveat"><title>Caveat</title>

     <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>
 </section>

 <section id="variables"><title>Setting variables</title>

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

     <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>

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

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

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

 <section id="conditional">
 <title>Conditional expressions</title>

     <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.
     <module>mod_include</module> 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>
 <example>
     &lt;!--#if expr="test_condition" --&gt;<br />
     &lt;!--#elif expr="test_condition" --&gt;<br />
     &lt;!--#else --&gt;<br />
     &lt;!--#endif --&gt;
 </example>

     <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 <module>mod_include</module>
     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>
 <example>
         BrowserMatchNoCase macintosh Mac<br />
         BrowserMatchNoCase MSIE InternetExplorer
 </example>

     <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>
 <example>
         &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;
 </example>

     <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>
 </section>
 </section>

 <section id="conclusion"><title>Conclusion</title>

     <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>
 </section>

 </manualpage>
	<?xml version='1.0' encoding='UTF-8' ?>
	<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
	<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
	<!-- $Revision: 1.12 $ -->

	<!--
	Copyright 2002-2004 The Apache Software Foundation

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<manualpage metafile="ssi.xml.meta">
	<parentdocument href="./">How-To / Tutorials</parentdocument>

	<title>Apache Tutorial: Introduction to Server Side Includes</title>

	<summary>
	<p>Server-side includes provide a means to add dynamic content to
	existing HTML documents.</p>
	</summary>

	<section id="related"><title>Introduction</title>
	<related>
	<modulelist>
	<module>mod_include</module>
	<module>mod_cgi</module>
	<module>mod_expires</module>
	</modulelist>

	<directivelist>
	<directive module="core">Options</directive>
	<directive module="mod_include">XBitHack</directive>
	<directive module="mod_mime">AddType</directive>
	<directive module="core">SetOutputFilter</directive>
	<directive module="mod_setenvif">BrowserMatchNoCase</directive>
	</directivelist>
	</related>

	<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>

	</section>

	<section id="what"><title>What are SSI?</title>

	<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>
	</section>

	<section id="configuring">
	<title>Configuring your server to permit SSI</title>

	<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>
	<example>
	Options +Includes
	</example>

	<p>This tells Apache that you want to permit files to be parsed
	for SSI directives. Note that most configurations contain
	multiple <directive module="core">Options</directive> directives
	that can override each other. You will probably need to apply the
	<code>Options</code> to the specific directory where you want SSI
	enabled in order to assure that it gets evaluated last.</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>
	<example>
	AddType text/html .shtml<br />
	AddOutputFilter INCLUDES .shtml
	</example>

	<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 <directive
	module="mod_include">XBitHack</directive> directive:</p>
	<example>
	XBitHack on
	</example>

	<p><directive module="mod_include">XBitHack</directive>
	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>
	<example>
	chmod +x pagename.html
	</example>

	<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 <directive
	module="mod_include">XBitHack</directive>. 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
	<module>mod_expires</module> to set an explicit expiration
	time on your files, thereby letting browsers and proxies
	know that it is acceptable to cache them.</li>
	</ol>
	</section>

	<section id="basic"><title>Basic SSI directives</title>

	<p>SSI directives have the following syntax:</p>
	<example>
	<!--#element attribute=value attribute=value ... -->
	</example>

	<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>

	<section id="todaysdate"><title>Today's date</title>

	<example>
	<!--#echo var="DATE_LOCAL" -->
	</example>

	<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>

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

	<section id="lastmodified"><title>Modification date of the file</title>

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

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

	<section id="cgi"><title>Including the results of a CGI program</title>

	<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>

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

	</section>
	</section>

	<section id="additionalexamples">
	<title>Additional examples</title>

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

	<section id="docmodified"><title>When was this document
	modified?</title>

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

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

	<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>
	</section>

	<section id="standard-footer">
	<title>Including a standard footer</title>

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

	<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>
	</section>

	</section>

	<section id="config">
	<title>What else can I config?</title>

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

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

	<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>
	</section>

	<section id="exec">
	<title>Executing commands</title>

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

	<p>or, on Windows</p>
	<example>
	<pre><br />
	<!--#exec cmd="dir" --><br />
	</pre>
	</example>

	<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>
	</section>

	<section id="advanced">
	<title>Advanced SSI techniques</title>

	<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>

	<section id="caveat"><title>Caveat</title>

	<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>
	</section>

	<section id="variables"><title>Setting variables</title>

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

	<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>

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

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

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

	<section id="conditional">
	<title>Conditional expressions</title>

	<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.
	<module>mod_include</module> 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>
	<example>
	<!--#if expr="test_condition" --><br />
	<!--#elif expr="test_condition" --><br />
	<!--#else --><br />
	<!--#endif -->
	</example>

	<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 <module>mod_include</module>
	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>
	<example>
	BrowserMatchNoCase macintosh Mac<br />
	BrowserMatchNoCase MSIE InternetExplorer
	</example>

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

	<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>
	</section>
	</section>

	<section id="conclusion"><title>Conclusion</title>

	<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>
	</section>

	</manualpage>