apache/docs/manual/howto/ssi.xml

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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>
<highlight language="config">
        Options +Includes
</highlight>

    <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>
<highlight language="config">
        AddType text/html .shtml<br />
        AddOutputFilter INCLUDES .shtml
</highlight>

    <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>
<highlight language="config">
        XBitHack on
</highlight>

    <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="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>
<highlight language="config">
        BrowserMatchNoCase macintosh Mac<br />
        BrowserMatchNoCase MSIE InternetExplorer
</highlight>

    <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="-T reqenv('Mac') &amp;&amp;
                         -T reqenv('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>