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Subject cvs commit: httpd-docs-1.3/htdocs/howto SSI-HOWTO.html
Date Sun, 05 Nov 2000 20:00:26 GMT
rbowen      00/11/05 12:00:26

  Added:       htdocs/howto SSI-HOWTO.html
  Adding SSI howto tutorial as the first one in the howto section.
  Revision  Changes    Path
  1.1                  httpd-docs-1.3/htdocs/howto/SSI-HOWTO.html
  Index: SSI-HOWTO.html
  <TITLE>Apache Tutorial: Introduction to Server Side Includes</TITLE>
  <LINK REV="made" HREF="">
  <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
  <!--#include virtual="header.html" -->
  <H1 ALIGN="CENTER">Apache Tutorial: Introduction to Server Side Includes</H1>
  <A NAME="__index__"></A>
  <!-- INDEX BEGIN -->
  	<LI><A HREF="#apache tutorial: introduction to server side includes">Apache
Tutorial: Introduction to Server Side Includes</A></LI>
  	<LI><A HREF="#related modules">Related modules</A></LI>
  	<LI><A HREF="#related directives">Related directives</A></LI>
  	<LI><A HREF="#what are ssi">What are SSI?</A></LI>
  	<LI><A HREF="#configuring your server to permit ssi">Configuring your server
to permit SSI</A></LI>
  	<LI><A HREF="#basic ssi directives">Basic SSI directives</A></LI>
  		<LI><A HREF="#today's date">Today's date</A></LI>
  		<LI><A HREF="#modification date of the file">Modification date of the file</A></LI>
  		<LI><A HREF="#including the results of a cgi program">Including the results
of a CGI program</A></LI>
  	<LI><A HREF="#additional examples">Additional examples</A></LI>
  	<LI><A HREF="#when was this document modified">When was this document modified?</A></LI>
  	<LI><A HREF="#including a standard footer">Including a standard footer</A></LI>
  	<LI><A HREF="#what else can i config">What else can I config?</A></LI>
  	<LI><A HREF="#executing commands">Executing commands</A></LI>
  	<LI><A HREF="#advanced ssi techniques">Advanced SSI techniques</A></LI>
  	<LI><A HREF="#caveat">Caveat</A></LI>
  	<LI><A HREF="#setting variables">Setting variables</A></LI>
  	<LI><A HREF="#conditional expressions">Conditional expressions</A></LI>
  	<LI><A HREF="#conclusion">Conclusion</A></LI>
  <!-- INDEX END -->
  <H1><A NAME="apache tutorial: introduction to server side includes">Apache Tutorial:
Introduction to Server Side Includes</A></H1>
  <P>This HOWTO first appeared in Apache Today (
  as a series of three articles. They appear here by arrangement with
  ApacheToday and</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 article, we'll talk about some of the somewhat
  more advanced things can be done with SSI, such as conditional statements
  in your SSI directives.</P>
  <H1><A NAME="related modules">Related modules</A></H1>
  <a href="../mod/mod_include.html">mod_include</a><p>
  <a href="../mod/mod_cgi.html">mod_cgi</a><p><P>
  <H1><A NAME="related directives">Related directives</A></H1>
  <a href="../mod/core.html#options">Options</a><p>
  <a href="../mod/mod_include.html#xbithack">XBitHack</a><p>
  <a href="../mod/mod_mime.html#addtype">AddType</a><p>
  <a href="../mod/mod_mime.html#addhandler">AddHandler</a><p>
  <a href="../mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><p><P>
  <H1><A NAME="what are ssi">What are SSI?</A></H1>
  <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
  <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>
  <H1><A NAME="configuring your server to permit ssi">Configuring your server
to permit SSI</A></H1>
  <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>
          Options +Includes</PRE>
  <P>This tells Apache that you want to permit files to be parsed for SSI 
  <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>,
  the following directives:</P>
          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>
          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>
          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
  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>
  <H1><A NAME="basic ssi directives">Basic SSI directives</A></H1>
  <P>SSI directives have the following syntax:</P>
          &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
  the results of the directive.</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>
  <H2><A NAME="today's date">Today's date</A></H2>
          &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>
          &lt;!--#config timefmt=&quot;%A %B %d, %Y&quot; --&gt;
          Today is &lt;!--#echo var=DATE_LOCAL --&gt;</PRE>
  <H2><A NAME="modification date of the file">Modification date of the file</A></H2>
          This document last modified &lt;!--#flastmod file=&quot;index.html&quot;
  <P>This element is also subject to <CODE>timefmt</CODE> format configurations.</P>
  <H2><A NAME="including the results of a cgi program">Including the results of
a CGI program</A></H2>
  <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>
          &lt;!--#include virtual=&quot;/cgi-bin/; --&gt;</PRE>
  <H1><A NAME="additional examples">Additional examples</A></H1>
  <P>Following are some specific examples of things you can do in your HTML
  documents with SSI.</P>
  <H1><A NAME="when was this document modified">When was this document modified?</A></H1>
  <P>In the last article, we mentioned that you could use SSI to inform the user when
  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 in the last article.</P>
          &lt;!--#config timefmt=&quot;%A %B %d, %Y&quot; --&gt;
          This file last modified &lt;!--#flastmod file=&quot;ssi.shtml&quot;
  <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>
          &lt;!--#config timefmt=&quot;%D&quot; --&gt;
          This file last modified &lt;!--#echo var=&quot;LAST_MODIFIED&quot; --&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>
  <H1><A NAME="including a standard footer">Including a standard footer</A></H1>
  <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>vitrual</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>
          &lt;!--#include virtual=&quot;/footer.html&quot; --&gt;</PRE>
  <P>I'll frequently combine the last two things, putting a <CODE>LAST_MODIFIED</CODE>
  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>
  <H1><A NAME="what else can i config">What else can I config?</A></H1>
  <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>
          [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>
          &lt;!--#config errmsg=&quot;[It appears that you don't know how to use SSI]&quot;
  <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>
  <H1><A NAME="executing commands">Executing commands</A></H1>
  <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
  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>
          &lt;!--#exec cmd=&quot;ls&quot; --&gt;
  <P>or, on Windows</P>
          &lt;!--#exec cmd=&quot;dir&quot; --&gt;
  <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>
  <H1><A NAME="advanced ssi techniques">Advanced SSI techniques</A></H1>
  <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>
  <H1><A NAME="caveat">Caveat</A></H1>
  <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>
  <H1><A NAME="setting variables">Setting variables</A></H1>
  <P>Using the <CODE>set</CODE> attribute, you can set variables for later
use. We'll need this
  later in the discussion, so we'll talk about this here. The syntax of this is
  as follows:</P>
          &lt;!--#set var=&quot;name&quot; value=&quot;Rich&quot; --&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 ($) before the name of 
  the variable.</P>
          &lt;!--#set var=&quot;modified&quot; value=&quot;$LAST_MODIFIED&quot;
  <P>To put a literal dollar sign into the value of your variable, you need to escape
  the dollar sign with a backslash.</P>
          &lt;!--#set var=&quot;cost&quot; value=&quot;\$100&quot; --&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>
          &lt;!--#set var=&quot;date&quot; value=&quot;${DATE_LOCAL}_${DATE_GMT}&quot;
  <H1><A NAME="conditional expressions">Conditional expressions</A></H1>
  <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 languages
  of sorts. <CODE>mod_include</CODE> provides an <CODE>if</CODE>,
<CODE>elif</CODE>, <CODE>else</CODE>, <CODE>endif</CODE>
  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>
          &lt;!--#if expr=&quot;test_condition&quot; --&gt;
      &lt;!--#elif expr=&quot;test_condition&quot; --&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>
          BrowserMatchNoCase macintosh Mac
          BrowserMatchNoCase MSIE InternetExplorer</PRE>
  <P>This will set environment variables ``Mac'' and ``InternetExplorer'' to true, if
  client is running Internet Explorer on a Macintosh.</P>
  <P>Then, in your SSI-enabled document, you might do the following:</P>
          &lt;!--#if expr=&quot;${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
  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 enviroment 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,
  functionality can let you do some pretty involved dynamic stuff without ever 
  resorting to CGI.</P>
  <H1><A NAME="conclusion">Conclusion</A></H1>
  <P>SSI is certainly not a replacement for CGI, or other technologies used for
  generating dynamic web pages. But it is certainly a great way to add small
  amounts of dynamic content to pages, without doing a lot of extra work.</P>

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