Apache Tutorial: Introduction to Server Side Includes
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.
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.
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.
What are SSI?
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.
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.
Configuring your server to permit SSI
To permit SSI on your server, you must have the following directive either in your
httpd.conf file, or in a
This tells Apache that you want to permit files to be parsed for SSI directives.
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
.shtml, with the following directives:
AddType text/html .shtml AddHandler server-parsed .shtml
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
.shtml extension, so that those directives would be executed.
The other method is to use the
XBitHack 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
chmod +x pagename.html
A brief comment about what not to do. You’ll occasionally see people recommending that you just tell Apache to parse all
.html files for SSI, so that you don’t have to mess with
.shtml file names. These folks have perhaps not heard about
XBitHack. 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.
Of course, on Windows, there is no such thing as an execute bit to set, so that limits your options a little.
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:
- Use the
XBitHack Fullconfiguration. 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.
- Use the directives provided by mod_expires to set an explicit expiration time on your files, thereby letting browsers and proxies know that it is acceptable to cache them.
Basic SSI directives
SSI directives have the following syntax:
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.
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
echo 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
If you don’t like the format in which the date gets printed, you can use the
config element, with a
timefmt attribute, to modify that formatting.
Modification date of the file
This document last modified
This element is also subject to
timefmt format configurations.
Including the results of a CGI program
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.”
Following are some specific examples of things you can do in your HTML documents with SSI.
When was this document modified?
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.
This file last modified
Of course, you will need to replace the
ssi.shtml 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
LAST_MODIFIED variable instead:
This file last modified
For more details on the
timefmt format, go to your favorite search site and look for
ctime. The syntax is the same.
Including a standard footer
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.
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
include SSI command. The
include element can determine what file to include with either the
file attribute, or the
virtual attribute. The
file attribute is a file path, relative to the current directory. That means that it cannot be an absolute file path (starting with /), nor can it contain ../ as part of that path. The
virtual 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.
I’ll frequently combine the last two things, putting a
LAST_MODIFIED 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.
What else can I config?
In addition to being able to
config the time format, you can also
config two other things.
Usually, when something goes wrong with your SSI directive, you get the message
[an error occurred while processing this directive]
If you want to change that message to something else, you can do so with the
errmsg attribute to the
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?)
And you can
config the format in which file sizes are returned with the
sizefmt attribute. You can specify
bytes for a full count in bytes, or
abbrev for an abbreviated number in Kb or Mb, as appropriate.
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
exec element. You can actually have SSI execute a command using the shell (
/bin/sh, to be precise – or the DOS shell, if you’re on Win32). The following, for example, will give you a directory listing.
or, on Windows
You might notice some strange formatting with this directive on Windows, because the output from
dir contains the string “” in it, which confuses browsers.
Note that this feature is exceedingly dangerous, as it will execute whatever code happens to be embedded in the
exec 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
exec feature, with the
IncludesNOEXEC argument to the
Advanced SSI techniques
In addition to spitting out content, Apache SSI gives you the option of setting variables, and using those variables in comparisons and conditionals.
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.
set 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:
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
LAST_MODIFIED, 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.
To put a literal dollar sign into the value of your variable, you need to escape the dollar sign with a backslash.
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.)
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.
mod_include provides an
endif structure for building conditional statements. This allows you to effectively generate multiple logical pages out of one actual page.
The structure of this conditional construct is:
A test_condition 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
mod_include documentation. Here are some examples of how one might use this construct.
In your configuration file, you could put the following line:
BrowserMatchNoCase macintosh Mac BrowserMatchNoCase MSIE InternetExplorer
This will set environment variables “Mac” and “InternetExplorer” to true, if the client is running Internet Explorer on a Macintosh.
Then, in your SSI-enabled document, you might do the following:
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
SetEnvIf directives, and other related directives, this functionality can let you do some pretty involved dynamic stuff without ever resorting to CGI.
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.