Server-side includes provide a means to add dynamic content to existing HTML documents.
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.
For example, you might place a directive into an existing HTML page, such as:
And, when the page is served, this fragment will be evaluated and replaced with its value:
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 - shown above. 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.
To permit SSI on your server, you must have the following
directive either in your httpd.conf
file, or in a
.htaccess
file:
This tells Apache that you want to permit files to be parsed
for SSI directives. Note that most configurations contain
multiple Options
to the specific directory where you want SSI
enabled in order to assure that it gets evaluated last.
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:
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
chmod
.
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
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:
XBitHack Full
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.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 function 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
The echo
function 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 set
function.
If you don't like the format in which the date gets printed,
you can use the config
function, with a
timefmt
attribute, to modify that formatting.
This function is also subject to timefmt
format
configurations.
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.
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.
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:
For more details on the timefmt
format, go to
your favorite search site and look for strftime
. The
syntax is the same.
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
If you want to change that message to something else, you
can do so with the errmsg
attribute to the
config
function:
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.
Here's something else that you can do with the exec
function. 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 ``<dir
>'' 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 Options
directive.
In addition to spitting out content, Apache SSI gives you the option of setting variables, and using those variables in comparisons and conditionals.
Using the 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 environment variables or the variables
discussed above (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.
if
,
elif
, else
, 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
For example, if you wish to customize the text on your web page based on the time of day, you could use the following recipe, placed in the HTML page:
Any other variable (either ones that you define, or normal environment variables) can be used in conditional statements. See Expressions in Apache HTTP Server for more information on the expression evaluation engine.
With Apache's ability to set environment variables with the
SetEnvIf
directives, and other related directives,
this functionality can let you do a wide variety of dynamic content
on the server side without resorting a full web application.
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.