Server Side Includes

Server Side Includes (let's use the term SSI from now on) are used in displaying pages served by web servers on the World Wide Web. They are relatively non-complex directives that can be inserted into specially marked HTML pages. SSI directives allow you to retrieve, act upon and display environment variables and include external files in your HTML pages when displayed in a browser that can be viewed without the need for any special programs or plug ins on the web page viewers side.

Before going further, please note that SSI only issues directives; not commands. SSI is like a macro, script or batch file - it only causes (directs) other actions to happen. I know, this is nit picking and you could even argue that SSI 'commands' other commands, but I think you can see where I'm going with this. I just felt this needed mention since some people have an unwarranted fear when the word 'command' is used. Plus, during this tutorial, I'll be using 'directive' when referring to an SSI function.

Most of you reading this tutorial are probably already aware of what SSI means, but for those who don't, the concept is really quite simple and the name Server Side Includes itself pretty much describes what SSI does.
Server Side - this is where all the actions take place, on the web server side of a web page connection.
Includes - after the server side directive completes, the web server includes the output of that action or the results of the program specified by the action into an HTML document and sends the final results to the requesting browser.

With me so far? Even if it's not completely clear, don't worry too much. After all, you don't have to understand how a CPU works in order to use a PC. Let's go on and see what SSI can do.

What Can SSI Do?
If you've ever seen a counter on someone's page that says "You are visitor #1000" or a personalized greeting that says "Welcome, visitor from yoursite.com", you have seen server-side-includes at work. They can be very useful for a number of different things, and if you've found yourself wanting to learn more about how to do "cool things" on your web pages, this tutorial might be what you're looking for.

Listed below are the basic SSI directives that almost all SSI capable web servers provide and they will probably satisfy the SSI needs of most people. They are as follows:
exec - directs scripts, programs or even a system command to execute.
config - configures the format of the SSI error-message, date/time and file size output strings.
echo - inserts the value returned by an SSI directive into the web page
include - inserts the content of a specified file into the web page
flastmod - inserts the date/time stamp of when a specified file was last updated
fsize - inserts the file size of a specified file into the web page

There are also eXtended SSI (XSSI) directives available on some servers which include the ability to perform operations based on conditions and logic, but the availability of each directive, the specific parameters/arguments and the syntax vary from server to server software, so I'll leave covering those for another tutorial or a future update. If you need information about XSSI, the best source of information would probably be the server software documentation.

I tried to keep this page brief because I do go much deeper into each directive later on in this tutorial, complete with syntax, the parameters and arguments that can be passed to each directive, example usage and some examples of results from various directives, but, hopefully, this is enough to whet your appetite enough to read on.
For a step by step summary of what happens during an SSI session, read the How It Works page.