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Using SSI to include the contents of an external file

The most common usage of SSI is to include the contents of an external file onto a page or across multiple pages on your site. Modify the external file, and all pages that have this file embedded is also updated with the modified information. For a site that uses the same header, navigational menu, or footer across pages, for example, this can save you countless time and energy. The syntax to embed the contents of an external file onto the current page is: 

<!--#include file="external.htm"-->
<!--#include virtual="/external.htm"-->

Which one to use depends on where "external.htm" is located. The first command assumes that the file is located in the same directory as the document containing it while the second syntax uses an absolute reference to "external.htm" starting from your root HTML directory. Typically you'll want to turn to the second syntax, as the external files to include most likely will be in a central directory while the pages including them are scattered across different directories. Here are a couple of more examples of the second syntax:

<!--#include virtual="/includes/navbar.txt"-->
<!--#include virtual="../navbar.txt"-->

With the first code, I'm telling the server to look for "navbar.txt" inside the "includes" directory that's directly beneath the root HTML directory (ie: http://www.mysite.com/includes), while in the second code, I'm simply telling it to look in the parent directory of the page that's including "navbar.txt"

As shown, you're also not limited to just including .htm files, but other static text files such as .txt. You cannot, however, include .cgi files using this syntax without additional configuration to your server. Just FYI, both the left menu and copyright footer on this page and across the site are dynamically included using SSI using two external files. To update the copyright notice for example, all I have to do is make one simple modification to one of these files, and the changes are reflected across the entire site.