Apache httpd Tutorial: Introduction to Server Side Includes

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+ +

Server-side includes provide a means to add dynamic content to +existing HTML documents.

Introduction
What are SSI?
Configuring your server to permit SSI
Basic SSI directives
Additional examples
What else can I config?
Executing commands
Advanced SSI techniques
Conclusion

Introduction

Related Modules	Related Directives
`mod_include` `mod_cgi` `mod_expires`	`Options` `XBitHack` `AddType` `SetOutputFilter` `BrowserMatchNoCase`

+ + +

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.

+ +

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:

+ +

+ Tuesday, 15-Jan-2013 19:28:54 EST +

+ +

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.

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 + .htaccess file:

Options +Includes

+ + +

This tells Apache that you want to permit files to be parsed + for SSI directives. Note that most configurations contain + multiple Options directives + that can override each other. You will probably need to apply the + 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:

AddType text/html .shtml
+AddOutputFilter INCLUDES .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 directive:

XBitHack on

+ + +

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.

+ 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 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.
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 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

+ +

Today's date

+ +

+  +

+ +

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.

+ +

+  + Today is  +

+ + +

Modification date of the file

+ +

+ This document last modified  +

+ +

This function 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.''

+ +

+  +

+ + +

Additional examples

+ + +

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 strftime. 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 + function 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 + 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.

Executing commands

+ + +

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.

+ <pre> +  + </pre> +

+ +

or, on Windows

+ <pre> +  + </pre> +

+ +

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.

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.

+ +

Setting variables

+ +

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.)

+  +

+ + +

Conditional expressions

+ + +

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 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 mod_include + documentation.

+ +

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:

+ +

+ Good +  + morning! +  + afternoon! +  +

+ +

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.

+ +

Conclusion

+ +

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.

Apache httpd Tutorial: Introduction to Server Side Includes

See also

Introduction

What are SSI?

Configuring your server to permit SSI

Basic SSI directives

Today's date

Modification date of the file

Including the results of a CGI program

Additional examples

When was this document +modified?

Including a standard footer

What else can I config?

Executing commands

Advanced SSI techniques

Setting variables

Conditional expressions

Conclusion

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