Conditional Expressions

SSI has the ability to include text based upon conditions. You can use this for a large number of applications:

Provide different format pages for different browsers.
Exclude parts of the page based upon spider (agent) type.

The format of the SSI "if" directive is as follows:

<!==#if "<op1>" operator "op2"> operation to perform -->

where:

<op1> is the first operand of the logical comparison

<op2> is the second operand

Note: <op1> and <op2> must be surrounded with && characters if they are reserved words. For example, &&HTTP_USER_AGENT&& is equated to the user agent (spider or browser name) while HTTP_USER_AGENT is simply the string "HTTP_USER_AGENT".

<operator> is the method of comparison (see table below)

operation to perform is what to do if the comparison is TRUE.

**Operators for Conditional Expressions**
Operator	Name	Result
==	equalto	TRUE if op1 is equal to op2
!=	notequalto	TRUE if op1 is not not equal to op2
<	lessthan	TRUE if op1 is less than op2
>	greaterthan	TRUE if op1 is greater than op2
!<	notlessthan	TRUE if op1 is not less than op2
!>	notgreaterthan	TRUE if op1 is not greater than op2
	hasstring	TRUE if text string op2 is contained in text string op1

There are a number of actions that you can take if the expression is true.

goto <label> where <label> is a string defined by the "label" directive. This is create for skipping around conditional information.

print <text> causes the text to he printed. This may be any text or HTML tag.

error causes the config.error text to be displayed.

break stops transmitting the HTML back to the client. In other words, it ends the page.

errorbreak causes the error and break directives to be executed.

printbreak causes print and break directives to be executed.

The "label" directive is closely related and has the following format.

 where <label> is the name of the label. Used as target for "goto".

Here are some examples of conditional SSI directives.

<p>Begin normal HTML document</p>

The above example uses SSI to modify the HTML code if the spider called EmailSiphon is identified as the user agent reading the page. In this case, a file called "poisonpill.htm" is included. Otherwise, the page is displayed normally (without the reference to poisonpill).

This example is much simpler. Find the "bad" referrer, simple stop processing.