Conditional Statements (Webmaster in a Nutshell, 3rd Edition)

13.5. Conditional Statements

Apache allows you to include only select portions of a server document using conditional statements. These conditional statements are based on the value of server-side variables initialized earlier using the SSI set command. The Apache flow-control statements allow you to effectively customize a document without adding more complex CGI programs to perform the same task.

There are four Apache flow-control statements:

<!--#if expr="expression" -->
<!--#elif expr="expression"-->
<!--#else-->
<!--#endif-->

Each works as you would expect from an ordinary scripting language. Note that each if must have a closing endif server-side statement. For example:

<!--#if expr="$myvar=activated" --> 
<B>The variable appears to be activated</B>
<!--#elif expr="$myvar=inactive"--> 
<B>The variable appears to be inactive</B>
<!--#else--> 
<B>The variable has an unknown value</B>
<!--#endif-->

Table 13-2 shows the allowed expressions, where the order of operations is as expected in a traditional programming language. Note that in some cases, var2 is allowed to be an egrep-based regular expression if it is surrounded by slashes (/) on both sides.

Table 13-2. XSSI conditional expressions

Expression	Meaning
var	True if the variable is not empty
var1=var2	True if the variables match
var1!=var2	True if the variables do not match
var1<var2	True if the first variable is less than the second
var1<=var2	True if the first variable is less than or equal to the second
var1>var2	True if the first variable is greater than the second
var1>=var2	True if the first variable is greater than or equal to the second
`(`expr`)`	True if the enclosed condition is true
`!`expr	True if the condition is false
expr1&&expr2	True if both expressions evaluate to true
expr1\|\|expr2	True if either expressions evaluates to true

Finally, you can place regular strings inside single quotes to preserve any whitespaces. If a string is not quoted, extra whitespaces are ignored. For example:

this   is   too  much    space

This string does not have quotes and will be collapsed to:

this is too much space

However, if you place the string in single quotes, the whitespace is preserved:

/'this   is   too  much    space/'

You can also place strings in double quotes, but you will have to escape each one while inside the expr="" expression, as shown here:

<!--#if expr="\" $HTTP_REFERER\" != " -->