13.5 Conditional Statements | Webmaster in a Nutshell, Third Edition


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	Webmaster in a Nutshell, 3rd Edition By Robert Eckstein, Stephen Spainhour
	Table of Contents

	Chapter 13. Server Side Includes

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
expr1expr2	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\" != " -->


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