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Expressions and Variable Manipulation
As we begin our dive into the deeper aspects of dialplans, it is time to introduce you
to a few tools that will greatly add to the power you can exercise in your dialplan.
These constructs add incredible intelligence to your dialplan by enabling it to make
decisions based on different criteria you define. Put on your thinking cap, and let’s get
started.
Throughout this chapter we use best practices that have been
developed over the years in dialplan creation. The primary one
you’ll notice is that all the first priorities start with the NoOp()
application (which simply means No Operation; nothing functional
will happen). The other one is that all following lines will start with
same => n, which is a shortcut that says, “Use the same extension
as was just previously defined.” Additionally, the indentation is two
spaces.
Basic Expressions
Expressions are combinations of variables, operators, and values that you string
together to produce a result. An expression can test values, alter strings, or perform
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mathematical calculations. Let’s say we have a variable called COUNT. In plain English,
two expressions using that variable might be [COUNT plus 1], or [COUNT divided by 2].
Each of these expressions has a particular result or value, depending on the value of
the given variable.
In Asterisk, expressions always begin with a dollar sign and an opening square
bracket and end with a closing square bracket, as shown here:
$[expression]
Thus, we would write our two examples like this:
$[${COUNT} + 1]
$[${COUNT} / 2]
When Asterisk encounters an expression in a dialplan, it replaces the entire expres‐
sion with the resulting value. It is important to note that this takes place after variable
substitution. To demonstrate, let’s look at the following code:1
exten => 321,1,NoOp()
same => n,Answer()
same => n,Set(COUNT=3)
same => n,Set(NEWCOUNT=$[${COUNT} + 1])
same => n,SayNumber(${NEWCOUNT})
In the second priority, we assign the value of 3 to the variable named COUNT.
In the third priority, only one application—Set()—is involved, but three things
actually happen:
1. Asterisk substitutes ${COUNT} with the number 3 in the expression. The expres‐
sion effectively becomes this:
same => n,Set(NEWCOUNT=$[3 + 1])
2. Asterisk evaluates the expression, adding 1 to 3, and replaces it with its computed
value of 4:
same => n,Set(NEWCOUNT=4)
3. The Set() application assigns the value 4 to the NEWCOUNT variable.
The third priority simply invokes the SayNumber() application, which speaks the cur‐
rent value of the variable ${NEWCOUNT} (set to the value 4 in priority two).
Try it out in your own dialplan.
1 Remember that when you reference a variable you can call it by its name, but when you refer to a variable’s
value, you have to use the dollar sign and brackets around its name.
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Chapter 10: Deeper into the Dialplan
Operators
When you create an Asterisk dialplan, you’re really writing code in a specialized
scripting language. This means that the Asterisk dialplan—like any programming
language—recognizes symbols called operators that allow you to manipulate vari‐
ables. Let’s look at the types of operators that are available in Asterisk:
Boolean operators
These operators evaluate the “truth” of a statement. In computing terms, that
essentially refers to whether the statement is something or nothing (nonzero or
zero, true or false, on or off, and so on). The Boolean operators are:
expr1 | expr2
This operator (called the “or” operator, or “pipe”) returns the evaluation of
expr1 if it is true (neither an empty string nor zero). Otherwise, it returns
the evaluation of expr2.