5.3.5. Using Variables

Variables can be used in an Asterisk dialplan to help reduce typing, add clarity, or add additional logic to a dialplan. If you have some computer programming experience, you probably already understand what a variable is. If not, don't worry; we'll explain what variables are and how they are used.

You can think of a variable as a container that can hold one value at a time. So, for example, we might create a variable called JOHN and assign it the value of Zap/1. This way, when we're writing our dialplan, we can refer to John's channel by name, instead of remembering that John is using the channel named Zap/1.

There are two ways to reference a variable. To reference the variable's name, simply type the name of the variable, such as JOHN. If, on the other hand, you want to reference its value, you must type a dollar sign, an opening curly brace, the name of the variable, and a closing curly brace. Here's how we'd reference the variable inside the Dial() application:

    exten => 555,1,Dial(${JOHN})

In our dialplan, whenever we write ${JOHN}, Asterisk will automatically replace it with whatever value has been assigned to the variable named JOHN.

	Tip
Note that variable names are case-sensitive. A variable named JOHN is different than a variable named John. For readability's sake, all the variable names in the examples will be written in upper case. You should also be aware that any variables set by Asterisk will be upper case as well. Some variables, such as CHANNEL or EXTEN are reserved by Asterisk. You should not attempt to set these variables

Tip

Note that variable names are case-sensitive. A variable named JOHN is different than a variable named John. For readability's sake, all the variable names in the examples will be written in upper case. You should also be aware that any variables set by Asterisk will be upper case as well. Some variables, such as CHANNEL or EXTEN are reserved by Asterisk. You should not attempt to set these variables

There are three types of variables we can use in our dialplan: global variables , channel variables , and environment variables. Let's take a moment to look at each type.

5.3.5.1. Global variables

As their name implies, global variables apply to all extensions in all contexts. Global variables are useful in that they can be used anywhere within a dialplan to increase readability and manageability. Suppose for a moment that you had a large dialplan and several hundred references to the Zap/1 channel. Now imagine you had to go through your dialplan and change all those references to Zap/2. It would be a long and error-prone process, to say the least.

On the other hand, if you had defined a global variable with the value Zap/1 at the beginning of your dialplan and then referenced that instead, you would only have to change one line.

Global variables should be declared in the [globals] context at the beginning of the extensions.conf file. They can also be defined programmatically, using the GLOBAL() dialplan function^[80]. Here is an example of how both methods look inside of a dialplan. The first shows setting a global variable named JOHN with a value of Zap/1. This variable is set at the time Asterisk parses the dialplan. The second example shows how a global varible can be set in the dialplan. In this case, the variable named George is being assigned the value of SIP/George when extension 124 is dialed in the [employees]context>.

    [globals]
    JOHN=Zap/1

    [employees]
    exten => 124,1,Set(GLOBAL(GEORGE)=SIP/George)

^[80]Don't worry! We'll cover dialplan functions in the next chapter.