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Dialplan variables

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Adapted from Asterisk variables -

Using Variables in Asterisk Dialplans

Variables are referenced in the dialplan (extensions.conf) using the syntax


where foo is the name of the variable, offset is an optional field indicating which characters should be excluded, and length is an optional field indicating the number of characters from the offset to be returned (see "Substrings" below for details and examples). A variable name may be any alphanumeric string beginning with a letter.

User-defined variable names are not case sensitive — ${FOO} and ${Foo} refer to the same variable — but Asterisk system variables are case-sensitive — ${EXTEN} works, but ${exten} doesn't.

There are four types of variables:

If you define a channel variable with the same name as a global variable (and remember: user-defined variable names are not case sensitive), references to that variable name will return the value of the channel variable. For example, let us say that you define a context "FooTest" with a single extension, 100, with the following definition:(:smile:)

   exten => 100,1,SetGlobalVar(FOO=5)
   exten => 100,2,NoOp(${FOO})
   exten => 100,3,NoOp(${foo})
   exten => 100,4,SetVar(foo=8)
   exten => 100,5,NoOp(${FOO})
   exten => 100,6,NoOp(${foo})

(Note the use of the NoOp command to assist in debugging.) If you dial extension 100 in context FooTest, and you have Asterisk running with a verbose console, you will see output similar to the following:

   — Executing SetGlobalVar("Zap/1-1", "FOO=5") in new stack
   — Setting global variable 'FOO' to '5'
   — Executing NoOp("Zap/1-1", "5") in new stack
   — Executing NoOp("Zap/1-1", "5") in new stack
   — Executing SetVar("Zap/1-1", "foo=8") in new stack
   — Executing NoOp("Zap/1-1", "8") in new stack
   — Executing NoOp("Zap/1-1", "8") in new stack

We discover that after the call to SetGlobalVar, ${FOO} and ${foo} returned the value of the global variable, giving the value 5. After the call to SetVar, the global variable "foo" was obscured by the channel variable "foo"; ${FOO} and ${foo} both gave the value 8. The value of the global variable remains unchanged at 5, however, and any other channels that refer to the global variable ${foo} would still get the value 5.

Inheritance of Channel Variables

Prepending a single _ character to a variables name in SetVar will cause that variable to be inherited by channels created by the main channel. eg. when using Dial(Local/...); once inherited these variables will not be further inherited. Prepending two _ characters will cause them to be inherited indefinitely.

Note:  for retrieval purposes these variable names do not include the underscores.

   exten => 100,1,SetVar(__FOO=5)
   exten => 100,2,Dial(Local/test@TestInherit)
   exten => test,1,NoOp(${FOO})

will result in FOO being inherited. Without the underscores, the new local channel would start with a clean slate.


   exten => 104,1,SetVar(FEE=fee)
   exten => 104,2,SetVar(_FIE=fie)
   exten => 104,3,SetVar(__FUM=fum)
   exten => 104,4,Dial(Local/105)

   exten => 105,1,NoOp(${FEE})
   exten => 105,2,NoOp(${FIE})
   exten => 105,3,NoOp(${FUM})
   exten => 105,4,Dial(Local/106)

   exten => 106,1,NoOp(${FEE})
   exten => 106,2,NoOp(${FIE})
   exten => 106,3,NoOp(${FUM})

results in

   — Executing SetVar("SIP/oberon-365e", "FEE=fee") in new stack
   — Executing SetVar("SIP/oberon-365e", "_FIE=fie") in new stack
   — Executing SetVar("SIP/oberon-365e", "__FUM=fum") in new stack
   — Executing Dial("SIP/oberon-365e", "Local/105") in new stack
   — Called 105
   — Executing NoOp("Local/105@default-7263,2", "") in new stack
   — Executing NoOp("Local/105@default-7263,2", "fie") in new stack
   — Executing NoOp("Local/105@default-7263,2", "fum") in new stack
   — Executing Dial("Local/105@default-7263,2", "Local/106") in new stack
   — Called 106
   — Executing NoOp("Local/106@default-49be,2", "") in new stack
   — Executing NoOp("Local/106@default-49be,2", "") in new stack
   — Executing NoOp("Local/106@default-49be,2", "fum") in new stack

(This did not work correctly prior to the 1.2 release.)

Using $

If you want to set a global variable containing the another variable name in the [globals] category of extensions.conf you have to do something like this:


This way the MY_VAR value is ${EPOCH}-${EXTEN}.gsm

Using it with the EVAL() function is very useful. I.e. if you want to record you can do this:

   exten => 104,1,SetVar(file=${EVAL(${MY_VAR})})
   exten => 104,2,MixMonitor($The attachment id given is not valid.)

Predefined Channel Variables

There are some channel variables set by Asterisk that you can refer to in your dialplan definitions. Asterisk-defined variables, in contrast to user-defined variables, are case sensitive. Note: Several of these builtin variables have been converted to functions in 1.2, to allow setting their values.
  (Note: this is not necessarily numeric as the name would indicate and can legitimately contain the space character. Commands acting on this variable (such as 'GotoIf', for example) should be aware of this).

Application-specific variables

Some applications take extra input or provide output using channel variables.

Macro-specific variables

When in a macro context, extra channel variables are available.

Environment Variables

You may access unix environment variables using the syntax:


String Handling Functions

String Length


returns the length of the string foo. For example,

   exten => 100,1,SetVar(Fruit=pear)
   exten => 100,2,NoOp(${LEN(Fruit)})
   exten => 100,3,NoOp(${LEN(${Fruit})})

The first NoOp would show a value of 5 (the length of the string "fruit"). The second NoOp would show a value of 4 (the length of the string "pear").

This is an excellent way to check for a NULL or empty string.



returns a substring of the string foo, beginning at offset offset and returning the next length characters. The first character is at offset 0.

   ${123456789:1}        - returns the string 23456789
   ${123456789:-4}       - returns the string 6789
   ${123456789:0:3}      - returns the string 123
   ${123456789:2:3}      - returns the string 345
   ${123456789:-4:3}     - returns the string 678

Examples of use:

   exten => _NXX.,1,SetVar(areacode=${EXTEN:0:3})   - get the first 3 digits of ${EXTEN}
   exten => _516XXXXXXX,1,Dial(${EXTEN:3})          - get all but the first 3 digits of ${EXTEN}
   exten => 100,1,SetVar(whichVowel=4)

   exten => 100,2,SetVar(foo=AEIOU:${whichVowel}:1) - sets ${foo} to the single letter 'U'

String Concatenation

To concatenate two strings, simply write them together:

Variable math

To perform math on variables e.g. increment, multiplication, addition simply write:

exten => s,1,SetVar(SOMEVAR=$[${SOMEVAR} + 1]) ; increment
exten => s,2,SetVar(SOMEVAR=$[2 * ${SOMEVAR}]) ; multiplication etc...
In times past, a single space was required between items in the $[...] expressions. This is no longer the case!

In late model Asterisks (1.2?), the MATH function is also available...

exten => s,1,Set(SOMEVAR=${MATH(${SOMEVAR}+1)}) ; increment
exten => s,2,Set(SOMEVAR=${MATH(2*${SOMEVAR})}) ; multiplication etc...

Version notes

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Last modified: March 12, 2019