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Now that you've learned a bit about variables, let's look at a few of the variables that Asterisk automatically creates.

Asterisk creates channel variables named CONTEXT, EXTEN, and PRIORITY which contain the current context, extension, and priority. We'll use them in pattern matching (below), as well as when we talk about macros in Section 308.10. Macros. Until then, let's show a trivial example of using ${EXTEN} to read back the current extension number.


If you were to add this extension to the [users] context of your dialplan and reload the dialplan, you could call extension 6123 and hear Asterisk read back the extension number to you.

Another channel variable that Asterisk automatically creates is the UNIQUEID variable. Each channel within Asterisk receives a unique identifier, and that identifier is stored in the UNIQUEID variable. The UNIQUEID is in the form of 1267568856.11, where 1267568856 is the Unix epoch, and 11 shows that this is the eleventh call on the Asterisk system since it was last restarted.

Last but not least, we should mention the CHANNEL variable. In addition to a unique identifier, each channel is also given a channel name and that channel name is set in the CHANNEL variable. A SIP call, for example, might have a channel name that looks like SIP/george-0000003b, for example.

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