Helpful hints when programming Asterisk telephony apps

I do not pretend that these are in any particular order, but given Asterisk's many features, I find this kind of aide memoire helpful.

• Where the heck do you 'program' behavior into this beast?
Short answer: /etc/asterisk/extensions.conf is what you are looking for in terms of so-called 'extension' programmability.
What does that mean? here is the quick and sweet:
You list a bunch of 'extensions' under an 'extension context' like this:


      [mycontext]
        exten => s,1,dothis
        exten => s,2,dothat
        exten => s,3,dotheother

The terminology is: 'extension context' mycontent has an extension 's' that has priority 1, 2, and 3; each priority does something, typically conducted in extension 'cmds' as listed on wiki.
It's kind of primitive, because there are crazy little things like conditionals that either branch to n+1 or n+101 on some condition. It's sort of macro assembly-language-ish (the program counter bit, I guess...)
Some 'extensions' are magical and used in specific places. The 's' extension is what seems to work to 'Start' a given extension context. The 't' extension is used for Timeout handling after Dial and other event-after-request dispatch.

• Why won't changes I made in extensions.conf affect behavior of system?

If you alter the extensions.conf file, the Asterisk server does not seem to automatically read the file. You can issue a command to the console of the Asterisk server to force the extensions in extensions.conf (and others??) to reload. This command inside Asterisk console is:

  extensions reload

• Does Asterisk support some kind of variable in the extension script? see 'Asterisk variables' on wiki
• Why did Asterisk hang up? see HANGUPCAUSE variable on wiki
• How do I get some info on what happened to a dial attempt? see DIALSTATUS variable on wiki I don't know enough about this, or Asterisk could use better support in the call-status-monitoring department....in fact, DIALSTATUS variable is in a fairly recent CVS HEAD so you have to be recent to get the feature....
• How do I execute an arbitrary command from extension context script? see 'System command' on wiki
• How do I "dial" an extension (or otherwise initiate a connection)? see 'Dial' command on wiki
• How do I Read DTMF data from the current call? see 'Read' command on wiki
• How do I record audio data during a connection? see 'Record' command on wiki
• How do I unconditionally branch in extension script? see 'Goto' on wiki
• How do I conditionally branch in extension script logic? see 'Gotoif' command on wiki
• How can I dump some debug info somewhere while my extension code is running? see 'NoOp' cmd on wiki