Asterisk: The Future of Telephony, 2nd Edition

Chapter 9. The Asterisk Gateway Interface (AGI)

Even he, to whom most things that most people would think were pretty smart were pretty dumb, thought it was pretty smart.

—Douglas Adams, The Salmon of Doubt

The Asterisk Gateway Interface, or AGI, provides a standard interface by which external programs may control the Asterisk dialplan. Usually, AGI scripts are used to do advanced logic, communicate with relational databases (such as PostgreSQL or MySQL), and access other external resources. Turning over control of the dialplan to an external AGI script enables Asterisk to easily perform tasks that would otherwise be difficult or impossible.

This chapter covers the fundamentals of AGI communication. It will not teach you how to be a programmer—rather, we’ll assume that you’re already a competent programmer, so that we can show you how to write AGI programs. If you don’t know how to do computer programming, this chapter probably isn’t for you, and you should skip ahead to the next chapter.

Over the course of this chapter, we’ll write a sample AGI program in each of the Perl, PHP, and Python programming languages. Note, however, that because Asterisk provides a standard interface for AGI scripts, these scripts can be written in almost any modern programming language. We’ve chosen to highlight Perl, PHP, and Python because they’re the languages most commonly used for AGI programming.

Fundamentals of AGI Communication

Instead of releasing an API for programming, AGI scripts communicate with Asterisk over communications channels (file handles, in programming parlance) known as STDIN, STDOUT, and STDERR. Most computer programmers will recognize these channels, but just in case you’re not familiar with them, we’ll cover them here.

What Are STDIN, STDOUT, and STDERR?

STDIN , STDOUT , and STDERR are channels by which programs in Unix-like environments receive information from and send information to external programs. STDIN, or standard input, is the information that is sent to the program, either from the keyboard or from another program. In our case, information coming from Asterisk itself comes in on the program’s STDIN file handle. STDOUT, or standard output, is the file handle that the AGI script uses to pass information back to Asterisk. Finally, the AGI script can use the STDERR (standard error) file handle to write error messages to the Asterisk console.

Let’s sum up these three communications concepts:

• An AGI script reads from STDIN to get information from Asterisk.

• An AGI script writes data to STDOUT to send information to Asterisk.

• An AGI script may write data to STDERR to send debug information to the Asterisk console.

Warning

At this time, writing to STDERR from within your AGI script writes the information only to the first Asterisk console—that is, the first Asterisk console started with the -c parameters.

This is rather unfortunate, and will hopefully be remedied soon by the Asterisk developers.

If you’re using the safe_asterisk program to start Asterisk (which you probably are), it starts a remote console on TTY9. (Try pressing Ctrl-Alt-F9, and see if you get an Asterisk command-line interface.) This means that all of the AGI debug information will print on only that remote console. You may want to disable this console in safe_asterisk to allow you to see the debug information in another console. (You may also want to disable that console for security reasons, as you might not want just anyone to be able to walk up to your Asterisk server and have access to a console without any kind of authentication.)

The Standard Pattern of AGI Communication

The communication between Asterisk and an AGI script follows a predefined pattern. Let’s enumerate the steps, and then we’ll walk through one of the sample AGI scripts that come with Asterisk.

When an AGI script starts, Asterisk sends a list of variables and their values to the AGI script. The variables might look something like this:

agi_request: test.py
agi_channel: Zap/1-1
agi_language: en
agi_callerid:
agi_context: default
agi_extension: 123
agi_priority: 2

After sending these variables, Asterisk sends a blank line. This is the signal that Asterisk is done sending the variables, and it is time for the AGI script to control the dialplan.

At this point, the AGI script sends commands to Asterisk by writing to STDOUT. After the script sends each command, Asterisk sends a response that the AGI script should read. These actions (sending commands to Asterisk and reading the responses) can continue for the duration of the AGI script.

You may be asking yourself what commands you can use from within your AGI script. Good question—we’ll cover the basic commands shortly.^[119]

Calling an AGI Script from the Dialplan

In order to work properly, your AGI script must be executable. To use an AGI script inside your dialplan, simply call the AGI() application, with the name of the AGI script as the argument, like this:

exten => 123,1,Answer()
exten => 123,2,AGI(agi-test.agi)

AGI scripts often reside in the AGI directory (usually located in /var/lib/asterisk/agi-bin), but you can specify the complete path to the AGI script.

AGI(), EAGI(), DeadAGI(), and FastAGI()

In addition to the AGI() application, there are several other AGI applications suited to different circumstances. While they won’t be covered in this chapter, they should be quite simple to figure out once you understand the basics of AGI scripting.

The EAGI() (enhanced AGI) application acts just like AGI() but allows your AGI script to read the inbound audio stream on file descriptor number three.

The DeadAGI() application is also just like AGI(), but it works correctly on a channel that is dead (i.e., a channel that has been hung up). As this implies, the regular AGI() application doesn’t work on dead channels.

The FastAGI() application allows the AGI script to be called across the network, so that multiple Asterisk servers can call AGI scripts from a central location.

In this chapter, we’ll first cover the sample agi-test.agi script that comes with Asterisk (which was written in Perl), then write a weather report AGI program in PHP, and finish up by writing an AGI program in Python to play a math game.

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^[119] To get a list of available AGI commands, type show agi at the Asterisk command-line interface. You can also refer to Appendix C for an AGI command reference.