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This is the Title of the Book, eMatter Edition

156

Chapter 9

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 pro-

grammer, 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 pro-

vides 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 Aster-

isk over communications channels (file pointers, in programming parlance) known

STDIN, STDOUT, and STDERR. Most computer programmers will recognize these chan-

nels, but just in case you’re not familiar with them we’ll cover them here.

,ch09.22228 Page 156 Wednesday, August 31, 2005 4:58 PM

This is the Title of the Book, eMatter Edition

Fundamentals of AGI Communication

157

What Are STDIN, STDOUT, and STDERR?

STDIN, STDOUT, and STDERR are channels by which programs in Unix-like environ-

ments 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 key-

board 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.

At this time, writing to STDERR from within your AGI script writes the

information only to the first Asterisk console—that is, the first Aster-

isk console started with the

-c or -r 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 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 pat-

tern. 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

,ch09.22228 Page 157 Wednesday, August 31, 2005 4:58 PM

This is the Title of the Book, eMatter Edition

158

Chapter 9: The Asterisk Gateway Interface (AGI)

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. This action (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.

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.

In this chapter, we’ll first cover the sample agi-test.agi script that comes with Aster-

isk (which was written in Perl), then write a weather report AGI program in PHP,

and then finish up by writing an AGI program in Python to play a math game.

* 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.

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.

,ch09.22228 Page 158 Wednesday, August 31, 2005 4:58 PM

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liqing0604

2012-10-31

很好的资源，asterisk的AGI，很详细，用于扩展asterisk的功能。
kingroa

2011-09-27

这不就是电话未来之路第九章的内容么
bingxin1111

2012-09-24

真是浪费我的积分，没想到这只是这书中的一章而已，早就看过了，浪费我的时间

ainisfen

粉丝: 2
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