Asterisk: The Future of Telephony

Writing AGI Scripts in Perl

Asterisk comes with a sample AGI script called agi-test.agi. Let’s step through the file while we cover the core concepts of AGI programming. While this particular script is written in Perl, please remember that your own AGI programs may be written in almost any programming language. Just to prove it, we’re going to cover AGI programming in a couple of other languages later in the chapter.

Let’s get started! We’ll look at each section of the code in turn, and describe what it does.

    #!/usr/bin/perl

This line tells the system that this particular script is written in Perl, so it should use the Perl interpreter to execute the script. If you’ve done much Linux or Unix scripting, this line should be familiar to you. This line assumes, of course, that your Perl binary is located in the /usr/bin/ directory. Change this to match the location of your Perl interpreter.

    use strict;

use strict tells Perl to act, well, strict about possible programming errors, such as undeclared variables. While not absolutely necessary, enabling this will help you avoid common programming pitfalls.

    $|=1;

This line tells Perl not to buffer its output—in other words, that it should write any data immediately, instead of waiting for a block of data before outputting it. You’ll see this as a recurrent theme throughout the chapter.

Warning

You should always use unbuffered output when writing AGI scripts. Otherwise, your AGI may not work as expected, because Asterisk may be waiting for the output ...