Page 238 - 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 recurring theme throughout the chapter.
# Set up some variables
my %AGI; my $tests = 0; my $fail = 0; my $pass = 0;
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 of your program, while your program thinks
it has sent the output to Asterisk and is waiting for a response.

Here, we set up four variables. The first is a hash called AGI, which is used to store the
variables that Asterisk passes to our script at the beginning of the AGI session. The next
three are scalar values, used to count the total number of tests, the number of failed
tests, and the number of passed tests, respectively.
while(<STDIN>) {
chomp;
last unless length($_);
if (/^agi_(\w+)\:\s+(.*)$/) {
$AGI{$1} = $2;
}
}

210 | Chapter 9: The Asterisk Gateway Interface (AGI)