Page 236 - Asterisk™: The Future of Telephony

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.

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

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