Callback Function

Callback Function

[BrainGeek]

With ALFRED I've noticed a parameter you can pass into it labeled "-c" which is described to be a callback. I've tried things like "-c poweroff" just to make it obvious that the callback worked, but to no avail. How can I properly use this feature and how does it work?

I'm currently running ALFRED and batman-adv on my Raspberry Pi 4. All the software is up-to-date.

Re: Callback Function

[Sven Eckelmann]

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On Wednesday, 28 April 2021 05:05:47 CEST BrainGeek wrote:
> With ALFRED I've noticed a parameter you can pass into it labeled "-c" which is described to be a callback. I've tried things like "-c poweroff" just to make it obvious that the callback worked, but to no avail. How can I properly use this feature and how does it work?

Please have a look at the manpage [1].

    Specify command to execute on data change. It will be called
    with data-type list as arguments.

As you can see, the called command will have additional values added to the 
end. So you have to keep in mind that some commands don't like it when they 
receive extra parameters at the end. Let us simulate this with poweroff:

    $ poweroff 1
    Too many arguments.

So this will not work. You have to add some kind of wrapper which evaluates 
the numbers and then calls poweroff at the appropriate time. Let us define 
a small test script on host 1.

    host1$ cat > /tmp/alfred_cmd.sh << "EOF"
    #! /bin/sh
    
    echo "$(date): $@" >> /tmp/alfred_cmd_log
    
    for i in "$@"; do
       if [ "$i" = "123" ]; then
          poweroff
       fi
    done
    EOF
    
    host1$ chmod +x /tmp/alfred_cmd.sh

And now start the alfred instances on both hosts:

    host1$  alfred -m -i bat0 -c /tmp/alfred_cmd.sh
    host2$  alfred -m -i bat0


You can now inject data on host2

    host2$ echo foobar | alfred -s 65

and see the changed data-types on host1

   host1$ touch /tmp/alfred_cmd_log
   host1$ tail -f /tmp/alfred_cmd_log


If you write something to data-type 123 and you alfred instance is running 
with high enough privileges then it might shut down the system.

Kind regards,
	Sven


[1] https://downloads.open-mesh.org/batman/manpages/alfred.8.html

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