So here is my solution:

First I write incoming command to a file called task:

mosquitto_sub -h SERVER -t '/TOPIC/' >> task

then I read the file, and execute the last command:

NUMOFLINES=$(wc -l < task)


while :
do

        NEWLINE=$(wc -l < task)

        if [ "$NEWLINE" -gt "$NUMOFLINES" ]
        then
                NUMOFLINES=$(wc -l < task)
                LASTCOMMAND=$(tail -n 1 task)
                eval ./$LASTCOMMAND
        fi

done

the Message of the topic equals the file name to execute.

This is working just fine for me

I would be happy to use a well posilded approach, but installing node red on an omega ( plus the then needed dock + an separate USB device) feels like a lot.

Especially because all I am trying to do is to set the relay extension from one state to an other.

And I am very surprised that nobody has created a "out of the box" solution with a small footprint for this kind of problem

Best wishes, and thanks for your support.

Thanks for your reply,

I have a node RED installation on a different device running.
All I am trying to do is to react to a message received by the MQTT Client

Message ON is received --> switch Relay to ON.
same thing for the OFF message.

Hi out there,

I have a very basic question but i am unable to find an answer that is helping me. Most likely I am thinking in a wrong direction, or messing something up.

But I am sure you are able to help me out.

I want to use the MQTT technology to react to events on the Onion Omega.

Everything is set up and seems to work just fine.

Mosquitto is installed, the broker is running on a different device and I can use this command:

mosquitto_sub -h MyServer -t 'omega2/'

to subscribe to the topic. If done so I see all the data that is published in my terminal connection. So far so good. I will send 2 messages to the device "on" and "off"

My question:
I want to react to a subscription (for example running a batch script)
How can I create the connection between "subscription" and execute script ?

In short:
Message ON received, start file A
Message OFF received, start file B

I hope this is not to confusing
Thanks to all