mosquitto_sub and grep to trigger command/action

Question

I can use mosquitto_sub to subscribe to broker for a specific topic, say test_topic,

mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic

I can also pipe grep for a specific keyword, say valve_closed,

mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_closed"

This would only output a newline with "valve_closed", every time that particular topic broadcasts "valve closed" message.

Input,

Line 1: mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_open"

Line 2: mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_closed"

Output,

Line 1: valve_closed

---

Now, my question is, how can I pipe a command such that if grep "valve_closed" is true, then execute another command.

I've tried chaining a command like echo "Okay so now Valve is Closed" with another | pipe command,

mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_closed" | echo "Okay, so now Valve is Closed"

But this simply runs the echo command with output "Okay, so now Valve is Closed", even when I group grep+echo within parentheses.

I also tried using AND operator &&,

mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_closed" && echo "Okay, so now Valve is Closed"

While this waits for grep to match string valve_closed as soon as it finds a match, it would execute echo "Okay, so now Valve is Closed".

I've also tried using if statement, however any inclusion of conditional statement seems to break the polling nature of message_sub which constantly monitors a given topic and prints out its output.

I'm kind of lost as to how I can use mosquitto_sub to retrieve messages and execute shell commands/scripts based on message content. Thanks for reading.

Answer 1

The problem here is that mosquitto_sub never returns (by default) so it's output never ends so can't be used to test against.

You will do MUCH better not trying to do this in a shell script and use something like python.

Edit:

If you REALLY MUST do this without writing something (trust me it is worth it) then you can use mqtt-exec (though it would require nodejs installing)

https://github.com/denschu/mqtt-exec

Answer 2

mosquitto_sub -h MOSQUITTO_BROKER_HOSTNAME -t test_topic | grep "valve_open" | some_other_script_to_handle_the_output.sh

#!/bin/bash
#content of some_other_script_to_handle_the_output.sh example script
echo "Input parameter: $1"
exit 0

That's one of the beautiful things about the pipe "|". They can be chained together.

That other script could effect changes elsewhere in a myriad of ways. Based on the frequency of these MQTT responses, you would want that script to finish quickly so things don't get backed up, but usually that's not a problem. Trial and error is your friend here and could lead you to a more efficient methodology down the line.