How to subscribe to multiple topics (at least 2) and republish the message recieved to a single topic with Mosquitto MQTT using linux shell comands

Iam trying to figure out a way to subscribe to multiple topics, and then after receiving a message the function should publish it to a different broker depending on the topic.

I've thought of implementing a function that goes like this: when a publisher publishes a message the value of the topic published get stored in a variable called "topic" and the message get stored in a variable called "message". here is my code:

#!/bin/bash
SOURCE_BROKER="192.168.1.37"
DEST_BROKER="192.168.1.25"
while true
do
#Subscribe to both topics ("0_0" and "0_1")
  mosquitto_sub -h $SOURCE_BROKER -t "0_0" -t "0_1" -u pi -P Pass123 | while read -r topic message
  do
    # This is a callback to be executed every time a client subscribes to a topic
    echo "Client subscribed to ${topic}"
    # Check the topic and redirect the subscription
    if [ "$topic" == "0_0" ]; then
      # Publish the message to the broker (SOURCE_BROKER)
      mosquitto_pub -h $SOURCE_BROKER -t "$topic" -m "$message" -u pi -P Pass123
    elif [ "$topic" == "0_1" ]; then
      # Publish the message to the new broker (DEST_BROKER)
      mosquitto_pub -h $DEST_BROKER -t "$topic" -m "$message" -u pi -P Pass123
    fi
  done
  sleep 10 # Wait 10 seconds until reconnection
done

then I've discovered that it would not work because mosquitto_sub does not return the topic name when subscribing to more than one topic at the same time. Is there is any other way to implement this function?

Solution

As you see, I subscribe to each topic separately using separate mosquitto_sub and the received messages are then passed to the handle_message function, which handles the logic based on the topic and I also use the & operator to run the first mosquitto_sub in the background, so that both subscriptions can be active simultaneously!

#!/bin/bash

SOURCE_BROKER="192.168.1.37"
DEST_BROKER="192.168.1.25"

# Function to handle the received message
handle_message() {
    topic="$1"
    message="$2"

    # This is a callback to be executed every time a client subscribes to a topic
    echo "Client subscribed to ${topic}"

    # Check the topic and redirect the subscription
    if [ "$topic" == "0_0" ]; then
        # Publish the message to the broker (SOURCE_BROKER)
        mosquitto_pub -h "$SOURCE_BROKER" -t "$topic" -m "$message" -u pi -P Pass123
    elif [ "$topic" == "0_1" ]; then
        # Publish the message to the new broker (DEST_BROKER)
        mosquitto_pub -h "$DEST_BROKER" -t "$topic" -m "$message" -u pi -P Pass123
    fi

    #exit the loop after forwarding the message
    exit
}

# Subscribe to both topics ("0_0" and "0_1")
mosquitto_sub -h "$SOURCE_BROKER" -t "0_0" -u pi -P Pass123 | while read -r message
do
    handle_message "0_0" "$message"
done &

mosquitto_sub -h "$SOURCE_BROKER" -t "0_1" -u pi -P Pass123 | while read -r message
do
    handle_message "0_1" "$message"
done