We will use the
mosquitto_pub command-line utility included in Mosquitto to generate a simple MQTT client that publishes a message to a topic. Open a Terminal in macOS or Linux, or a Command Prompt in Windows, go to the directory in which Mosquitto is installed and run the following command:
mosquitto_pub -V mqttv311 -t sensors/drone01/altitude -m "10 f" -d
The previous command will create an MQTT client that will establish a connection with the local MQTT server and then will make the client publish a message to the topic specified after the
sensors/drone01/altitude. We specify the payload for the message after the
"10 f". We specify the version of the MQTT protocol that we want to use when the client ...