This is two MQTT version 5.0/3.1.1/3.1 command line clients. mosquitto_pub can be used to publish messages to a broker and mosquitto_sub can be used to subscribe to a topic to receive messages.

MQTT provides a method of carrying out messaging using a publish/subscribe model. It is lightweight, both in terms of bandwidth usage and ease of implementation. This makes it particularly useful at the edge of the network where a sensor or other simple device may be implemented using an arduino for example.