Mosquitto

Very nice IOT messaging protocol.

How to publish and subscribe to mqtt messages:

Publish

mosquitto_pub -h <host-ip> -p <host-port> -u <username> -P <password> -t <topic> -m '<message>'

Subscribe

mosquitto_sub -h <host-ip> -p <host-port> -u <username> -P <password> -t <topic>