A plain "GET" subscribes to the topic and prints all published messages. Doing a "POST" publishes the post data to the topic and exits.

Subscribe to the temperature in the "home/bedroom" subject published by example.com:

curl mqtt://example.com/home/bedroom/temp

Send the value '75' to the "home/bedroom/dimmer" subject hosted by the example.com server:

curl -d 75 mqtt://example.com/home/bedroom/dimmer

What does curl deliver as a response to a subscribe

It outputs two bytes topic length (MSB | LSB), the topic followed by the payload.

Caveats

Remaining limitations in curl's MQTT support as of September 2020:

• No username support

• Only QoS level 0 is implemented for publish

• No way to set retain flag for publish

• No username/password support

• No TLS (mqtts) support