mosquitto_sub

Subscribe to topic

$ mosquitto_sub -t [sensors/temperature]

$ mosquitto_sub -t '#'

$ mosquitto_sub -t '[sensors/+/temperature]'

$ mosquitto_sub -h [broker.example.com] -t [topic]

$ mosquitto_sub -h [broker] -u [username] -P [password] -t [topic]

$ mosquitto_sub -h [broker] --cafile [ca.crt] -t [topic]

$ mosquitto_sub -v -t '[sensors/#]'

$ mosquitto_sub -t [topic] -C 1

mosquitto_sub [-h host] [-p port] [-t topic] [-u user] [-P pass] [options]

mosquitto_sub subscribes to MQTT topics and prints received messages. MQTT is a lightweight publish/subscribe messaging protocol for IoT and telemetry.
Topics use hierarchical naming with / separators. Wildcards expand subscriptions: + matches one level, # matches all remaining levels. For example, sensors/+/temperature matches any sensor's temperature.
Quality of Service levels control delivery guarantees: QoS 0 is fire-and-forget, QoS 1 ensures at-least-once delivery, QoS 2 guarantees exactly-once delivery.
Retained messages are stored by the broker and sent immediately on subscription. This provides last-known values to new subscribers.
TLS encryption protects communication. Client certificates enable mutual authentication. The broker must be configured to accept TLS connections.

-t TOPIC

Topic to subscribe to (can repeat).

Broker hostname.

Broker port (default 1883).

Username.

Password.

Client ID.

Verbose (print topic with message).

Exit after receiving count messages.

Quality of service (0, 1, 2).

CA certificate for TLS.

Client certificate.

Client private key.

Output format string.

Don't append newline to messages.

Only show retained messages.

Subscribing to # on busy brokers produces high volume. QoS 2 has significant overhead. Passwords on command line visible in process list.

mosquitto_sub is part of the Eclipse Mosquitto project, an MQTT broker and client suite. Mosquitto was created by Roger Light starting around 2009 and joined the Eclipse Foundation in 2014.

mosquitto_pub(1), mosquitto(8), mosquitto_passwd(1)