How do I subscribe to all topics of a MQTT broker

Subscribing to # gives you a subscription to everything except for topics that start with a $ (these are normally control topics anyway).

It is better to know what you are subscribing to first though, of course, and note that some broker configurations may disallow subscribing to # explicitly.

You can use mosquitto_sub (which is part of the mosquitto-clients package) and subscribe to the wildcard topic #:

mosquitto_sub -v -h broker_ip -p 1883 -t '#'