How to fix Mosquitto 'exited with code 13'

When mosquitto exits with code 13 such as a in a docker based setup, you will often see not error messsage:

Attaching to mosquitto_mosquitto_1
mosquitto_mosquitto_1 exited with code 13

However, there **will be an error message in mosquitto.log.**So, ensure that you have configured a log_dest file in your mosquitto.conf such as:

log_dest file /mosquitto/log/mosquitto.log

and check that file. In my case it showed these error messages:

1637860284: mosquitto version 2.0.14 starting
1637860284: Config loaded from /mosquitto/config/mosquitto.conf.
1637860284: Error: Unable to open pwfile "/mosquitto/conf/mosquitto.passwd".
1637860284: Error opening password file "/mosquitto/conf/mosquitto.passwd".

In my case, the path of the password file was mis-spelled (conf instead of config)

Note that you need to create the password file in order for mosquitto to start up!

See How to setup standalone mosquitto MQTT broker using docker-compose for example commands on how to create the user and the password file