我正在使用安装了 mosquitto 代理的 raspberry pi 图像。为了排除故障，指令是运行：

pi@raspberrypi:/ $ mosquitto_sub -v -t "gateway/+/rx"

然而，这导致：

-bash: mosquitto_sub: command not found

即使 mosquitto 正在运行：

pi@raspberrypi:/ $ sudo service mosquitto status
● mosquitto.service - LSB: mosquitto MQTT v3.1 message broker
   Loaded: loaded (/etc/init.d/mosquitto; generated; vendor preset: enabled)
   Active: active (running) since Sun 2018-09-30 21:43:51 UTC; 5h 14min ago
     Docs: man:systemd-sysv-generator(8)
  Process: 347 ExecStart=/etc/init.d/mosquitto start (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/mosquitto.service
           └─432 /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf

搜索问题并没有产生容易找到的答案。