ubuntu搭建mqtt服务器及验证

1. 安装mosquitto

   sudo apt install mosquitto
   sudo apt install libmosquitto-dev
   sudo apt install mosquitto-clients
   

2. 启动mosquitto
   默认安装mosquitto后就会自动启动mosquitto进程，其配置文件在/etc/mosquitto下，默认监听端口为1883
   /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf
   查看mosquitto的启动命令

   # mosquitto -h
   mosquitto version 1.4.15 (build date Tue, 18 Jun 2019 11:42:22 -0300)
   
   mosquitto is an MQTT v3.1.1/v3.1 broker.
   
   Usage: mosquitto [-c config_file] [-d] [-h] [-p port]
   
    -c : specify the broker config file.   (配置文件路径)
    -d : put the broker into the background after starting. （后台运行）
    -h : display this help. （显示帮助）
    -p : start the broker listening on the specified port.
         Not recommended in conjunction with the -c option. （指定端口）
    -v : verbose mode - enable all logging types. This overrides
         any logging options given in the config file. （打印日志）
   
   See http://mosquitto.org/ for more information.
   

   执行：mosquitto -p 5678 -v，在本地端口5678上启动mosquitto服务，并在前台运行打印日志

3. 订阅
   使用mosquitto_sub命令启动订阅，查看mosquitto_sub的启动命令

   # mosquitto_sub --help
   mosquitto_sub is a simple mqtt client that will subscribe to a single topic and pr