下面开始正式讲systemctl命令，这里我就以一个firewalld的服务作为解释！！

systemctl  start  firewalld                              启动服务
systemctl  stop  firewalld                               关闭服务
systemctl  status  firewalld                            查看状态 
systemctl  enable   firewalld                          开机自启动
systemctl  disable   firewalld                         关闭开机自启动
systemctl  is-enable   firewalld                      查看是否开机自启动状态

systemctl list-unit-files                             ##列出所有的服务systemctl list-units --all --type=service             ##查看所有的service