systemd的原理：

https://www.linux.com/learn/tutorials/527639-managing-services-on-linux-with-systemd

http://ktaraghi.blogspot.com/2013/11/what-is-systemd-and-how-it-works-part-1.html

http://ktaraghi.blogspot.com/2013/12/what-is-systemd-and-how-it-works-part-2.html

使用方法：https://fedoraPRoject.org/wiki/Systemd/zh-cn