Reference:

• systemctl 命令完全指南 https://linux.cn/article-5926-1.html
• How To Check All Running Services In Linux https://www.2daygeek.com/how-to-check-all-running-services-in-linux/

• Systemd 入门教程：命令篇 http://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-commands.html

• Systemd 入门教程：实战篇 http://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-part-two.html