systemctl操作
啟動
systemctl start xxx
systemctl stop xxx
systemctl restart xxx
查看狀態
systemctl status xxx
設置自啟動
systemctl enable xxx
systemctl disable xxx
修改了unit之后
# 如/usr/lib/systemd/system、/etc/systemd/system中xxx.service，xxx.service.d的東西
systemctl daemon-reload
查看開機項
systemctl list-unit-files  (查看開機啟動項)
systemctl list-unit-files  |  grep 程序名稱   （查看某些服務開機啟動狀態）
systemctl list-unit-files |  grep enable （查看哪些為開機啟動服務）
從失敗的列表中刪除unit
systemctl stop [servicename]
systemctl disable [servicename]
要重置所有狀態為失敗的unit：
systemctl reset-failed
或指定特定unit：
systemctl reset-failed xxx.service
CentOS修改了系統啟動文件后執行systemctl daemon-reload報錯　

Error getting authority: Error initializing authority: Error calling StartServiceByName for org.freedesktop.PolicyKit1: Timeout was reached (g-io-error-quark, 24)

或
[bin]$ systemctl daemon-reload

** (pkttyagent:31514): WARNING **: Unable to register authentication agent: Timeout was reached
Error registering authentication agent: Timeout was reached (g-io-error-quark, 24)
Failed to execute operation: Connection timed out
解決辦法安裝polkit，再重載即可

yum -y install polkit