service mysqld start 然后报：
==== AUTHENTICATING FOR org.freedesktop.systemd1.manage-units ===
Authentication is required to start 'mysqld.service'.
Authenticating as: lll,,, (lll)
Password:
polkit-agent-helper-1: pam_authenticate failed: Authentication failure
==== AUTHENTICATION FAILED ===
Failed to start mysqld.service: Access denied
See system logs and 'systemctl status mysqld.service' for details.
其实是没有权限访问之类的，所以用ｓｕｄｏ就可以啦。