[root@training ~]# systemctl start mysql Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.
Use systemctl status to check the status:
[root@training ~]# systemctl status mysql ● mysqld.service - LSB: start and stop MySQL Loaded: loaded (/etc/rc.d/init.d/mysqld; bad; vendor preset: disabled) Active: failed (Result: exit-code) since Sun 2018-05-06 10:06:53 CST; 26min ago Docs: man:systemd-sysv-generator(8) Process: 29770 ExecStart=/etc/rc.d/init.d/mysqld start (code=exited, status=1/FAILURE) May 06 10:06:52 training systemd[1]: Starting LSB: start and stop MySQL... May 06 10:06:53 training mysqld[29770]: Starting MySQL.The server quit without updating PID file (/usr/local/mysql/data/mysql/training.pid).[FAILED] May 06 10:06:53 training systemd[1]: mysqld.service: control process exited, code=exited status=1 May 06 10:06:53 training systemd[1]: Failed to start LSB: start and stop MySQL. May 06 10:06:53 training systemd[1]: Unit mysqld.service entered failed state. May 06 10:06:53 training systemd[1]: mysqld.service failed.
The specific reasons are still under analysis