MySQL server PID file could not be found!

解決方法：
1.透過ss -tnl命令檢視服務
[root@s1 Downloads]# ss -tnl
State       Recv-Q Send-Q   Local Address:Port   Peer Address:Port
LISTEN      0      50               *:3306                 *:* 

2，停止mysql服務
[root@s1 Downloads]# killall mysqld
3.重啟mysql服務

4.發現mysql資料目錄許可權異常
[root@s1 Downloads]# mysql
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

5.修改mysql資料目錄許可權
[root@s1 Downloads]# chown -R mysql.mysql /var/lib/mysql/
6.重啟服務，即可以正常登陸mysql