[root@localhost ~]# systemctl start mysqld       启动失败

Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.

查看系统日志报错为：

May 29 19:04:53 localhost mysqld: 2018-05-29T11:04:53.723812Z 0 [ERROR] InnoDB: The innodb_system data file 'ibdata1' must be writable
May 29 19:04:53 localhost mysqld: 2018-05-29T11:04:53.723862Z 0 [ERROR] InnoDB: The innodb_system data file 'ibdata1' must be writable
May 29 19:04:53 localhost mysqld: 2018-05-29T11:04:53.723871Z 0 [ERROR] InnoDB: Plugin initialization aborted with error Generic error
May 29 19:04:54 localhost mysqld: 2018-05-29T11:04:54.325440Z 0 [ERROR] Plugin 'InnoDB' init function returned error.
May 29 19:04:54 localhost mysqld: 2018-05-29T11:04:54.325475Z 0 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
May 29 19:04:54 localhost mysqld: 2018-05-29T11:04:54.325483Z 0 [ERROR] Failed to initialize builtin plugins.
May 29 19:04:54 localhost mysqld: 2018-05-29T11:04:54.325485Z 0 [ERROR] Aborting

报错原因大致为：你的数据文件不可写

导致原因：yum安装的mysql的拥有者是root，而mysql要求执行身份为mysql，权限问题导致数据文件不可写

　　解决方法如下：

[root@localhost ~]# chown -R mysql /var/lib/mysql

[root@localhost ~]# systemctl start mysqld

[root@localhost ~]# tailf /var/log/messages &

日志打印：

May 29 19:09:15 localhost mysqld: Version: '5.7.22'  socket: '/var/lib/mysql/mysql.sock'  port: 3306  MySQL Community Server (GPL)

启动成功

登录 MySQL，此时不需要密码，直接回车:

# mysql -u root -p

更改 root 密码 为 123456：

mysql> use mysql; mysql> update user set authentication_string=password("123456") where user='root'; mysql> flush privileges; # 刷新权限，不刷新的话可能导致mysql回车直接就会登录

注意密码字段名 5.7 版本的是 authentication_string，之前的为 password。

修改完后，记得注释掉 my.cnf 中的 skip-grant-tables 参数，重启 MySQL 服务，就可以用你设置的密码登录了。