mysql问题排查

系统删除了mysql用户，导致mysql重启报错，内容如下：
161025 21:24:55 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
/usr/libexec/mysqld: Can't find file: './mysql/plugin.frm' (errno: 13)
161025 21:24:55 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
161025 21:24:55 InnoDB: Initializing buffer pool, size = 8.0M
161025 21:24:55 InnoDB: Completed initialization of buffer pool
161025 21:24:55 InnoDB: Operating system error number 13 in a file operation.
InnoDB: The error means mysqld does not have the access rights to
InnoDB: the directory.
InnoDB: File name ./ibdata1
InnoDB: File operation call: 'open'.
InnoDB: Cannot continue operation.
161025 21:24:55 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended

考虑到mysql服务使用mysql用户启动的，首先增加mysql用户，useradd -r mysql
修改目录权限：cd /var/lib/mysql
chown -R mysql.mysql *
启动还是报错，显示信息：
161025 21:57:47 [ERROR] /usr/libexec/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 13)
161025 21:57:47 [ERROR] Can't start server: can't create PID file: Permission denied
这个很明显了，mysql.pid没有权限生成
赋予权限：chown -R mysql.mysql /var/run/mysqld/
启动mysql，成功。