一，問題描述：

ubuntu18.02 新裝了mysql-server-5.7,

問題一，普通使用者不能進mysql：

~$ mysql -u root -p
Enter password: 
ERROR 1698 (28000): Access denied for user 'root'@'localhost'

問題二，su root ，直接用root許可權登入就不用密碼：

~&su root 
~$ mysql
 Success!

二，解決步驟：

1、停止mysql服務

 ~$ sudo service mysql stop

2、以安全模式啟動MySQL

~$ sudo mysqld_safe --skip-grant-tables &
 

3、MySQL啟動之後就可以不用密碼登陸了

~$ mysql -u root
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.10 MySQL Community Server (GPL)

檢視一下user表，錯誤的起因就是在這裡， root的plugin被修改成了auth_socket，用密碼登陸的plugin應該是mysql_native_password。

mysql> select user, plugin from mysql.user;
+-----------+-----------------------+
| user      | plugin                |
+-----------+-----------------------+
| root      | auth_socket           |
| mysql.sys | mysql_native_password |
| dev       | mysql_native_password |
+-----------+-----------------------+
3 rows in set (0.01 sec)
 

關於auth_socket，在官方有說明： https://dev.mysql.com/doc/mysql-security-excerpt/5.5/en/socket-authentication-plugin.html ，反正現在暫時不用它， 那就把這裡改了。

mysql> update mysql.user set authentication_string=PASSWORD('newPwd'), plugin='mysql_native_password' where user='root';
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
 

重啟服務，問題就解決了

~$ sudo service mysql stop
...
 \* MySQL Community Server 5.7.10 is stopped
~$ sudo service mysql start
..
 \* MySQL Community Server 5.7.10 is started
~$ mysql -u root -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.10 MySQL Community Server (GPL)