在阿里云ECS下的Ubuntu 16.04安装Mysql，紧接着就用mysql_secure_installation安装，并且设置了root的密码。但是完毕后居然不能登录，百思不得其解。
$ mysql -uroot -p
Enter password:
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
无奈只好重置root密码。步骤如下：
1. 把mysql停止。
$ sudo /etc/init.d/mysql stop
[ ok ] Stopping mysql (via systemctl): mysql.service.
2. 启动mysqld_safe。
$ sudo mysqld_safe --skip-grant-tables &
[1] 28860
（如果遇到错误
mysqld_safe Directory '/var/run/mysqld' for UNIX socket file don't exists.
则执行下面命令后再次启动mysqld_safe。
sudo mkdir -p /var/run/mysqld
sudo chown mysql:mysql /var/run/mysqld
）
3. 进入mysql，执行两条命令：
$ mysql -u root
mysql> UPDATE mysql.user SET authentication_string = PASSWORD('NewPassword') , password_expired = 'N' WHERE User = 'root' AND Host = 'localhost';
Query OK, 0 rows affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 1
mysql> update user set plugin="mysql_native_password";
Query OK, 1 row affected (0.00 sec)
Rows matched: 3 Changed: 1 Warnings: 0
注意Mysql 5.7版本要修改的字段为authentication_string，而不是Password。另外，plugin字段的值要置成"mysql_native_password"。
4. 退出mysql
mysql> quit
Bye
5. 停止Mysql
$ sudo /etc/init.d/mysql stop
[ ok ] Stopping mysql (via systemctl): mysql.service.
$ sudo kill -9 $(pgrep mysql)
6. 再次启动mysql
$ sudo /etc/init.d/mysql start
[ ok ] Starting mysql (via systemctl): mysql.service.
再次用密码可以登录。
$ mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.19 MySQL Community Server (GPL)
参考文档：
https://dev.mysql.com/doc/refman/5.7/en/resetting-permissions.html
https://www.linuxquestions.org/questions/linux-software-2/mysql-mariadb-install-root-password-not-working-4175602071/
http://www.jb51.net/article/77858.htm