笔者在CentOS7上安装MySQL 5.7版本，安装完成后，登录的时候，提示登录被拒绝：

[root@cdh1 ~]# mysql -uroot -p
Enter password: 
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

以前安装5.6版本的时候，root用户的默认密码为空， 提示输入密码时，敲回车即能登录。但是此次安装的5.7版本按默认密码为空的方式登录，却不能登录。原因是5.7版本中，系统会随机为root用户设置一个密码。此时，若想进入mysql命令行模式，需先进行root用户的密码重置操作。

root用户密码重置非常简单：

1、修改文件：/etc/my.cnf ：

 找到 mysql配置的代码块，添加一行配置 skip-grant-tables ， 绕过密码验证，如下图：

修改完，保存退出，并重启mysql

[root@cdh1 etc]# systemctl restart mysqld.service

2、进入mysql命令行， 修改root用户的密码：

[root@cdh1 etc]# mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.21 MySQL Community Server (GPL)

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
 
mysql> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed

mysql> update user set authentication_string = password('123'), password_expired = 'N', password_last_changed = now() 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)

mysql> quit;
Bye

此处需要注意，在5.6版本中，密码字段是 password ， 在5.7版本中，密码字段是 authentication_string 。

3、将/etc/my.cnf 文件恢复原状，即删除第一步中添加的配置：skip-grant-tables ， 保存退出。

4、重启mysql服务，即可使用新密码进行登录：

[root@cdh1 etc]# systemctl restart mysqld.service
[root@cdh1 etc]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.21 MySQL Community Server (GPL)

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

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