1. mysql安装后默认没有密码，初始化安装后默认密码登录，需要马上修改root密码。
[root@mysql ~]# cat /root/.mysql_secret    --查看root账号密码
# The random password set for the root user at Tue Dec 13 18:59:06 2016 (local time): 7MhtyX3aZEl9OUf8
[root@mysql ~]# mysql -uroot -p    -- 登录mysql
Enter password:               --（此处复制粘贴 7MhtyX3aZEl9OUf8）
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.6.25
Copyright (c) 2000, 2015, 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.
说明：  出现mysql>   表示一切顺利成功。

2.  修改密码
mysql> SET PASSWORD = PASSWORD('123456');   --设置密码为123456
[root@mysql etc]# mysql -uroot -p123456     --修改密码后，登录
Warning: Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 39
Server version: 5.6.25 MySQL Community Server (GPL)
mysql>     --登录成功

3. 忘记mysql数据库root的密码后，如何修改密码。
[root@mysql etc]# service mysql stop   --停止mysql进程
Shutting down MySQL.... SUCCESS! 
[root@mysql etc]# mysqld_safe --skip-grant-tables&  --启动mysql时不启动grant-tables，授权表。执行后出现下面画面，另起一个新会话。
[1] 3574
[root@mysql etc]# 161215 06:13:53 mysqld_safe Logging to '/var/lib/mysql/mysql.err'.
161215 06:13:53 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
[root@mysql ~]# ps -ef|grep mysql   --查看mysql进程是否起来
root       3467   3440  0 05:29 pts/2    00:00:00 mysql -uandy -px xx
root       3574   2845  0 06:13 pts/0    00:00:00 /bin/sh /usr/bin/mysqld_safe --skip-grant-tables
mysql      3677   3574  2 06:13 pts/0    00:00:00 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-
mysql> UPDATE user SET password=PASSWORD('cccccc') WHERE user='root';   --更改root密码
Query OK, 4 rows affected (0.04 sec)
Rows matched: 4  Changed: 4  Warnings: 0
mysql> FLUSH PRIVILEGES;    --权限更新
Query OK, 0 rows affected (0.00 sec)
[root@mysql ~]# mysql -uroot -p123456    --使用旧密码登录，发现不行，说明密码已经更新
Warning: Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
[root@mysql ~]# mysql -uroot -pcccccc   --新密码登录成功
Warning: Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
mysql>