1.修改root密码
方法1：使用mysqladmin命令
--适用于记得root旧密码，修改root密码
语法：
mysqladmin -u用户名 -p旧密码 password 新密码
例如：
# mysqladmin -u root -proot password mysql

--注意：如当旧密码输入错误时会报如下错误
# mysqladmin -u root -proot1 password mysql
mysqladmin: connect to server at 'localhost' failed
error: 'Access denied for user 'root'@'localhost' (using password: YES)'

方法2：直接更新user表password字段
--适用于忘记root密码，而对root密码进行重置
Step 1: 修改MySQL的登录设置
# vi /etc/my.cnf
--windows系统是my.ini文件
--在[mysqld]的段中加上一句：skip-grant-tables，如没有[mysqld]字段，可手动添加上
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
skip-name-resolve
skip-grant-tables

Step 2: 重新启动mysql
[root@gc ~]# service mysql restart
Shutting down MySQL..[确定]
Starting MySQL...[确定]

Step 3: 登录并修改MySQL的root密码
--此时直接用mysql即可无需密码即可进入数据库了
[root@gc ~]# mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.5.24 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, 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;
Database changed
mysql> update user set password=password('new_password') where user='root';
Query OK, 5 rows affected (0.00 sec)
Rows matched: 5  Changed: 5  Warnings: 0

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

--注意：如果没做step1，直接用mysql登录时会报如下错误
[root@gc ~]# mysql
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

Step 4: 将MySQL的登录设置修改回来
再删除/etc/my.cnf文件中的skip-grant-tables

Step 5: 重新启动mysql
[root@gc ~]# service mysql restart
Shutting down MySQL..[确定]
Starting MySQL...[确定]