mysql登录不了及修改密码

安装mariadb，默认是无密码的，但一般是指要设置密码的。在设置密码时出现各种问题，可能还是不太明白其原理。
一下我尝试了两种方法，但都失败了。下面这两个是我尝试的方法：

一、网上最多的方法是
1、输入下面的指令
mysqladmin -u root -p password newpassword
但是我已输入提示输入密码，我输入密码了但提示错误，见下
[root@localhost sakila]# mysqladmin -u root -p password newpasswd
Enter password:
mysqladmin: connect to server at 'localhost' failed
error: 'Access denied for user 'root'@'localhost' (using password: YES)'

2、停掉数据库服务
systemctl stop mariadb.service
3、对所有的表设置成无密码登陆。
mysqld_safe --skip-grant-tables &
执行完这条语句提示一下错误

[root@localhost sakila]# mysqld_safe --skip-grant-tables &
[1] 14275
[root@localhost sakila]# 150424 10:13:41 mysqld_safe Logging to '/var/log/mariadb/mariadb.l
og'.150424 10:13:41 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

4、尝试退出数据库，然后重启服务，再进入
[root@localhost sakila]# systemctl restart mariadb.service
[root@localhost sakila]# mysqladmin -u root -p password 'passwd'
Enter password:
mysqladmin:
You cannot use 'password' command as mysqld runs
with grant tables disabled (was started with --skip-grant-tables).
Use: "mysqladmin flush-privileges password '*'" instead

二、正确的操作
1、进到数据库进行操作
[root@localhost etc]# mysql
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 7
Server version: 5.5.41-MariaDB MariaDB Server

Copyright (c) 2000, 2014, Oracle, MariaDB Corporation Ab and others.

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

MariaDB [(none)]> mysql
-> ;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds
to your MariaDB server version for the right syntax to use near 'mysql' at line 1MariaDB [(none)]> update user set password=password("123456")where user='root';
ERROR 1046 (3D000): No database selected

2、选择数据库
MariaDB [(none)]> 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

修改密码

UPDATE user SET authentication_string=PASSWORD('aaaAAA111') WHERE user='root';

FLUSH PRIVILEGES;