默认情况下，新安装的 mariadb 的密码为空，在shell终端直接输入 mysql 就能登陆数据库。

如果是刚安装第一次使用，请使用 mysql_secure_installation 命令初始化。

# mysql_secure_installation

NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MariaDB
      SERVERS IN PRODUCTION USE!  PLEASE READ EACH STEP CAREFULLY!

In order to log into MariaDB to secure it, we'll need the current
password for the root user.  If you've just installed MariaDB, and
you haven't set the root password yet, the password will be blank,
so you should just press enter here.

Enter current password for root (enter for none):
OK, successfully used password, moving on...

Setting the root password ensures that nobody can log into the MariaDB
root user without the proper authorisation.

Set root password? [Y/n] y
New password:
Re-enter new password:
Password updated successfully!
Reloading privilege tables..
 ... Success!


By default, a MariaDB installation has an anonymous user, allowing anyone
to log into MariaDB without having to have a user account created for
them.  This is intended only for testing, and to make the installation
go a bit smoother.  You should remove them before moving into a
production environment.

Remove anonymous users? [Y/n] y
 ... Success!

Normally, root should only be allowed to connect from 'localhost'.  This
ensures that someone cannot guess at the root password from the network.

Disallow root login remotely? [Y/n] y
 ... Success!

By default, MariaDB comes with a database named 'test' that anyone can
access.  This is also intended only for testing, and should be removed
before moving into a production environment.

Remove test database and access to it? [Y/n] y
 - Dropping test database...
 ... Success!
 - Removing privileges on test database...
 ... Success!

Reloading the privilege tables will ensure that all changes made so far
will take effect immediately.

Reload privilege tables now? [Y/n] y
 ... Success!

Cleaning up...

All done!  If you've completed all of the above steps, your MariaDB
installation should now be secure.

Thanks for using MariaDB!

这里针对的是知道 root 密码，而需要修改的情况。

两种修改方法：

1、直接在shell命令行使用 mysqladm 命令修改。

# mysqladmin -uroot -poldpassword password newpassword

这种方法的弊端在于会明文显示密码。

2、登陆数据库修改密码。

# mysql -uroot -p

2.1 更新 mysql 库中 user 表的字段：
MariaDB [(none)]> use mysql;  
MariaDB [mysql]> UPDATE user SET password=password('newpassword') WHERE user='root';  
MariaDB [mysql]> flush privileges;  
MariaDB [mysql]> exit;

2.2 或者，使用 set 指令设置root密码：
MariaDB [(none)]> SET password for 'root'@'localhost'=password('newpassword');  
MariaDB [(none)]> exit; 

如果是忘记了 root 密码，则需要以跳过授权的方式启动 mariadb 来修改密码。

1、先停掉服务。

# systemctl stop mariadb

2、使用跳过授权的方式启动 mariadb。

# mysqld_safe --skip-grant-tables &
[1] 1441
[root@centos7 ~]# 170531 02:10:28 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
170531 02:10:28 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

# ps -ef | grep 1441
root      1441   966  0 02:10 pts/0    00:00:00 /bin/sh /usr/bin/mysqld_safe --skip-grant-tables
mysql     1584  1441  0 02:10 pts/0    00:00:00 /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --user=mysql --skip-grant-tables --log-error=/var/log/mariadb/mariadb.log --pid-file=/var/run/mariadb/mariadb.pid --socket=/var/lib/mysql/mysql.sock

3、当跳过授权启动时，可以不需要密码直接登陆数据库。登陆更新密码即可。

# mysql
MariaDB [(none)]> use mysql;  
MariaDB [mysql]> UPDATE user SET password=password('newpassword') WHERE user='root';  
MariaDB [mysql]> flush privileges;   
MariaDB [mysql]> exit; 

更新密码后，在跳过授权启动时也不能空密码直接登陆了。

4、关闭跳过授权启动的进程：

# kill -9 1441 

5、正常启动 mariadb：

# systemctl start mariadb