MySQL ver5.7でrootのパスワードを忘れた場合の再設定方法 ref: http://qiita.com/kiimiiis/items/a8a47bc3065779e1dc0d

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[root@vagrant-centos65 log]# service mysqld stop
[root@vagrant-centos65 log]# mysqld_safe --skip-grant-tables &
mysql> use mysql;
mysql> UPDATE mysql.user
    ->     SET authentication_string = PASSWORD('新パスワード'), password_expired = 'N'
    ->     WHERE User = 'root' AND Host = 'localhost';
mysql> FLUSH PRIVILEGES;