mysql how to fix Access denied for user 'root'@'localhost'

Follow the steps below.

1. Start the MySQL server instance or daemon with the --skip-grant-tables option (security setting).

   $ mysqld --skip-grant-tables
   

2. Execute these statements.

   $ mysql -u root mysql
   $mysql> UPDATE user SET Password=PASSWORD('my_password') where USER='root';
   $mysql> FLUSH PRIVILEGES;
   

If you face the unknown field Password error above use:

update user set authentication_string=password('my_password') where user='root';

3. Finally, restart the instance/daemon without the --skip-grant-tables option.

   $ /etc/init.d/mysql restart
   

You should now be able to connect with your new password.

$ mysql -u root -p

Enter password: my_password

Fix for MySQL “Unable to lock ibdata1” error

sudo mv /usr/local/mysql/data/ibdata1 /usr/local/mysql/data/ibdata1.bak
sudo mv /usr/local/mysql/data/ib_logfile0 /usr/local/mysql/data/ib_logfile0.bak
sudo mv /usr/local/mysql/data/ib_logfile1 /usr/local/mysql/data/ib_logfile1.bak
sudo cp -a /usr/local/mysql/data/ibdata1.bak /usr/local/mysql/data/ibdata1
sudo cp -a /usr/local/mysql/data/ib_logfile0.bak /usr/local/mysql/data/ib_logfile0
sudo cp -a /usr/local/mysql/data/ib_logfile1.bak /usr/local/mysql/data/ib_logfile1
sudo /etc/init.d/mysql restart

None of the above were helpful for me. I found I needed to clear the plugin method. In 5.6, I could do:

sudo mysql -u root
use mysql;
[mysql] update user set plugin='' where User='root';
[mysql] flush privileges;

In 5.7, I found I needed to:

sudo mysql -u root
use mysql;
[mysql] update user set plugin='mysql_native_password' where User='root';
[mysql] flush privileges;

According to the docs, with plugin set to an empty string, it should have effectively defaulted to mysql_native_password, but may be getting confused by an empty password hash. For more nuance, you can read the documentation here: https://dev.mysql.com/doc/refman/5.7/en/native-authentication-plugin.html

Also make sure needed record in table user has empty plugin field (there can be, for example, "unix_socket").

Since version 5.5.7 mysql has various auth plugins support https://dev.mysql.com/doc/refman/5.6/en/authentication-plugins.html

So if you have non-empty plugin field then password would be ignored and there would be warning at mysql error log (for me it's /var/log/mysql/error.log):

[Warning] 'user' entry 'root@localhost' has both a password and an authentication plugin specified. The password will be ignored.