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.

grep 'temporary password' /var/log/mysqld.log
Sort date (newest date)

You may see something like this;

[root@SERVER ~]# grep 'temporary password' /var/log/mysqld.log
2016-01-16T18:07:29.688164Z 1 [Note] A temporary password is generated for root@localhost: O,k5.marHfFu
2016-01-22T13:14:17.974391Z 1 [Note] A temporary password is generated for root@localhost: b5nvIu!jh6ql
2016-01-22T15:35:48.496812Z 1 [Note] A temporary password is generated for root@localhost: (B*=T!uWJ7ws
2016-01-22T15:52:21.088610Z 1 [Note] A temporary password is generated for root@localhost: %tJXK7sytMJV
2016-01-22T16:24:41.384205Z 1 [Note] A temporary password is generated for root@localhost: lslQDvgwr3/S
2016-01-22T22:11:24.772275Z 1 [Note] A temporary password is generated for root@localhost: S4u+J,Rce_0t
[root@SERVER ~]# mysql_secure_installation

Securing the MySQL server deployment.

Enter password for user root: 

The existing password for the user account root has expired. Please set a new password.

New password: 

Re-enter new password:

If you see it says

... Failed! Error: Your password does not satisfy the current policy requirements
That means your password needs to have a character such as ! . # - etc...
mix characters well, upper case, lower case, ! . , # etc...

New password: 

Re-enter new password: 
The 'validate_password' plugin is installed on the server.
The subsequent steps will run with the existing configuration
of the plugin.
Using existing password for root.

Estimated strength of the password: 100 
Change the password for root ? ((Press y|Y for Yes, any other key for No) : Y

New password: 

Re-enter new password: 

Estimated strength of the password: 100 
Do you wish to continue with the password provided?(Press y|Y for Yes, any other key for No) : Y
By default, a MySQL installation has an anonymous user,
allowing anyone to log into MySQL 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? (Press y|Y for Yes, any other key for No) : 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? (Press y|Y for Yes, any other key for No) : Y
Success.

By default, MySQL 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? (Press y|Y for Yes, any other key for No) : 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? (Press y|Y for Yes, any other key for No) : Y
Success.

All done! 
[root@SERVER ~]# mysql -u root -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 11
Server version: 5.7.10 MySQL Community Server (GPL)

Watch the last 10 minutes of this video, it teaches you how you do it.

Try it:

mysql --no-defaults --force --user=root --host=localhost --database=mysql 
UPDATE user SET Password=PASSWORD('NEWPASSWORD') where USER='root';
FLUSH PRIVILEGES;