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mysqladmin command to change root password
If you have never set a root password for MySQL, the server does not require a password at all for connecting as root. To setup root password for first time, use mysqladmin command at shell prompt as follows:

$ mysqladmin -u root password NEWPASSWORD
However, if you want to change (or update) a root password, then you need to use following command

$ mysqladmin -u root -p'oldpassword' password newpass
For example, If old password is abc, and set new password to 123456, enter:

$ mysqladmin -u root -p'abc' password '123456'
Change MySQL password for other user
To change a normal user password you need to type (let us assume you would like to change password for vivek):

$ mysqladmin -u vivek -p oldpassword password newpass
Changing MySQL root user password using MySQL sql command
This is another method. MySQL stores username and passwords in user table inside MySQL database. You can directly update password using the following method to update or change password for user vivek:
1) Login to mysql server, type following command at shell prompt:

$ mysql -u root -p
2) Use mysql database (type command at mysql> prompt):

mysql> use mysql;
3) Change password for user vivek:

mysql> update user set password=PASSWORD("NEWPASSWORD") where User='vivek';
4) Reload privileges:

mysql> flush privileges;
mysql> quit
This method you need to use while using PHP or Perl scripting.
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