User
On Windows, you can run the server as a Windows service using a normal user account.

On Linux, for installations performed using a MySQL repository or RPM packages, the MySQL server mysqld should be started by the local mysql operating system user. Starting by another operating system user is not supported by the init scripts that are included as part of the MySQL repositories.

On Unix (or Linux for installations performed using tar.gz packages) , the MySQL server mysqld can be started and run by any user. However, you should avoid running the server as the Unix root user for security reasons. To change mysqld to run as a normal unprivileged Unix user user_name, you must do the following:

Stop the server if it is running (use mysqladmin shutdown).

Change the database directories and files so that user_name has privileges to read and write files in them (you might need to do this as the Unix root user):

shell> chown -R user_name /path/to/mysql/datadir
If you do not do this, the server cannot access databases or tables when it runs as user_name.

If directories or files within the MySQL data directory are symbolic links, chown -R might not follow symbolic links for you. If it does not, you must also follow those links and change the directories and files they point to.

Start the server as user user_name. Another alternative is to start mysqld as the Unix root user and use the --user=user_name option. mysqld starts, then switches to run as the Unix user user_name before accepting any connections.

To start the server as the given user automatically at system startup time, specify the user name by adding a user option to the [mysqld] group of the /etc/my.cnf option file or the my.cnf option file in the server’s data directory. For example:

[mysqld]
user=user_name
If your Unix machine itself is not secured, you should assign passwords to the MySQL root account in the grant tables. Otherwise, any user with a login account on that machine can run the mysql client with a --user=root option and perform any operation. (It is a good idea to assign passwords to MySQL accounts in any case, but especially so when other login accounts exist on the server host.) See Section 2.10.4, “Securing the Initial MySQL Account”.