Below command will connect to MySQL using root user

manoj@manoj:~$ mysql -b -h localhost -u root -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 36
Server version: 5.5.24-0ubuntu0.12.04.1 (Ubuntu)

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Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the current input statement.

mysql>

I have used Ubuntu 12.04 Desktop and MySQL version is 5.5.24

The same command will work in Windows – DOS.