How to: Log In to an Instance of SQL Server (Command Prompt)
To test connectivity to an instance of SQL Server, use the sqlcmd utility.
To log in to the default instance of SQL Server
From a command prompt, enter the following command to connect by using Windows Authentication:
sqlcmd [ /E ] [ /S servername ]
To log in to a named instance of SQL Server
From a command prompt, enter the following command to connect by using Windows Authentication:
sqlcmd [ /E ] /S servername\instancename