Logging on to the DBM Server: -u 

Use

Option when Calling the Database Manager CLI

If you use –u without specifying <userid>,<password>, the Database Manager CLI prompts you to enter the operator name and password. In this way you can avoid the operator password is visible in the command line, command history, and the operating system process list.

If you do not specify a <user_key> when using –U, the Database Manager CLI uses the data under the DBMUSR XUSER key.

User details can be stored for various tasks with the help of the program XUSER and used for logging on.

Have a look at the significance of upper and lower case lettering on the meaning of the command.
-u: Explicit specification of the user with the <userid> and <password>
-U: Logon with an operator stored in the XUSER file.

See also:

User Manual: SAP DB

Prerequisites

You are using the option -d at the same time to specify a database instance.

Syntax

-u [<userid>,<password>]

or logon with XUSER: -U [<user_key>]