Just for my curiosity, where is the username and associated password stored in Ubuntu? And how it is verified at the login time?
Passwords can be found (in hashed form) in /etc/shadow and the users in /etc/passwd.
The login program is used to establish a new session with the system. It is normally invoked automatically by responding to the login: prompt on the user´s terminal. login may be special to the shell and may not be invoked as a sub-process. Typically, login is treated by the shell as exec login which causes the user to exit from the current shell. Attempting to execute login from any shell but the login shell will produce an error message.
The user is then prompted for a password, where appropriate. Echoing is disabled to prevent revealing the password. Only a small number of password failures are permitted before login exits and the communications link is severed. If password aging has been enabled for your account, you may be prompted for a new password before proceeding. You will be forced to provide your old password and the new password before continuing. Please refer to passwd(1) for more information.