Just for my curiosity ,where is the username and associated password is 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.