From LQWiki
Login (or log in) is the process of authenticating oneself to a computer system, usually by typing in a user name and password when prompted. It is a standard process in computer security.
The login program (usually /bin/login) is the name of a small program on Unix systems that prompts a user for a username and password, then checks if the user account so named exists, and has the proper password. Some systems support a more comprehensive array of authentication methods, such as PAM, smart cards, and biometrics. The login program itself usually invokes some other program or library to handle such things.
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