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.
Most (all?) Linux distributions incorporate this from the [util-linux] project.