Moo is also a an old game available on UNIX. It is similar to the game Mastermind.

Modified from the UNIX man page
The computer picks a number consisting of 5 decimal digits. The player guesses 5 digits being scored on each guess. A "cow" is a correct digit in an incorrect position. A "bull" is a correct digit in a correct position. The game continues until the player guesses the number (a score of 5 bulls).