stdin, stdout, and stderr

In computing, standard streams are preconnected input and output communication channels between a computer program and its environment. Specifically, stdin (standard input) is the channel through which a program receives input, typically from the keyboard or another program. stdout (standard output) is the channel through which a program writes its normal output, usually displayed on the screen. stderr (standard error) is the channel through which a program writes error messages and diagnostic information, also typically displayed on the screen, but kept separate from stdout to allow for easier handling of errors.