TTYs

TTY stands for teletypewriter, originally a mechanical device that could print and receive text over a communications line. In computing, the term has broadened to refer to any device or interface that provides character-based input and output for a program. Physical TTYs included early electromechanical terminals and serial line consoles; today the term also covers software terminals, which simulate a TTY for text terminals.

Historically, teletypewriters connected to mainframes and minicomputers via serial lines. As display terminals emerged, they replaced paper printing with video screens but retained the same character-oriented communication model. In Unix and Unix-like systems, TTY was repurposed as a generic term for terminal devices, with devices named /dev/tty*, and a process's controlling terminal.

In Unix-like systems, a TTY can be a physical serial device (such as /dev/ttyS0), a virtual console, or a pseudo-terminal pair (master and slave) used by terminal emulators. The system's terminal driver handles line discipline, which controls input editing, echoing, and special characters. Applications access TTYs through standard streams; the termios API allows configuring modes (canonical vs raw), input processing, and signal handling. The controlling terminal is the TTY associated with a session, typically managed by login or an SSH session via a pty.

Modern usage includes serial console access to routers and embedded devices, terminal emulators within desktop environments, and remote access through SSH. Pseudo-terminals enable programs to run with interactive input/output in situations where no physical terminal is present. TTYs remain a core abstraction for text-based interaction in multi-user systems and programming environments.