Understanding Linux TTY, PTY, and PTS: A Complete Guide to Terminal Devices

Basic Concepts

1> tty (terminal device): The term tty comes from teletypes, originally referring to electromechanical typewriters that communicated over serial lines. Modern terminals are character devices, commonly abbreviated as tty.

2> pty (pseudo‑terminal): When you remote‑login via telnet or use an xterm, a virtual terminal (pty) provides the interactive interface.

3> pts/ptmx (pts and ptmx): pts (pseudo‑terminal slave) works together with ptmx (pseudo‑terminal master) to implement a pty.

Terminal Device Files in /dev

1. Serial port terminal (/dev/ttySn): Serial terminals are character devices such as /dev/ttyS0, /dev/ttyS1, corresponding to COM1, COM2, etc. Data can be sent by redirecting output, e.g., echo test > /dev/ttyS1.

2. Pseudo‑terminal (/dev/pty/): A pseudo‑terminal consists of a master–slave pair; operations on the master are reflected on the slave. Examples are /dev/ptyp3 (master) and /dev/ttyp3 (slave).

3. Controlling terminal (/dev/tty): Represents the terminal that controls the current process. The command tty shows which device it is.

4. Console terminal (/dev/ttyn, /dev/console): The physical console (e.g., tty1‑tty6) is a virtual terminal; /dev/console is used by the OS to output messages, especially in single‑user mode.

5. Virtual terminal (/dev/pts/n): Used in graphical environments (X‑windows); for example, /dev/pts/1 appears when using a terminal emulator like Konsole.

6. Other types: Linux also defines various other terminal device files for specific hardware such as ISDN (/dev/ttyIn).

In summary, tty devices include virtual consoles, serial ports, and pseudo‑terminals. /dev/tty refers to the current terminal, so echo "hello" > /dev/tty prints directly to the active terminal.