PTY

• PTY (pseudoterminal)

  • A kernel-provided pair of virtual file descriptors — a controller end (/dev/ptmx) and a subordinate end (/dev/pts/N) — that together emulate a hardware serial terminal.
  • The subordinate end looks like a real tty device to any process that opens it: isatty() returns true, the process can query terminal size (TIOCGWINSZ), and it receives and sends ANSI escape sequences normally.
  • The controller end is held by the terminal emulator (or any host process); it reads the subordinate's output, interprets escape codes, and writes input back.

  • Why it matters

    • CLI programs check isatty() to decide whether to emit ANSI color and paging. Attached to a PTY → color on; attached to a pipe → color off.
    • nvim's :terminal command allocates a PTY, so programs inside it behave as if in a real terminal (see vim/split/v for the :vsplit | terminal pattern).
    • A plain :!cmd subprocess gets a Unix/Pipe, not a PTY — isatty() returns false.

  • Relationship to TTY

    • A TTY originally referred to a physical teletype or serial terminal. Modern usage treats TTY and PTY as near-synonyms in software; PTY specifically denotes the virtual (kernel-emulated) variety.

  • See also

    • PTY/Q/What is a proper PTY buffer and how is it different from a non-PTY subprocess?
    • The TTY demystified (Linus Åkesson)