PTY/Q/What is a proper PTY buffer and how is it different from a non-PTY subprocess?

• What is a "proper PTY buffer" and how is it different from a "non-PTY subprocess"?

  • AI Answer

    • Short answer: A PTY (pseudoterminal) makes a program think it is running inside a real terminal; a non-PTY subprocess is attached to a plain pipe, so the program knows it is not in a terminal and usually disables ANSI color output.

    • What a PTY is

      • A PTY is a kernel-provided pair of virtual file descriptors: a controller end (held by the terminal emulator or host process, /dev/ptmx) and a subordinate end (exposed to the child program as its stdin/stdout/stderr, /dev/pts/N).
      • The subordinate side behaves exactly like a real hardware terminal device (/dev/tty), so the child program's isatty() call returns true.
      • Because isatty() is true, programs that auto-detect terminals — ls --color=auto, glow, less, most CLI tools — will emit ANSI escape sequences and enable paging, color, and cursor movement.
      • The controller end receives those escape sequences and renders them visually (e.g., nvim's :terminal buffer acts as the controller).

    • What a non-PTY subprocess is

      • When a process is launched via :!cmd in vim, a shell pipe (cmd | other), or most subprocess.run() calls, its stdin/stdout are connected to anonymous pipes, not a PTY.
      • isatty() returns false for a pipe, so well-behaved programs strip ANSI codes, disable paging, and output plain bytes.
      • Even if the program still emits escape sequences, the parent process receives them as raw text — it does not interpret or render them unless it explicitly parses ANSI.

    • Why this matters for nvim :terminal vs :!cmd

      • :vsplit | terminal glow -p % — nvim allocates a PTY for the terminal buffer; glow sees isatty() == true, enables color and paging, and the buffer renders ANSI output correctly.
      • :!glow % — nvim runs glow as a non-PTY subprocess; glow detects no terminal, disables ANSI, and the command-line area shows plain text (or garbled escape codes if the program ignores the isatty check).

    • Sources

      • The TTY demystified (Linus Åkesson)
      • POSIX pseudoterminal interface — openpty(3)