.. _cmd-isatty:

isatty - test if a file descriptor is a terminal
================================================

Synopsis
--------

::

    isatty [FILE DESCRIPTOR]

Description
-----------

``isatty`` tests if a file descriptor is a terminal (as opposed to a file). The name is derived from the system call of the same name, which for historical reasons refers to a teletypewriter (TTY).

``FILE DESCRIPTOR`` may be either the number of a file descriptor, or one of the strings ``stdin``, ``stdout``, or ``stderr``. If not specified, zero is assumed.

If the specified file descriptor is a terminal device, the exit status of the command is zero. Otherwise, the exit status is non-zero. No messages are printed to standard error.


Examples
--------

From an interactive shell, the commands below exit with a return value of zero:



::

    isatty
    isatty stdout
    isatty 2
    echo | isatty 1


And these will exit non-zero:



::

    echo | isatty
    isatty 9
    isatty stdout > file
    isatty 2 2> file

