isatty - test if a file descriptor is a tty.
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Synopsis
--------

isatty [FILE DESCRIPTOR]


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

`isatty` tests if a file descriptor is a tty.

`FILE DESCRIPTOR` may be either the number of a file descriptor, or one of the strings `stdin`, `stdout`, or `stderr`.

If the specified file descriptor is a tty, 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:

\fish
isatty
isatty stdout
isatty 2
echo | isatty 1
\endfish

And these will exit non-zero:

\fish
echo | isatty
isatty 9
isatty stdout > file
isatty 2 2> file
\endfish