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


\subsection isatty-synopsis Synopsis
\fish{synopsis}
isatty [FILE DESCRIPTOR]
\endfish

\subsection isatty-description 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.


\subsection isatty-examples 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