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\section isatty isatty - test if a file or file descriptor is a tty.
\subsection isatty-synopsis Synopsis
\fish{syn}
isatty [FILE | DEVICE | FILE DESCRIPTOR NUMBER]
\endfish
\subsection isatty-description Description
`isatty` tests if a file or file descriptor is a tty.
The argument may be in the form of a file path, device, or file descriptor
number. Without an argument, `standard input` is implied.
If the resolved file descriptor is a tty, the command returns zero. Otherwise, the command exits one. 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 /dev/fd/1
\endfish
And these will exit non-zero:
\fish
echo | isatty
isatty /dev/fd/9
isatty stdout > file
isatty 2 2> file
\endfish
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