isatty(1)

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

  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:

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