isatty(1)                            fish                            isatty(1)

       isatty - test if a file descriptor is a tty.

       isatty [FILE DESCRIPTOR]

       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.

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

       isatty stdout
       isatty 2
       echo | isatty 1

       And these will exit non-zero:

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

Version 3.0.2                   Tue Mar 26 2019                      isatty(1)