fish-shell/share/functions/isatty.fish

function isatty -d "Tests if a file descriptor is a tty"
    set -l fd 0
    if count $argv >/dev/null
        switch $argv[1]

            case -h --h --he --hel --help
                __fish_print_help isatty
                return 0

            case stdin ''
                set fd 0

            case stdout
                set fd 1

            case stderr
                set fd 2

            case '*'
                set fd $argv[1]

        end
    end

    # Use `command test` because `builtin test` doesn't open the regular fd's.
    # See https://github.com/fish-shell/fish-shell/issues/1228
    command test -t "$fd"

end