isatty            226 ext/readline/readline.c     if (!isatty(0) && errno == EBADF) rb_raise(rb_eIOError, "closed stdin");
isatty           2937 io.c               if (isatty(fptr->fd) == 0)
isatty           4225 io.c               if (isatty(fptr->fd))
isatty           5801 io.c               if (!isatty(fd)) {