primaries is of the form `/dev/fd/N', then file descriptor N is checked. If the file argument to one of th... v/stdin', `/dev/stdout', or `/dev/stderr', file descriptor 0, 1, or 2, respectively, is checked.
-a file... a size greater than zero.
-t fd
True if file descriptor fd is open and refers to a terminal.
-u file ... is enabled. The list of options appears in the description of the `-o' option to the set builtin (see sec