Reading from /dev/random does not necessarily ensure that /dev/urandom is seeded because it relies on an implementation detail. But reading from /dev/random once and using that numbers to explicitly seed /dev/urandom by writing to it will work without relying on implementation details. That is why I would prefer doing it explicitly by writing to /dev/urandom instead of implicitly by reading from /dev/random although it makes no (big) difference given the current implementation. But this is what you suggested elsewhere anyway.