Being friendly, but they could also be the the start of a conversation in which you could share interests with someone - and in a conversation you also listen to the other person (right?).
In no way is it being disrespectful (unless of course you violently shake their hand and you use a rather derogatory tone with them, and are incredibly rude when saying "hello" and smile disgustingly.
To me it depends on a lot of things because actually it could be all four of the options. So I'd answer the question properly if I was able to see the smile, hear the "hello" and see the handshake (but even then you never know what goes on in people's heads). - Sorry for time wasting but I attempted to answer the question!
