How do you do? How are you today?

Note that 'How are you?' isn't really a greeting in itself. 'Good morning' , 'Hello', and 'Hi' (in descending order of formality) are greetings. The greeting may then be followed by a brief simple exchange such as: 'How are you?' 'Very well, thanks. How are you?' 'Fine, thanks.' This exchange - or a variation on it - is what I'd always use with friends, neighbours, relatives, colleagues, business contacts. It's a polite, neutral way to begin any conversation And to answer your question - yes, 'How are you?' is acceptable in any situation. You are right, that there are many variants. 'How are you doing?' is more informal, so you wouldn't say that to a client in a business context, for example. 'How's it going?' is more informal still. Some variants are regional. For example, some younger Americans might say 'Wassup, bro?' to each other, just as many British men might say 'Alright, mate?' to each other. But please don't assume that any of these informal and regional variants are standard English - if you were to use them in the wrong context you could confuse or even offend the other person. If you want to enquire about somebody's health or wellbeing, your best option is simply to say 'How are you?' - it's standard English, polite and neutral, and acceptable everywhere. Also note that you don't have to say this to a person you are meeting for the first time. When you are first introduced to someone, 'Hello. It's nice to meet you' is fine.

I always heard young people say"what's up?"as greeting.

yo whats up homie

Do you want polite or friendly? During my time in Hong Kong, I found all the polite greetings people learned in school were kind awkward. Polite, but awkward. I don't think having awkward conversations.