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At the beginning of conversation, people often say:
- How are you doing?
- how's it going?
- what's going on?
- what's up?
- what are you doing now?
- Are you doing Ok?
Could you tell me the usage and meaning of these questions?
What question we often say in formal situation and in formal situation?
They are "general pleasantries", with no specific meaning [as a rule], but meant to indicate a positive approach to the other person while not expecting a direct reply to the question.
Formally you would say "How do you do?"; the examples you have given are all informal.
Greeting 
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