There are 2 ways that you can use:
What are you up to? or
What have you been up to?
These two got very similar meanings, but the difference is the time line. Both mean what have you been doing? Is there any new? any plan? What are your situations right now.
When someone asks " what have you been up to", this means that you and person you are having a conversation with, have not met or talked for a while.
For what you are up to,, it mostly concerns with your current situation. these two questions expect you to answer your conversation partner? It is not like " How's it going, what's up or how are you doing", these types of greeting do not require you to respond back. Just smile and moving on. But if you answer,, nothing is wrong with that.
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