干啥呢/干什么呢 = what are you doing?
干啥/干什么 = what's up?
If someone call your name in a sudden, you can reply him/her with '干啥'. (the same meaning as '怎么了')
It implies the meaning '你叫我干啥?'='what's up?/why do you call me?'
but if '干啥/干什么' is not used as a single sentence, it means 'to do something'. for example,
你在干啥?= What are you doing?
你要我干啥?= What do you want me to do?
我得干点啥。= I need to do something.