干嘛

Highly colloquial. Two uses: (1) as a question word meaning 'what are you doing' (你在干嘛), (2) as an exasperated 'why on earth' (干嘛这样). Tone is casual and can sound challenging.

mistakes

干嘛 and 干什么 are near-equivalents, but 干嘛 is chattier and shows more emotion. Mainland writers often use the variant 干吗; both spellings share the same rising-tone má reading. Note the 嘛 here is rising tone, not the toneless particle 嘛 of sentences like 他是学生嘛.