Answer:
I have a straight back and sharp teeth to cut objects.
Everyone in the world needs me. They generously give me but never take me. What am I?
I am a band that does not perform sing or act.
Fashions change, but what can a person wear that is never out of style?
I can be long or short. I can be grown or bought. I can be painted or left bare. My tip can be round or square. (one word)
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.