Is zakat obligatory on the profits of shares purchased for an Islamic bank?

Zakat is obligatory on shares if they are trade goods or were purchased with the intention of reselling them. In this case, zakat is due on their principal value and their profits. However, if the purpose of the shares is to benefit from their income rather than selling them when their price rises, and their activity is not in trade goods, then zakat is not obligatory on those shares. Rather, it becomes obligatory on their yield if it reaches the nisab (minimum threshold) and a hawl (full lunar year) has passed over it.