I'm pretty sure that there are exactly zero units of the Iranian military that have independent authority to, for example, start shooting missiles at US Navy warships. They may be able to act independently once they understand that it's time to open fire. But to decide to start... that's a central command decision.
But they will start shooting when they learn that a conflict has started. You know, if a US attack in the strait begins it will become obvious pretty pretty soon. Also they don't need a sophisticated link from the top to start an attack, just a simple command.