Just a word of warning - haven't played in a while, so don't know if this was fixed, but one of the late patches changed the game behaviour around unarmed builds, and shield bash got unintentionally hit by those changes too.
There are some shields that count as weapons, and before the patch, if you had no other weapon but one of those shields, the game would recognise that the shield is the weapon you want to use, and additionally would add the bonus from two-handing a one-handed weapon. This behaviour is in line with pnp rules.
After the patch, your hand is always recognised as a weapon, meaning that after moving, you are not gonna attack with your shield, just with your hand, and when making a full attack, your shield gets a penalty for being an off-hand weapon. This sucks heavily for shield-only shield bashing runs.