S h i e l d M a s t e r
You use shields not just for protection but also for
offense. You gain the following benefits while you are
wielding a shield:
• If you take the Attack action on your turn, you can use
a bonus action to try to shove a creature within 5 feet
of you with your shield.
• If you aren’t incapacitated, you can add your shield’s AC
bonus to any Dexterity saving throw you make against
a spell or other harmful effect that targets only you.
• If you are subjected to an effect that allows you to
make a Dexterity saving throw to take only half dam -
age, you can use your reaction to take no damage if
you succeed on the saving throw, interposing your
shield between yourself and the source of the effect.
With this feat do you have to attack first? Can you declare the attack action, then use the bonus to shove and then attack?
If so, you can then knock someone prone and attack with advantage.
If you read under the section on bonus actions (PHB 189):
"You choose when to take a bonus action during your turn,
unless the bonus action's timing is specified, ..."
Since the bonus action's timing is not specified by the Shield Master feat, you can use the Shove ability as a bonus action before or after the attack as long as when your turn arrives you state you are using the Attack action. Here are two examples showing how the timing of the bonus action would be beneficial in that case:
Example: You state on your turn you are attacking. You use your bonus action to attempt to knock your target prone, and then take your attack at advantage if you succeeded on knocking the target prone.
Example: You state on your turn you are attacking. You attack your target and then use your bonus action to shove it 5 feet away. You then use your movement to move away from the target without provoking an opportunity attack since your are outside of its reach (unless the target's reach was greater than 5 feet, of course).
If the developers meant for you to only shove after attacking, the feat would have been written "
After you take the Attack action, you can use a bonus action..." not "
If you take the Attack action..." Seems pretty clear to me.
Also, nothing explicitly states you can
only move between attacks. It simple says than you can move between attacks, but in no way indicates prohibiting other actions. Especially when if you read under the Shove option for attacking. Without the bonus action, the shove counts as one of your attacks if you have the extra attack feature.
For example, a Fighter with three attacks could use one to shove a target (knocking it prone), attack with advantage (say killing it), then move up to 30 feet, and attack another target.
In another scenario the same Fighter might make an attack, then use one attack to shove and knock a target prone, then use his next to either attack at advantage on the prone target, go back to the first target, or attack a different target completely.
Obviously, the shove option as an attack can interrupt the other normal attacks. There is nothing to state the same Fighter, say as an Eldritch Knight, could not use his bonus action to cast Magic Weapon, even doing so in the middle of extra attacks granted by that feature.
So, again, unless the bonus action's timing is specified under its use, you can use it any time during your turn provided any other conditions for your turn (such as using the Attack action) are met sometime during the turn as well.
As for the Shield Master feat, it basically allows you one Shove at the cost of a bonus action when you are attacking instead of the Shove costing you an attack roll.