It is not in the PHB, it is a from a JC comment. As I said earlier, I am not going to waste my time finding it just to satisfy you. Instead I will simply direct you to the latest SA on the Shield Master feat:
Shield Master
[NEW] The Shield Master feat lets you shove someone as a bonus action if you take the Attack action. Can you take that bonus action before the Attack action? No. The bonus action provided by the Shield Master feat has a precondition: that you take the Attack action on your turn. Intending to take that action isn’t sufficient; you must actually take it before you can take the bonus action. During your turn, you do get to decide when to take the bonus action after you’ve taken the Attack action. This sort of if-then setup appears in many of the game’s rules. The “if” must be satisfied before the “then” comes into play.
As long as the attack action is comes before the actually attacking (as the sanctuary example helped reveal) then you can take the attack action, shield master shove, attack then attack again. Nothing in our interpretation goes against this quote.
The text "During your turn, you do get to decide when to take the bonus action after you’ve taken the Attack action." clearly indicates you can decide when to take the bonus action AFTER you've taken the Attack action. The timing has now been specified: after you've taken the Attack action. You are no longer free to take it whenever you want.
I agree. But the attack action happens before your actual attacks. That's the key to piecing it back together.
You don't. How can you have taken the Attack action without Making an Attack and proceeding to the Steps to do so? To say you are taking the Attack action and then ignore what needs to follow to constitute taking the Attack action is nonsensical.
Because the sanctuary discussion revealed that you can take the attack action and never actually attack.
Because the disengage discussion revealed that you must take the disengage action and have it end before you can move again.
The individual evidences may not be wholly convincing. I get that. But the evidences all considered as a whole together definitely make a strong case that the actions in general and more importantly, the attack action all happen before the attacks it provides.
No, cause and effect are not simultaneous. Of course they are not. No one ever said they were, did they? But to satisfy the cause of taking the attack action you must make an attack, otherwise you have not taken the Attack action.
Sanctuary proves otherwise. You don't make an attack with sanctuary, you lose your attack.
You have not taken the Attack action until you choose a target for your attack (Step 1 from Making an Attack).
Okay, then I'll just shield master shove after choosing the target for my attack. You agree I can take the shove anytime on my turn after I've taken the attack action right? And since I've taken the attack action after choosing my target then I should be able to shield bash before determining modifiers and resolving the attack right?
There is no time in-between taking the Attack action and making your attack.
Do you have a rule or any evidence for this? Our side has provided evidence that actions come before their effects, including the attack action. What supports your belief that it doesn't?
If you say you are taking the Attack action, then Shove before Making an Attack, you are setting yourself up for paradox as in the scenario I proposed in post #952.
No. As long as you have taken the attack action then whether you actually can attack or not is irrelevant. In our scenario it would be take attack action. shield master shove. get stunned. SHOOT I lost my attacks. DM reviews situation and finds nothing that broke the rules as the attack action was taken before the shield master shove.
What I don't understand is the point of all this at this stage. Can you simply not accept how this works? Do you feel you need to debate it to justify the fact that you want to be able to utilize the Shove bonus action before making your attacks on your turn?
I can't answer for Arial but I don't want to be able to use it on my turn. I wanted to be right. My original belief had always been that the rules only allow the shield master shove after the attacks. I was thrilled when JC joined my side. So no, I don't want to use shield master after the attack action. I would much rather have been right.
However, I have found compelling evidence in this thread that I was wrong.
It is simply enough to just house-rule it. As JC states, the bonus action comes AFTER the Attack action. How can you be after the Attack action if you have not taken it. To start to take it, is not to have taken it.
Bad arguments deserve that response. The argument in this case is very persuasive.
For our group we'll probably just house-rule it to allow the Shove after you have made at least one attack. I'm done debating it. If you want to rule it your way feel free. If I have the time and find the JC quote, I WILL POST IT just to satisfy you and others like you.

Until then have a great game!
Yep, even if JC had been right in this case then I would have houserulled it. But my reason was that I'd played it by his original ruling for so long that I wasn't changing it now. It is nice knowing I don't have to houserule it though.