In the army do you have to call at ease for the 1SG when the commander is already in the building?

No. The commander is a higher rank the the 1SG. Same thing would apply if just a LT was in the room

No if the Commander leaves and the 1SG walks in then you would call at ease. In the real army you wouldn't call at ease for a 1SG only your CSM. You call attention for the Commander and you don't go to parade rest you stand at attention until he says at ease.

Check out FM 7-21.13, under Courtesies.

As for your question, it depends - there are many scenarios I can give you. But if they are the senior ranking NCO entering or exiting the building, you will call AT EASE regardless of which officer may already be in the building. Just think of the times a BN or BDE CSM walks in...

If they are the senior ranking NCO who works in the building, you will call AT EASE when they first enter and then again when they last exit. And again, this will be done regardless of what officer may be in the building.

If this takes place in a room, then this will be based on rank of the Soldiers in the room. In your case, if there's a senior 1SG, CSM or CW3-CW5/CPT or above, then no (2LTs and 1LTS are just commissioned PVTs and WO1s and CW2s are commisioned SGTs/SSGs mostly, and in my opinion they should also render that senior NCO this courtesy).

TEN HUT! When an Officer regardless if their rank is 2nd LT or a Full Bird Colonel you call Attention regardless whether the 1sg is there or not, and you snap to attention and you salute and you hold that salute until that salute is returned Your Drill SGT teaches you that in basic training

This is what the FM 7-21.13 has to say. It depends on previous instructions from Top and the CO. They may not want it done during they day as they go in and out of the building and into their respective offices.

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When an officer of superior rank enters a room, the first soldier to recognize the officer calls personnel in the room to attention but does not salute. A salute indoors is rendered only when reporting.

When an NCO of superior rank enters the room, the first soldier to recognize the NCO calls the room to "At ease."

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AR 600-20 has more.

Never hurts when unsure though. Better to over do it then under do it.

SSG US Army 73-82

You only call attention and at ease for the commander or a higher level commander when they enter the building or a room. You don't stand for enlisted soldiers.

No. The Commander is already there. He is higher ranking and you should already be at ease by his command.

It depends on the 1SG.

Generally, No.

You don't call at ease the officer does whether the 1sgt is there or not.