I have taken out a shower and the wall is down to the studs. I want to tile the wall and want to know if drywall is required under the hardy board.

No drywall is not required nor needed if using hardy board

To be specific, you only need the hardi in wet areas, which are areas within the shower or bath with shower head, up to the point of the shower head. (It is different for a steamer, though) You do not need cement board in tiled wall areas of the bathroom outside the areas defined as wet areas.

Your cement board will not waterproof your shower, nor will your thinset, tile, grout, or sealer. You have two waterproofing options. First option is to use a moisture barrier behind the cement board that overlaps the flange of the tub or shower, most commonly used are 4 mil plastic sheeting or 15 lb roofing felt, the second option is surface waterproofing applied to the cement board, such as Custom's redgard, or Laticrete's Hydroban (at Lowe's, Laticrete merchandises "Floor and wall watertight") which are liquid membranes or you can use sheet style membranes such as Nobleseal TS or Kerdi.

You do need some sort of waterproofing, but generally only one kind is to be used to prevent trapping moisture.

Make sure you insulate any exterior walls between the studs. Use unfaced insulation or cut slashes in the paper faced insulation. I insulated all the walls in my shower for sound deadening mostly. I then used 15lb. Felt on the walls followed by 1/2" hardi backerboard. Mark where your studs are on the wall above so you can screw the backerboard on. Use the screws for hardi board and also use the special fiberglass tape for hardi board. Apply modified thinset to the joints and then the tape. Scrape smooth with a putty knife. I let the thinset cure and then applied red guard to the joints as a precautionary measure.

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I agree and go one step further, I insulate all walls in a bathroom to keep sound down.

I wish I could insulate all the walls in my house but... that's why I did the shower area since I tore those walls out. It does help on the sound from taking a shower especially around the water pipes.