Oh shell jackets were worn by most if not all of the Infantry.
The Confederate shell jacket was not an officer, cavalry or artillery item, it was an item for all branches uniform. The single breasted shell jacket was born out of the necessity, that became fashionable, its single most valuable attribute was it saved cloth, which was in too short a supply to meet the Army regulation for a frock coat for everyone, and used less cloth than sack coats as well, which outside of civilian ones sent from home, were a rarity in the ranks. The vast majority of sack coats at reenactments are made on patterns that really didn't exist by the way.
Most Confederate soldiers preferred cloths made at home over government issue uniforms, but the Richmond Depot was in full swing by the time of Gettysburg, and if a soldier was from say Texas, in the Texas Brigade, a care package from home with a nice "homespun" sack coat or frock coat was a pipedream by that time, same goes with troops from a lot of other States , as there wasn't enough material to make 3 frock coats and trousers by some mother to send to her sons, so the order of the day at Gettysburg was gonna "Commutation" shell jackets (home made/made by a bunch of ladies in sewing societies for the Government),and type 2 Richmond Depot jackets for everybody at Gettysburg infantry or not.
There will always be exceptions, but as a rule you can expect the majority of everybody in single-breasted shell jacket, more than likely without branch of service trim, which was comparatively rare in the Eastern Theater.
To be honest I'm surprised at this inquiry, I'd about decided it was common knowledge. But one doesn't know until they ask.