Will most all stores in Strasbourg be shut down on a Sunday in the summer? If so what will be open?
French and German stores all close on Sundays.
Small tourist area shops catering to tourists may be open. All tourist attractions will be open except on religious holidays.
Restaurants will be open.
Shops catering to local residents will be closed.
What are you trying to buy?
Most likely will window shop but picturing a ghost town. Petite France looks lovely.
It won't be a ghost town - at least some restaurants and cafes will be open, as will all cultural sites, so both locals and tourists will be on the street.
Germany (along with Austria and Switzerland) has strict laws about store hours, and almost all stores are closed on Sundays - year round. If you need something on a Sunday in one of these countries, go to the train station, as stores there have much looser rules and can be open Sundays (that's why there's usually a supermarket in German train stations - so it can be open Sundays).
Restaurants, cafes, museums, tourist attractions, etc will all be open Sundays. Museums are much more likely to close Mondays than any other day, but if you're interested in a particular museum or sight, check its hours online to avoid disappointment.
Small towns with a tourist economy (e.g. those in the general region of Strasbourg), tend to close down on Mondays. I have been in small towns in France where I couldn't find a cup of coffee on Monday. Tourist attractions will be open on Sundays but shops except for tourist tat won't be most places.
Bakeries are almost always open on Sunday morning. Larger grocery stores are too, but not after noon.
Usually bakeries in an area get together and decide which days they take off so there will always be at least one bakery in an area open Sat or Sun mornings with others taking the day off and any given bakery might be closed on some other day of the week as well. Near an apartment we spent several months in in the 17th there were 3 bakers quite near us, all of which closed on Sunday morning, but there was one just a couple of blocks further on open. I discovered it by noticing people carrying baguettes and asking where they got them.
From mid July to mid August the European parliament will not be in session so more Sunday closings might be in effect during that period.
Be careful, many restaurants, if they open, will only be open in the evening on Sundays.
I just realized you asked about Strasbourg, which is in France, not Germany. Sorry - brain fart.