Convenience stores are dime a dozen in Shanghai. On every corner you are guaranteed to find a 7-11, Family Mart or an All Days. These are usually open 24 hours and you can get most things you would need in a pinch here: water, booze, eggs, candy, bread, Band-Aids and more.
Some of the stores are better than others and my personal favorite is Family Mart (is it weird to have a favorite convenience store?). These are usually cleaner and better lit with more goods. I especially like the Family Marts that have picnic tables outside where, when the weather is nice, it turns into a Family Mart Pub – I can sit outside with friends and some cheap beers.
Below are some pictures of the All Days a few steps from my apartment. Candy, gum and (bad) booze galore.