It's a joke on scarcity of food etc. No (availability of) shit to eat on most days > he doesn't have to eat shit. On other days shit is available but no shovel

It's a joke on scarcity of food etc. No (availability of) shit to eat on most days > he doesn't have to eat shit. On other days shit is available but no shovels are available > he doesn't have to eat shit because there's no shovel to measure it/eat it with. He knew this would happen because in Russia the food stores were more often than not empty and tools would be often "misplaced"/stolen.