There is no guarantee that they will get paid. For that to happen, congress will have to pass a spending bill that not only authorized future spending but also explicitly gives back pay for the time they were working without paying after the shutdown.

So far this has always happened, but I do remember some noises about not doing it around the last shutdown.

So especially after those comments last time, there is no certainty that if you come in to work for the government during a shutdown you will be paid.

There's a reason the country lost credit rating, and it's because people are slightly less confident that it will actually pay its debts, including simple payroll.