Government Shutdown 2018 Explained
What Happens When the Government Shuts Down
The discretionary budget funds most federal departments. But those that provide essential services are not shut down. Essential services are those that include defense, national safety, and security. Many of these agencies are set up so they can operate for weeks without a funding bill. The Defense Department warned it wouldn't pay military personnel during a shutdown.
Border Protection and Immigration, air traffic controls, and the Transportation Security Administration remain open. The Justice Department remains open, but gun permits will not be issued during the shutdown. The Postal Service has a separate source of funds, so mail continues to be delivered.
https://www.thebalance.com/government-shutdown-3305683