How to Schedule a Future Reboot in Ubuntu Using the shutdown Command

Why Schedule a Reboot?

In a server environment or a headless Ubuntu workstation, you often need to install kernel updates or apply heavy system configurations that require a full system restart. However, if you are working during active business hours, restarting the server immediately will disconnect all active users and interrupt database queries. Instead of staying awake until 3:00 AM to manually reboot the machine, you can use the built-in shutdown command in the Linux terminal to schedule an automated reboot for a future time, ensuring minimal downtime for your users.

Using the shutdown -r Command

The shutdown command handles both powering off and restarting the machine. To schedule a restart, you must use the -r (reboot) flag.

1. Open your Terminal (Ctrl + Alt + T), or connect to your Ubuntu server via SSH.
2. The syntax requires root privileges, the -r flag, and a time variable. The time variable can be a specific hour and minute (in 24-hour format) or a relative number of minutes from now.

Method A: Reboot at a Specific Time

To schedule the server to reboot at exactly 3:00 AM local time, use the following command:

1. sudo shutdown -r 03:00
2. Press Enter and provide your administrator password. The terminal will confirm that the reboot has been scheduled.

Method B: Reboot After a Delay

If you just sent out a warning email to your team and want to give them exactly 45 minutes to save their work before the server bounces, use the + symbol:

1. sudo shutdown -r +45
2. Press Enter. The system will count down 45 minutes and then automatically restart.

Adding a Warning Message

It is best practice to broadcast a warning message to any users currently logged into the server via SSH. You can append a custom message to the end of your command in quotes:

1. sudo shutdown -r +15 "Applying kernel updates. System going down in 15 mins!"

How to Cancel a Scheduled Reboot

If you made a mistake or need to delay the maintenance window, you can easily abort any pending shutdown or reboot command.

1. Simply run the shutdown command with the -c (cancel) flag:
2. sudo shutdown -c
3. The scheduled reboot is instantly canceled, and the server will remain online.