Unattended-Upgrade::Automatic-Reboot default value

The default for automatically rebooting after unattended-upgrades is FALSE.

You can confirm on your own system by looking where the default value is stored, in the script itself:

grep 'Automatic-Reboot"' /usr/bin/unattended-upgrade

The default could change with a software upgrade, although that seems unlikely. To be certain your systems don't reboot by after automatic upgrades, explicitly disable rebooting in your configuration.

If you don't want it to auto-reboot, make that section look like this:

// Automatically reboot *WITHOUT CONFIRMATION*
//  if the file /var/run/reboot-required is found after the upgrade 
Unattended-Upgrade::Automatic-Reboot "false";

I have it set that way and it works.

This page says that not rebooting is the default, but I'm pretty sure that's wrong. I think the first time my system autoinstalled a kernel (before I changed the config), it rebooted. But in either case, the config above will prevent it.