How do I know if my ubuntu server needs a restart?

A file at /var/run/reboot-required is created that's used at login to tell you a restart is needed in the MOTD. You can check for the presence of this file to know if the updates you just did require a restart

To do something with this information we can wrap this in an if statement in BASH e.g:

if [ -f /var/run/reboot-required ]
then
	echo 'Restart required' 
fi

Or as a one liner:

if [ -f /var/run/reboot-required ]; then echo 'Restart required'; fi

This just prints "Restart required" but you can do something else in place of the echo if you want.

Another tip, if you use Byobu, it will flag when a reboot is required.