How to tell when to reboot afer updating when doing so remotely??

Use the command:

last | less

It shows login times and when then system rebooted. It also shows logoff times.

As long as you have not cron'd a reboot, you will know who rebooted the system.

Thank you actionparsnip. Useful command to know, indeed. But my question was rather when is it necessary to reboot the OS after performing an apt-get upgrade? I read online that there will be a glowing red icon, suggesting that a reboot is necessary for say updating a kernel. In most cases, there is not a need. I need to know if there is a way to tell remotely by command prompt?

Since you brought up cron'd, can you provide me a script to reboot in the early morning hours?

I appreciate your efforts.

You only really need to reboot to boot to a newly installed kernel. Kernel 4.0 is taking steps to remove this so reboots will be a thing of the past. Otherwise you can run the system indefinitely and be fine. Linux is geared around uptime.

AFAIK if a reboot is required for activating a new kernel, or core package like dbus, then a file /var/run/reboot-required is created.

So you could test for the presence of
/var/run/reboot-required
and eventually
/var/run/reboot-required.pkgs
(the latter shows the names of the packages that require the reboot)

Yes, indeed. Last night, one of the boot up messages was that a restart is required. So I rebooted. Checking /var/run this morning, there were no files of that nature. However, there are new security updates that also appeared this morning. After doing an apt-get dist-upgrade, those files reboot-required and reboot-required.pkgs in /var/run appeared. Thank you.

I see a crond.reboot file in the /var/run. But it is empty. What is this for?