The reason you see the *** System restart required *** notice when you sign into Ubuntu/Debian is that you need to reboot your system.
You must reboot your Linux box when you install a new kernel or update critical libraries such as libc. This happens in both Ubuntu and Debian Linux systems where they take the initiative to tell you if the system needs a reboot when you log in to your box as a root or sudoer user.
The System restart required message indicates the presence of the file /var/run/reboot-required. The presence of that file indicates there is a package that has requested a system reboot.
Generally, a reboot is usually required when an update such as security patches to the Linux kernel has been installed that only come into effect after reboot.
You can run this command to find the packages that requested the reboot.
$ cat /var/run/reboot-required.pkgs
The result is a list of packages awaiting reboot for example:
You typically don’t need to restart your Linux system the moment you see the restart message. It is important to note that you will be vulnerable to any security problems fixed in the new kernel until you reboot.
To get rid of the System Restart Required message without restarting, you can remove the reboot flag file using this command.
$ sudo rm /var/run/reboot-required
We do not advise this as you may tend to forget to reboot the system, therefore, keeping the *** System restart required *** notice alive.