A new feature of Ubuntu 18.04 LTS that wasn't in Ubuntu 16.04 LTS is that you can configure it to automatically install security updates. You can see that here on the installer screen:

I must confess, though, that I have mixed feelings about this. I mean, it's nice that the security updates get installed without me having to do anything, but a lot of those updates require that the system be rebooted before they can take effect. By default, Ubuntu systems don't automatically reboot after an update is installed. If you keep it that way, you'll see a message about it when you log in to the system. But if you prefer, you can set Ubuntu to automatically reboot after it automatically updates itself. Here's how to do this:

1. Go into the /etc/apt/apt.conf.d directory and open the 50unattended-upgrades file in your favorite text editor. In the vicinity of line 68, you'll see a line that says the following:

//Unattended-Upgrade::Automatic-Reboot "false";

2. Uncomment the line by removing the leading slashes, and change false to true, like so:

Unattended-Upgrade::Automatic-Reboot "true";

3. With this new configuration, Ubuntu will now reboot itself immediately after the automatic update process has completed. If you'd rather have the machine reboot at a specific time, scroll down to about line 73, where you'll see this line of code:

//Unattended-Upgrade::Automatic-Reboot-Time "02:00";

4. Since this line is commented out with its pair of leading slashes, it currently has no effect. To have the machine reboot at 2:00 A.M., just uncomment this line. To have it reboot at, say, 10:00 P.M., uncomment the line and change the time to 22:00, like so:

Unattended-Upgrade::Automatic-Reboot-Time "22:00";

Of course, there's that old, basic precept that thou shalt not install system updates on a production system without first testing them on a test system. Any operating system vendor can occasionally supply you with problematic updates, and that has included Ubuntu. (I know what you're saying: Preach it, Donnie.) Ubuntu's automatic update feature is in direct opposition to that basic precept. If automatic updates have been enabled, disabling them is quite easy, if you choose to do so:

1. To disable automatic updates, just go into the /etc/apt/apt.conf.d directory and open the 20auto-upgrades file in your favorite text editor. What you'll see is this:

APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";

2. Change the parameter for that second line to 0, so that the file will now look like this:

APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "0";

Now, the system will still check for updates and show a message at the login screen when any are available, but it won't automatically install them. And of course, it should go without saying that you need to check your systems on a regular basis to see if updates are available. If you do prefer to leave automatic updates enabled, be sure to either enable automatic rebooting or to log in to the system at least a couple of times a week to see if it needs to be rebooted.

3. If you want to see if there are any security-related updates available, but don't want to see any non-security updates, use the unattended-upgrade command, like so:

sudo unattended-upgrade --dry-run -d

4. To manually install the security-related updates without installing non-security updates, just run the following line of code:

sudo unattended-upgrade -d

You can also use the apt command to install only the security updates, but it would require piping the apt output into a convoluted set of text filters in order to mask the non-security updates. Using the unattended-upgrade command is much easier.