RHEL/Centos
Start off by installing yum-cron.
# Install yum-cron
$ sudo yum install yum-cron
# Configure yum-cron
$ sudo vim /etc/yum/yum-cron.conf
Set the following
update_cmd = security
apply_updates = yes
Finally start yum-cron and enable it so it’ll start automatically on boot.
$ sudo systemctl enable yum-cron
$ sudo systemctl start yum-cron
Debian
# Install unattended-upgrades
$ sudo apt install unattended-upgrades
# Select "yes" to confirm that you want to install stable packages.
$ sudo dpkg-reconfigure --priority=low unattended-upgrades
# Run it manually the first time to see that everything works
$ sudo unattended-upgrades -v