HOWTO Stop Ubuntu from Automatically Updating on it's own
Newer versions of Ubuntu will automatically check for updates and download and install them on it's own. This can be extremely annoying if you start Ubuntu on a computer which has not been used in a while and you actually need to get work done. It is specially annoying if you have a metered or slow tethered Internet connection. Stopping it from automatically updating when you absolutely do not want it to is easy enough.
Removing The Update Trigger[edit]
Ubuntu uses a systemd feature called "timers" to trigger it's automatic actions. A list of all the jobs that are started by timers on machines using systemd for boot and service management can be generated with the command
systemctl list-timers
This will produce quite the list on a default Ubuntu installation. Two jobs are relevant for automatic updates: apt-daily.timer and apt-daily-upgrade.timer
The apt-daily.timer will update the package cache (apt-get update). The job apt-daily-upgrade.timer is the job that actually upgrades the packages that are installed on the system. These timers can be disabled by running systemctl disable but the ultimate solution is to mask them so they are never ever started ever again:
sudo systemctl mask apt-daily.timer
sudo systemctl mask apt-daily-upgrade.timer
You will, of course, have to manually update your system a few times a year or month with apt-get update && apt-get upgrade if you disable automatic updates.
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