etc/cron.daily/apt-compat

#!/bin/sh

set -e

# Systemd systems use a systemd timer unit which is preferable to
# run. We want to randomize the apt update and unattended-upgrade
# runs as much as possible to avoid hitting the mirrors all at the
# same time. The systemd time is better at this than the fixed
# cron.daily time
if [ -d /run/systemd/system ]; then
    exit 0
fi

check_power()
{
    # laptop check, on_ac_power returns:
    #       0 (true)    System is on main power
    #       1 (false)   System is not on main power
    #       255 (false) Power status could not be determined
    # Desktop systems always return 255 it seems
    if which on_ac_power >/dev/null 2>&1; then
        if on_ac_power; then
            :
        elif [ $? -eq 1 ]; then
            return 1
        fi
    fi
    return 0
}

# sleep for a random interval of time (default 30min)
# (some code taken from cron-apt, thanks)
random_sleep()
{
    RandomSleep=1800
    eval $(apt-config shell RandomSleep APT::Periodic::RandomSleep)
    if [ $RandomSleep -eq 0 ]; then
	return
    fi
    if [ -z "$RANDOM" ] ; then
        # A fix for shells that do not have this bash feature.
	RANDOM=$(( $(dd if=/dev/urandom bs=2 count=1 2> /dev/null | cksum | cut -d' ' -f1) % 32767 ))
    fi
    TIME=$(($RANDOM % $RandomSleep))
    sleep $TIME
}

# delay the job execution by a random amount of time
random_sleep

# ensure we don't do this on battery
check_power || exit 0

# run daily job
exec /usr/lib/apt/apt.systemd.daily