check_upgrade.sh

#!/usr/bin/env bash

packages_to_upgrade=()
regex="The following packages will be upgraded:\s(.*)[0-9]+ upgraded, [0-9]+ newly installed, [0-9]+ to remove"

check_update () {
    sudo lxc-attach -n $1 -- apt-get update -qq
    res=$(sudo lxc-attach -n $1 -- apt-get upgrade --show-upgraded --assume-no)
    res="${res//[$'\t\r\n']}"
    if [[ "$res" =~ $regex ]]; then
	packages_to_upgrade+=${BASH_REMATCH[1]}
    fi
}

# Check upgrades for all active containers
for cx in `sudo lxc-ls --active --line`; do
    check_update $cx
done

# Check upgrade for the Host
apt-get update -qq
res=$(apt-get upgrade --show-upgraded --assume-no)
res="${res//[$'\t\r\n']}"
if [[ "$res" =~ $regex ]]; then
    packages_to_upgrade+=${BASH_REMATCH[1]}
fi

# Send email to root if there are upgrades available
if [ ! -z "$packages_to_upgrade" ]; then
    res=$(echo $packages_to_upgrade | tr ' ' '\n' | sort | uniq -c)
    echo -e "Packages waiting for upgrade\n\n$res" | mail -s "Proxmox upgrade list" root
fi