Upgrade a GNU/Linux Debian or Ubuntu system with only one command

update.sh

#/bin/sh

if [ "`whoami`" != 'root' ]; then
        echo ''
        echo 'Only root can execute this script'
        echo ''

        exit 1
fi

export DEBIAN_FRONTEND=noninteractive
export NEEDRESTART_MODE=a
export DEBIAN_PRIORITY=critical

apt clean
apt autoclean

apt update
apt \
        -o Dpkg::Options::="--force-confdef" \
        -o Dpkg::Options::="--force-confold" \
        -o APT::Get::Always-Include-Phased-Updates=true \
        --assume-yes \
        --allow-unauthenticated \
        --allow-downgrades \
        dist-upgrade

apt autoremove --assume-yes