Detect Whether Running as Root

# Bail if not root
if [[ $EUID -ne 0 ]]; then
    echo "This script must be run as root (or via sudo)." >&2
    exit 1
fi

# Alternative: auto re-exec with sudo
if [[ $EUID -ne 0 ]]; then
    exec sudo -E "$0" "$@"
fi

# Bail if IS root (some scripts shouldn't run as root)
if [[ $EUID -eq 0 ]]; then
    echo "Don't run this as root." >&2
    exit 1
fi

# Reusable
is_root() { [[ $EUID -eq 0 ]]; }
need_root() { is_root || { echo "Need root" >&2; exit 1; }; }

need_root
apt update && apt upgrade -y