debian offline update

offline-update.sh

#!/bin/bash
export DEBIAN_FRONTEND=noninteractive
export DEBCONF_NONINTERACTIVE_SEEN=1
export TERM=linux
if [ $$ -ne 1 ] ; then
    set -e
    apt update
    apt install kexec-tools -yq
    apt full-upgrade -yq --download-only
    sync
    if [ ! -f /usr/libexec/offline-update ] ; then
        install -Dm755 $0 /usr/libexec/offline-update
    fi
    kexec \
      -l /boot/vmlinuz-$(uname -r) \
      --initrd=/boot/initrd.img-$(uname -r) \
      --append="$(cat /proc/cmdline) init=/usr/libexec/offline-update"
    kexec -e
else
    mount -o remount,rw /
    apt full-upgrade -yq
    sync
    reboot -f
fi