raspi-alpine-builder/resources/scripts/ab_flash

 #!/bin/bash
set -e

image_file=$1

if [ -z $image_file ]; then
    echo "USAGE: $0 [IMAGE_PATH]"
    return 1
fi

# change to directory containing update file
cd $(dirname $image_file)

# check integrity of image
sha256sum -c ${image_file}.sha256

# get current partition index
current_idx=$(rdev | sed 's#/dev/mmcblk0p\([^ ]*\).*#\1#')

if [ $current_idx -eq 2 ]; then
    echo "Start update for partition B"
else
    echo "Start update for partition A"
fi

flash_device="/dev/mmcblk0p${flash_idx}"

# flash device
gunzip -c ${image_file} | dd of=${flash_device} status=progress


# switch active partition
mount -o remount,rw /uboot
/sbin/uboot_tool part_switch
sync
mount -o remount,ro /uboot

echo "Update complete -> please reboot"