raspi-alpine-builder/resources/scripts/ab_active

 #!/bin/bash
set -e

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

if [ $current_idx -eq 2 ]; then
    echo "Active partition: A"
else
    echo "Active partition: B"
fi