Raspi-image-spec/rpi3-resizerootfs

#!/bin/sh

roottmp=$(lsblk -l -o MAJ:MIN,MOUNTPOINT | grep '/$')
rootpart=/dev/block/${roottmp% /}
rootdev=${rootpart%:*}:0

flock $rootdev sfdisk -f $rootdev -N 2 <<EOF
,+
EOF

sleep 5

udevadm settle

sleep 5

flock $rootdev partprobe $rootdev

mount -o remount,rw $rootpart

resize2fs $rootpart

exit 0