peach-img-builder/rpi3-resizerootfs

#!/bin/sh

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

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

partprobe

resize2fs $rootpart