Raspi-image-spec/rootfs/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs

#!/bin/sh
set -e

#
# List the soft prerequisites here.  This is a space separated list of
# names, of scripts that are in the same directory as this one, that
# must be run before this one can be.
#
PREREQS=""
case $1 in
    prereqs) echo "$PREREQS"; exit 0;;
esac

. /scripts/functions

# Given the root partition, get the underlying device and partition number
rootpart=$(realpath $ROOT)
rootpart_nr=$(blkid -sPART_ENTRY_NUMBER -o value -p $rootpart)
rootdev="/dev/$(lsblk -no pkname "$rootpart")"

# Parted will detect if the GPT label is messed up and fix it
# automatically, we just need to tell it to do so.
parted -s $rootdev print 2>&1 | grep -z "fix the GPT" && {
    echo "Fix" | parted ---pretend-input-tty $rootdev print
}

# Check if there's free space at the end of the device
free_space="$(parted -m -s $rootdev print free | tail -n1 | grep free)"
if test -z "$free_space"; then
    # Great, we already resized; nothing left to do!
    exit 0
fi

log_begin_msg "$0 resizing $ROOT"

# Unmount for safety
umount "${rootmnt}"

# Expand the partition size to fill the entire device
parted -s $rootdev resizepart $rootpart_nr 100%

wait_for_udev 5

# Now resize the filesystem
partprobe $rootdev
resize2fs $rootpart

# Remount root
if ! mount -r ${FSTYPE:+-t "${FSTYPE}"} ${ROOTFLAGS} "${ROOT}" "${rootmnt?}"; then
    panic "Failed to mount ${ROOT} as root file system."
fi

log_end_msg