SD Card image shrinker

sd_shrink.sh

#!/bin/bash

######################################################
# sd_shrink.sh
# Usage: sudo bash sd_shrink.sh /dev/DEVICE FILENAME
# Backs up and shrinks DEVICE to FILENAME_YYYYmmdd.img
######################################################

set -euf -o pipefail

BACKUP_DIR=${BACKUP_DIR:-/mnt/n8storage/Tech/Linux}

err() {
  (echo >&2 "$0 ERROR: $1")
  exit 1
}

USAGE="Usage: sudo bash $0 /dev/DEVICE FILENAME"

if [ -z "${1-}" ]; then
  err "$USAGE"
fi

if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
  echo "$USAGE"
  exit
fi

if [ $# -ne 2 ]; then
  err "Wrong number of arguments.
Usage: sudo bash $0 /dev/DEVICE FILENAME"
fi

if [ $(id -u) -ne 0 ]; then
  err "Must run as root."
fi

if [ -b "$1" ]; then
  DEVICE="$1"
else
  err "$1 is not a block device"
fi

FILENAME="$2"
IMG="${FILENAME}_$(date "+%Y%m%d")".img

if [ -e "${IMG}" ]; then
  err "${IMG} already exists"
else
  pushd "${BACKUP_DIR}" > /dev/null
  echo "Backing up SD card to ${PWD}/${IMG}"
  dd if="${DEVICE}" of="${IMG}" bs=4M status=progress conv=fsync
  sync
fi

echo "Shrinking SD card"

# https://gist.github.com/toddtreece/56c16aa06905998d7c94d638cc9cd5fb
P_START=$(fdisk --list --units=sectors "${IMG}" | awk '/Linux/ {gsub("*", ""); print $2}')            # Start of 2nd partition in 512 byte sectors
P_SIZE=$(($(fdisk --list --units=sectors "${IMG}" | awk '/Linux/ {gsub("*", ""); print $3}') * 1024)) # Partition size in bytes
modprobe loop
loopdevice="$(losetup -f)"
losetup "${loopdevice}" "${IMG}" -o $(($P_START * 512)) --sizelimit $P_SIZE
fsck -fy "${loopdevice}"
resize2fs -M "${loopdevice}" # Make the filesystem as small as possible
fsck -fy "${loopdevice}"
P_NEWSIZE=$(dumpe2fs "${loopdevice}" 2> /dev/null | awk '/^Block count:/ {print $3}') # In 4k blocks
P_NEWEND=$(($P_START + ($P_NEWSIZE * 8) + 1))                                         # in 512 byte sectors
losetup -d "${loopdevice}"
echo -e "p\nd\n2\nn\np\n2\n$P_START\n$P_NEWEND\np\nw\n" | fdisk "${IMG}"
I_SIZE=$((($P_NEWEND + 1) * 512)) # New image size in bytes
truncate -s $I_SIZE "${IMG}"
sync

echo "Compressing ${IMG}"
gzip "${IMG}"

echo "Done"
echo "Output file:"
ls -lh "${IMG}".gz
popd > /dev/null