shrink_image

#!/bin/sh
# Part of spindle http://asbradbury.org/projects/spindle
#
# See LICENSE file for copyright and license details

# TODO: this script is very rough and ready. Currently shrinks ext4 image to 
# minimum size. Should really be refactored and merged with grow_image

set -x

if [ $(id -u) -ne 0 ]; then
  printf "Script must be run as root\n"
  exit 1
fi

if [ ! "$1" ]; then
  printf "Usage: ./shrink_image IMAGEFILE\n"
  exit 1
fi
IMAGEFILE="$1"

# Partition resizing code same as in http://github.com/asb/raspi-config
PART_START=$(parted "$IMAGEFILE" -ms unit s p | grep "^2" | cut -f 2 -d:)
if [ ! "$PART_START" ]; then
  printf "Failed to extract root partition offset\n"
fi

# Just use losetup to create a loopback device. fdisk will operate on files, 
# but it complains about the number of cylinders not being set

LOOP_DEV=$(losetup -f)

# We know the offset anyway so let's just use losetup directly rather than 
# bothering with kpartx
PART_START_BYTES=$((${PART_START%s}*512))
losetup --offset "$PART_START_BYTES" -v $LOOP_DEV "$IMAGEFILE"

# Sanity check
file -s $LOOP_DEV

# Run resize2fs
e2fsck -f $LOOP_DEV
resize2fs -M $LOOP_DEV

EXT4_BLCKCNT=$(sudo tune2fs -l /dev/loop0 | grep "^Block count" | cut -d ":" -f 2 | tr -d ' ')
EXT4_BLKSIZE=$(sudo tune2fs -l /dev/loop0 | grep "^Block size" | cut -d ":" -f 2 | tr -d ' ')
EXT4_FS_BYTES=$(($EXT4_BLCKCNT * $EXT4_BLKSIZE))

# Unmount it all
losetup -d $LOOP_DEV

NEW_IMG_BYTES=$(($EXT4_FS_BYTES + $PART_START_BYTES + 4194304))

# Shrink the image file
dd if=/dev/null of="$IMAGEFILE" bs=$NEW_IMG_BYTES seek=1

losetup -v $LOOP_DEV "$IMAGEFILE"

fdisk -cu $LOOP_DEV <<EOF
p
d
2
n
p
2
$PART_START

p
w
EOF

losetup -d $LOOP_DEV

printf "Success! (hopefully)\n"