#!/bin/zsh

usage() { echo "Usage: $0 imagefile.img [newimagefile.img]"; exit 1; }

cleanup() {
  [[ -n "$loopback" ]] && hdiutil detach "$loopback" >/dev/null 2>&1
}
trap cleanup EXIT

#Args
img="$1"

#Usage checks
[ -z "$img" ] && usage
[ ! -f "$img" ] && { echo "ERROR: $img is not a file..."; exit 2; }

#Check that what we need is installed
for command in tune2fs e2fsck resize2fs; do
  command -v $command 2>&1 >/dev/null
  [ $? -ne 0 ] && { echo "ERROR: $command is not installed."; exit 4; }
done

#Copy to new file if requested
if [ -n "$2" ]; then
  echo "Copying $1 to $2..."
  cp "$1" "$2"
  [ $? -ne 0 ] && { echo "ERROR: Could not copy file..."; exit 5; }
  img="$2"
fi

#Gather info
beforesize=$(ls -lh "$img" | awk '{print $5}')
partnum=$(fdisk "$img" | tr -d ":" |  awk '$2=="83" {print $1}')
partstart=$(fdisk -d "$img" |  awk -F "," '/0x83/ {print $1}')
loopback=$(hdiutil attach -nomount "$img" | awk '/Linux/ {print $1}')
tune2fs_output=$(tune2fs -l "$loopback")
currentsize=$(echo "$tune2fs_output" | awk  '/Block count:/ {print $NF}')
blocksize=$(echo "$tune2fs_output" |  awk  '/Block size:/ {print $NF}')

#Make sure filesystem is ok, force yes on all questions
echo "Checking filesystem on partition $partnum ..."
e2fsck -y -f "$loopback"
minsize=$(resize2fs -P "$loopback" | awk -F ': ' '{print $2}')

# add 200 MB of extra space, the system may need it to run, minsize is in blocks unit
minsize=$(($minsize + 200 * 1048576 / $blocksize))

[ $minsize -gt $currentsize ] && { echo "ERROR: Image already shrunk to smallest size"; exit 3; }

#Shrink filesystem
echo
echo "Shrinking partition $partnum starting at block $partstart on $img..."
resize2fs -p "$loopback" $minsize
[ $? -ne 0 ] && { echo "ERROR: resize2fs failed..."; exit 7; }
sleep 1

#Shrink partition
hdiutil detach $loopback
# fdisk uses a block size of 512 Bytes!
partnewsize=$(($minsize * $blocksize / 512))
newpartend=$(($partstart + $partnewsize))

# now use fdisk to change the partition
echo
echo "Updating partition table to new size ..."
fdisk -e "$img" 2>/dev/null <<EOF2
e $partnum



$newpartend
w
q

EOF2
sleep 1

#Truncate the file
endresult=$(($newpartend * 512))
truncate -s $endresult "$img"
aftersize=$(ls -lh "$img" | awk '{print $5}')
echo
echo
echo "Shrunk $img from $beforesize to $aftersize"