#!/bin/bash

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

#Args
img="$1"

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

#Check that what we need is installed
for command in tune2fs e2fsck resize2fs; do
  which $command 2>&1 >/dev/null
  if (( $? != 0 )); then
    echo "ERROR: $command is not installed."
    exit -4
  fi
done

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

#Gather info
beforesize=$(ls -lh "$img" | tr -s " " | cut -d " " -f 5)
partnum=$(fdisk "$img" | grep Linux | cut -d ":" -f1 | tr -d " ")
partstart=$(fdisk -d "$img" | grep 0x83 | cut -d "," -f1)
loopback=$(hdiutil attach -nomount "$img" | grep Linux | cut -d " " -f1)
tune2fs_output=$(tune2fs -l "$loopback")
currentsize=$(echo "$tune2fs_output" | grep '^Block count:' | tr -d ' ' | cut -d ':' -f 2)
blocksize=$(echo "$tune2fs_output" | grep '^Block size:' | tr -d ' ' | cut -d ':' -f 2)

#Make sure filesystem is ok, force yes on all questions
e2fsck -y -f "$loopback"
minsize=$(resize2fs -P "$loopback" | cut -d ':' -f 2 | tr -d ' ')

# add ~200 MB of extra space, the system may need it to run
minsize=$(($minsize + 50000))

if [[ $minsize -gt $currentsize ]]; then
echo "ERROR: Image already shrunk to smallest size"
exit -6
fi

#Shrink filesystem
resize2fs -p "$loopback" $minsize
if [[ $? != 0 ]]; then
  echo "ERROR: resize2fs failed..."
  hdiutil detach $loopback
  exit -7
fi
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
fdisk -e "$img" <<EOF2
e $partnum



$newpartend
w
q

EOF2
sleep 1

#Truncate the file
endresult=$(($newpartend * 512))
endresult=$(($endresult + 1))
truncate -s $endresult "$img"
aftersize=$(ls -lh "$img" | tr -s " " | cut -d " " -f 5)

echo "Shrunk $img from $beforesize to $aftersize"