shrink-image.sh

#!/bin/bash

set -e
shopt -s lastpipe extglob

if [[ $# != 1 ]]; then
	echo "Usage: ${BASH_SOURCE} <device or image file>" >&2
	false
fi

dev="$1"

/sbin/dumpe2fs -h "$1" | grep 'time:'
/sbin/e2fsck -f -p "$1" || [[ $? == 1 ]]
/sbin/resize2fs -M "$1"

declare -A info
/sbin/dumpe2fs -h "$dev" | grep -P ': *\d+$' | \
	while IFS=: read -r k v; do
		info["${k// /_}"]="${v##*( )}"
	done

bytes=$(( ${info[Block_size]:?} * ${info[Block_count]:?} ))
sectors=$(( bytes / 512 ))
if [[ -f "$dev" ]]; then
	echo "truncating file to $bytes bytes..."
	truncate --no-create --size=$bytes "$dev"
else
	echo "new size is $sectors sectors"
fi