A shell script to defragment Linux ext4 partitions and fill empty space with zeroes for compact backup of virtual machine. See https://supratim-sanyal.blogspot.com/2016/12/zero-out-free-disk-space-on-virtual.html

zerofree.sh

#!/bin/bash -x

# * ----------------------------------------------------------------------------
# * "THE BEER-WARE LICENSE" (Revision 42):
# * Supratim Sanyal wrote this file. As long as you retain this notice you
# * can do whatever you want with this stuff. If we meet some day, and you think
# * this stuff is worth it, you can buy me a beer in return.
# * https://www.emailmeform.com/builder/form/65c8aanX6uJ8RVa
# * ----------------------------------------------------------------------------

echo THIS MUST BE RUN IN SINGLE-USER MODE AND WILL TAKE A LONG TIME maybe a day
echo DID YOU DO telinit 1 BEFORE RUNNING THIS?
echo Press ^C to abort or any other key to continue ...
read -n 1
sync;sync;sync

# Defrag the data partitions
for i in 1 2 3 4 5
do
	e4defrag /dev/sda1
	sync;sync;sync
	sleep 3

	e4defrag /dev/sdc1
	sync;sync;sync
	sleep 3
done

# zerofree swap partitons
swapoff /dev/sda2
dd if=/dev/zero of=/dev/sda2 bs=4096
sync;sync;sync
mkswap /dev/sda2
swapon /dev/sda2
sleep 3

swapoff /dev/sdb1
dd if=/dev/zero of=/dev/sdb1 bs=4096
sync;sync;sync
mkswap /dev/sdb1
swapon /dev/sdb1
sleep 3


# zerofree the data partitions
umount /dev/sdc1
zerofree /dev/sdc1
sync;sync;sync
sleep 3

mount -o remount,ro /dev/sda1
zerofree /dev/sda1
sync;sync;sync
mount -o remount,rw /dev/sda1
sleep 3

echo NOW SHUTDOWN, then
echo vboxmanage modifyhd --compact the vdi virtual disks and take a backup.
sync;sync;sync
exit 0