VirtualBox: compact vdi file

In Guest OS

sudo dd if=/dev/zero of=/zero bs=1M 
sudo rm /zero
sudo shutdown now

Check to make sure there is free space. If no free space, then it will fail to reboot. In Host OS

$ cd
$ VBoxManage modifyhd /path/to/disk.vdi --compact
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

In case accidentally forgot to rm /zero and shutdown. Do the following steps to recover it. Boot in safe-mode (Press Left Shift key during boot) Then,

mount -o remount,rw /
sudo rm /zero
reboot

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