With this handy little app you can shred files and folders, but more conveniently you can shred your entire free space. Just make sure you’ve erased all the files you want to shred. Empty the trans, issue:

sudo apt-get autoclean && localepurge

Download scrub from Sourceforge:

http://sourceforge.net/projects/diskscrub

Its a simply source so configure, make and install. It will install a binary in the global path. You can run it at a user or as root. Run scrub accordingly to shred all free space:

sudo scrub -X ~/scrub

This will create a directory, in your home folder, called scrub and fill it with files created from free space. It will then shred them substantially. You can use the -r option, with scrub, to attempt to delete the scrub folder afterward, but it never worked for me. Just make a script as follows:

#!/bin/bash
sudo scrub -X ~/scrub
rm -r ~/scrub

Make it executable, and this will ensure that the folder is deleted after the process is complete. You can make this a cronjob to run it at night. Like at 3 in the morning. It will take hours on a relatively large drive. Maybe an hour per 50 gigs.