From: http://www.commandlinefu.com

$ shred targetfile

GNU shred is provided by the core utils package on most Linux distribution (meaning, you probably have it installed already), and is capable of wiping a device to DoD standards.

You can give shred any file to destroy, be it your shell history or a block device file (/dev/hdX, for IDE hard drive X, for example). Shred will overwrite the target 25 times by default, but 3 is enough to prevent most recovery, and 7 passes is enough for the US Department of Defense. Use the -n flag to specify the number of passes, and man shred for even more secure erasing fun.

Note that shredding your shell history may not be terribly effective on devices with journaling filesystems, RAID copies or snapshot copies, but if you’re wiping a single disk, none of that is a concern. Obviously shredding a disk would take quite some time.