From Soup To Nuts - Linux Tips
Need to Delete a File for Good -- no one can recover.

You have a file "secret". The following makes it so no one can read it. If the file was 12 bytes, it's now 4096 after it has been over written 100 times. There's no way to recover this:

$ shred -n 100 -z secret

Want to remove the file? Use the "u" option.
$ shred -n 100 -z -u test2

It can be applied to a device
$ shred -n 100 -z -u /dev/fd0