best fedora solution to securely erase folders/clean free space?

[M. Fioretti]

On Sat, Mar 30, 2013 13:10:57 PM +0100, Reindl Harald wrote:
> 
> dd if/dev/urandom of=random.bin bs=16M; sync; rm -f random.bin

thanks.
I assume there should be an equal sign after the "if", right?
Anyway:

- shouldn't this be done several times, to make sure that whatever was
  in the free space really is unrecoverable?

- apart from that, probably it's a mix of typos and English not being
  my native language, but honestly I don't understand the last two
  lines of your answer, that is the part after "as the same":

> creates a file with random-stuff until the disk is full, syncs it
> really to the disk and removes the file as the same with dd
> if=/dev/zero will zerofill free space to make it possible shink thin
> provisioned virtual-disks as example

may I ask you to rephrase them, please?

Marco