Jessie or Wheezy with Raspberry on space free out Wipe
When you run the cat script the created file eats up all free space by creating a huge file. Then the contents of this file are written in a number of steps - ensuring that all areas of the disk which were previously free have had their contents erased. Once completed the huge file have to be removed, meaning you have free disk space again.
cat /dev/zero > /zero.fill