Shred utility overwrites a file to hide its contents, and optionally delete it if needed. The idea is pretty simple as it overwrites the specified FILE(s) repeatedly, in order to make it harder for even very expensive hardware probing to recover the data. By default file is overwritten 25 times. I’ve seen cases where law enforcement agencies had successfully recovered data from 5 year old *not so* working hard disk as evidence. Also when you move your rented server you should consider running file shredding; otherwise new owner can get data including passwords.