# Secure wipe individual file
shred -v -n 3 ./file
Thou might not be secure if your filesystem use COW (copy-on-write) (like btrfs,zfs), make shadow copies (windows, ntfs), uses RAID.
# Secure wipe entire drive
shred -v -n 3 /dev/HDD
# Or if you are paranoid - slower
shred -v -n 7 /dev/HDD
Long. Expect waiting a day or two.Or…
via Casual Blog