# 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…