sdX = hdd you want to wipe

# shred -vfz -n 7 /dev/sdX
# for i in {1..5}; do dd if=/dev/zero of=/dev/sdX status=progress; done
# for i in {1..5}; do dd if=/dev/urandom of=/dev/sdX status=progress; done
# shred -vfz -n 7 /dev/sdX