A really basic bash script to loop dd in order to writes random data to a disk 8 times over.

killd.sh

#!/bin/bash
#Basic while loop to whipe a harddisk.

# Show us the disks.
sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL

# Prompt for disk to kill.
echo What disk to you want to kill \(ex: sdb\)
read killdisk
killdisk="/dev/$killdisk"

# This can take a long time for large disks.
counter=1
while [ $counter -le 8 ]
do
        sudo dd if=/dev/urandom of="$killdisk" bs=1M # Change to not be sdb
        ((counter++))
done
echo All done