Secure HDD wipe
Selling off an old hard drive, I wanted to wipe it clean.
Found the tip here
shred -vfz -n 100 /dev/hda
Here /dev/hda is my whole hard disk. And I am asking shred to make (-n) 100 passes by overwriting the entire hard disk with (-z) zeros. And shred program (-f) forces the write by changing the permissions wherever necessary.