How to erase the harddisk completly using linux ?

Erasing the entire hard disk using Linux:

Run the following command to fill your entire hard disk with zeros

$ dd if=/dev/zero of=/dev/sda bs=1M

The above command will whole hard disk with zeros. Note: this will take long time to complete.

Erasing/filling the hard Disk with random data:

We can fill the hard disk with random data in case of any security requirement to prevent data loss. Run the below

$ dd if=/dev/random of=/dev/sda bs=1M

Erasing the MBR(Master Boot Record) of the Hard Disk.

To Erase only code area in your MBR:

Run

$ dd if=/dev/zero of=/dev/sda bs=446 count=1

To Erase entire MBR :

Run

$ dd if=/dev/zero of=/dev/sda bs=512 count=1

Note : If linux is not installed, Boot into Linux image from Live CD to perform above operations on the hard disk.