The secure removal of data is not as easy as you may think. When you delete a file using the default commands of the operating system (for example ???rm??? in Linux/BSD/MacOS/UNIX or ???del??? in DOS or emptying the recycle bin in WINDOWS) the operating system does NOT delete the file, the contents of the file remains on your hard disk.
The only way to make recovering of your sensitive data nearly impossible is to overwrite (???wipe??? or ???shred???) the data with several defined patterns. For erasing hard disk permanently, you can use the standard dd command. However, I recommend using shred command or wipe command or scrub command. In this tutorial, I will explain various open source software that can help you permanently erase the hard disk drive.