shred (delete a file securely)
When you delete a file using →rm, the file's content is unlinked from its file name. This means that the file is no longer accessible and can be overwritten. However, the file's content is still there, which means that a malicious individual might be able to retrieve it. If you want to obliterate a file from your hard drive, use the following command:
shred -u -n 5 file
The switch -n 5 means that shred will overwrite file a total of five times, so it will be nearly impossible to retrieve it. If you leave out this option, the number of overwrites defaults to 3. The more often a file is overwritten, the more difficult it will be to retrieve its contents. The -u option means that the file will be deleted (unlinked) after it has been overwritten.
Please bear in mind that no degree of security is absolute. Even a file that has been overwritten several times and unlinked may be recoverable. Still, be careful with shred. If you accidentally delete a file with rm, you might be able to get it back. If you overwrite it ten times using shred, you may need the help of a data forensic laboratory to recover it, if it's possible at all.