How To Securely Erase Free Space Using Terminal? A friend told me that when you delete a file on your Mac in the normal way it actually stays where it is, but it is just no longer usable by your Mac. The addition of more files to your Mac causes them to overwrite the space taken by those unusable files that you deleted previously.

If this is true then I'm thinking if your Mac got into the wrong hands, that individual(s)  could actually recover files that you have deleted previously (which could be potentially harmful).

Therefore how do you delete them securely first to prevent such potential catastrophe (preferably using the Terminal command) ?
From the Terminal app you can use the program srm (secure remove).
Simply start Terminal, then type:

srm &lt;filename&gt;


For more info, take a look at the man page.
Excerpt:


  srm  removes  each  specified file by overwriting, renaming, and truncating it before unlinking. This prevents other people from undeleting or recovering any information about the file from  the command line.