Have you ever wanted to delete “permanently” a file from your file system?. Well, you can do it with this command: shred . First, a litle about how a file system works. Simply put, the system keeps a table with references to each file. When you delete a file what really happens is that the operating system removes the reference to the file from the table of references, but the data is still there, so is possible to recover the data. If you really want to delete a file you can use shred. This utility overwrite with random data the file so its unrecoverable. Example: (the u option deletes the file after overwriting)
shred -u filename
As always, see the documentation for more options and use cases.