You have a file "test".The following command makes it so no one can read it.If the file was 12 bytes, it's now 4096 after it has been over written 100 times. There's no way to recover this.

$ shred -n 100 -z test

Want to remove the file? Use the "u" option.

$ shred -n 100 -z -u test

It can be applied to a device also
For more pls visit