title: How to extract ramdisk.img
body: <p>use ubuntu</p>
  
  <p>I type <strong>file ramdisk.img</strong> and the Terminal display <strong>ramdisk.img</strong>: data,
  I use <code>cpio</code> and get nothing extracted!
  How can I <code>extract</code> such <strong>ramdisk.img</strong>?</p>