title: In Unix,why directory is a file and an allocation bitmap is not
body: <p>I'm currently preparing an entrance of a graduate school.</p>
  
  <p>And one of a past-year-question is:</p>
  
  <p>In. Unix system,which of the following is not a file ?</p>
  
  <p>A.a device special file </p>
  
  <p>B.a directory</p>
  
  <p>C.a symbolic link</p>
  
  <p>D.an allocation bitmap</p>
  
  <p>E.the standard input(console)</p>
  
  <p>And the official answer is D.</p>
  
  <p>I understand A is a file .But why B and E are also files?</p>
  
  <p>And why bitmap is not a file?</p>