title: list words of length 3 without duplication
body: <p>how to list all word of length 3 without duplication ?</p>
  
  <p>using <code>tr ' ' '\n' &lt; cca1.txt | grep '^.\{3\}$'</code></p>
  
  <p>list all word of length 3 </p>
  
  <p>but when  add <code>sort -u</code> to be <code>tr ' ' '\n' &lt; cca1.txt | grep '^.\{3\}$' |sort -u</code></p>
  
  <p>to list words of length 3 without duplication </p>
  
  <p>it list part of words not whole words of length 3</p>
  
  <p>any suggestion?</p>