title: Count the frequency of a number in a file with awk
body: <p>I have the following file:</p>
  
  <pre><code>1
  1
  2
  3
  3
  3
  4
  4
  5
  5
  5
  5
  </code></pre>
  
  <p>I would like to count how many times a number appear and how many times this is frequent: for example, the number 1 appear 2 times, the number 2 one time, the number 3 three times, the number 4 two times the 5 four times; the output would be a two columns file in which the first column how many times the number appear in the column, the second column represent the times in which a number is repeated, :</p>
  
  <pre><code>2 2  %(because the number 1 and number 4 appear 2 times and there are only 2 number that appear this often)
  1 3
  1 1
  1 4
  </code></pre>
  
  <p>I hope that the output example file can help to understand...</p>