title: sorting with multiple keys with Linux sort command
body: <p>Say I have this file.</p>
  
  <pre><code>$ cat a.txt
  c 1002 4
  f 1001 1
  d 1003 1
  a 1001 3
  e 1004 2
  b 1001 2
  </code></pre>
  
  <p>I want to sort it by the second column and then by the third column. Column two are numbers, while column 3 can be treated as string. I know the following command works well.</p>
  
  <pre><code>$ sort -k2,2n -k3,3 a.txt
  f 1001 1
  b 1001 2
  a 1001 3
  c 1002 4
  d 1003 1
  e 1004 2
  </code></pre>
  
  <p>However, I think <code>sort -k2n a.txt</code> should also work, while it does not.</p>
  
  <pre><code>$ sort -k2n a.txt
  a 1001 3
  b 1001 2
  f 1001 1
  c 1002 4
  d 1003 1
  e 1004 2
  </code></pre>
  
  <p>Seems like it sorts by column two, and then by column one instead of column three. Why is this happening? Is it a bug or not? Cause <code>sort -k2 a.txt</code> works ok with above data since those numbers are just fixed width.</p>
  
  <p>My sort version is <code>sort (GNU coreutils) 8.15</code> in cygwin.</p>