title: Merge files with sort -m and give error if files not pre-sorted?
body: <p>need some help out here.</p>
  
  <p>I have two files,</p>
  
  <pre><code>    file1.txt &gt;
      5555555555
      1111111111
      7777777777
  
      file2.txt &gt;
      0000000000
      8888888888
      2222222222
      4444444444
      3333333333
  </code></pre>
  
  <p>when I run,
          <code>$ sort -m file1.txt file2.txt &gt; file-c.txt</code>
  the output file-c.txt get the merged within file1 and file2 but it is not sorted.</p>
  
  <pre><code>    file-c.txt &gt;
      0000000000
      5555555555
      1111111111
      7777777777
      8888888888
      2222222222
      4444444444
      3333333333
  </code></pre>
  
  <p>When it happens I need an error saying that the files (file1 and file2) is not sorted and the merge can't merge the files before it has been sorted. So when I run <code>$ sort -m file1.txt file2.txt &gt; file-c.txt</code> I have to get an error saying that it cannot merge file1 and file2 to file-c because they are not yet sorted.</p>
  
  <p>Hope you guys understand me :D  </p>