title: join multiple files
body: <p>I am using the standard join command to join two sorted files based on column1.
  The command is simple join file1 file2 > output_file.</p>
  
  <p>But how do I join 3 or more files using the same technique ?
  join file1 file2 file3 > output_file
  Above command gave me an empty file.
  I think sed can help me but I am not too sure how ?</p>