title: Padding Empty Field in Unix Join Operation
body: <p>I have two files where I want to perform union operation
  based on 1st column:</p>
  
  <p>file1.txt</p>
  
  <pre><code>foo 1
  bar 2
  qux 3
  </code></pre>
  
  <p>file2.txt</p>
  
  <pre><code>foo x
  qux y
  boo z
  </code></pre>
  
  <p>The result I hope to get is like this:</p>
  
  <pre><code>foo  1 x
  bar  2 -
  qux  3 y
  boo  - z
  </code></pre>
  
  <p>where the empty fields of column 1 is padded
  with "-".</p>
  
  <p>But why this join command doesn't work as I expected?</p>
  
  <pre><code>$ join -a1 -a2 -e"-" file1.txt file2.txt
  </code></pre>
  
  <p>What's the right way to do it?</p>