title: Combine two lists into csv
body: <p>I have two files, a.txt and b.txt. Let's say:</p>
  
  <p><code>a.txt</code> contains:</p>
  
  <pre><code>123
  234
  345
  456
  </code></pre>
  
  <p>and <code>b.txt</code> contains:</p>
  
  <pre><code>012
  023
  034
  045
  </code></pre>
  
  <p>I need to combine them into the following:</p>
  
  <pre><code>123,012
  234,023
  345,034
  456,045
  </code></pre>
  
  <p>Any idea on how can I do it in basic sh?</p>