title: how to combine contents of two files based on key columns in linux
body: <p>I've two files with 3 identical columns and 4th a different one. </p>
  
  <p><strong>File A</strong></p>
  
  <pre><code>a b c 100
  e f g 50
  h i j 25
  </code></pre>
  
  <p><strong>File B</strong></p>
  
  <pre><code>a b c 200
  e f g 20
  h i j 15
  </code></pre>
  
  <p>How can files <code>A</code> and <code>B</code> be combined to look like file <code>C</code>?</p>
  
  <p><strong>File C</strong></p>
  
  <pre><code>a b c 100 200
  e f g 50 20
  h i j 25 15
  </code></pre>
  
  <p>--UPDATE--</p>
  
  <p>I've used the solutions provided by Jotne an Kent but both of the script's output have . (dot) instead of comma. it looks like</p>
  
  <pre><code>  a,b,c,100.200 
    e,f,g,50.20 
  </code></pre>