title: combining the text files in to one text file
body: <p>i have a requirement like the following.
  i am using linux</p>
  
  <p>i have a set of text files like <code>text1.txt ,text2.txt, text3.txt</code>. 
  now i am combining into one final text file.</p>
  
  <p><strong>text1.txt</strong> </p>
  
  <pre><code>1    
  NULL    
  NULL    
  4
  </code></pre>
  
  <p><strong>text2.txt</strong></p>
  
  <pre><code>1    
  2    
  NULL    
  4
  </code></pre>
  
  <p><strong>text3.txt</strong></p>
  
  <pre><code>a    
  b    
  c    
  d
  </code></pre>
  
  <p>i am using the following command :</p>
  
  <pre><code>paste -d ' ' text1.txt text2.txt text3.txt  &gt;&gt; text4.txt
  </code></pre>
  
  <p>i am getting the :</p>
  
  <p><strong>text4.txt</strong></p>
  
  <pre><code>1 1 a   
  2 b    
  c  
  4 4 d
  </code></pre>
  
  <p>but i want the output like the following</p>
  
  <p><strong>text4.txt</strong></p>
  
  <pre><code>1 1 a
  NULL 2 b
  NULL NULL c
  4 4 d
  </code></pre>
  
  <p>NOTE :- NULL means space </p>
  
  <p>i am passing this text4 to another loop as a input so here there i am reading the variable by positionl</p>
  
  <p>thanks in advance</p>