title: How to tar a set of files?
body: <p>I am new to the <code>tar</code> command. I have many files in a directory:
  <code>1_1_1.txt</code>,  <code>1_1_2.txt</code>, 
  <code>2_1_1.txt</code>,  <code>2_1_2.txt</code>..</p>
  
  <p>I know there is a command: </p>
  
  <pre><code>tar cf file.tar *.txt
  </code></pre>
  
  <p>which will <code>tar</code> all the four files into <code>file.tar</code>. ButI need to tar <code>1_1_1.txt</code> and <code>1_1_2.txt</code> into one tar file and <code>2_1_1.txt</code> and <code>2_1_2.txt</code> into another tar file.
  How can I accomplish this?</p>