title: replace a certain string in a file - sed
body: <p>The problem is there is a file with many strings like <code>$VARx$</code> and <code>x</code> stands for a number from 1 to 9: <code>$VAR1$</code>, <code>$VAR2$</code>, <code>$VAR3$</code>, etc.</p>
  
  <p>I want to quote these strings like <code>$VARx$</code> → <code>"$VARx$"</code>. I read or know that this could be possible with sed out of the command line but I have no idea how. Something like that but don't work:</p>
  
  <pre><code>sed -i "s/\\\$VAR1\$/\"\$VAR1\"\$/g" file.txt
  </code></pre>
  
  <p>The special part would be to include the variable <code>x</code>.</p>