title: Expand variables in sed
body: <p>I need use sed into bash script, for add lines after any line numer of script with some pair of values (below work)</p>
  
  <pre><code>sed -i.bak '14i\some_text=some_text' file
  </code></pre>
  
  <p>But I need on script bash (sh) for expand variables (below not work)</p>
  
  <pre><code>sed -i.bak '$number_linei\$var1=$var2' $var3
  </code></pre>