title: Shell script - How do I insert data into a separate file at a specific line?
body: <p>In the following shell script, how do I insert <code>${today}</code> into a separate existing file index.html at line 4? (Line 1-3 in index.html already has some code. Line 4 is empty. Line 5-EOL has some html code.) </p>
  
  <pre><code>#!/bin/sh
  Today=$(date "+%Y.%m.%d-%H.%M.%S")
  
  #insert ${today} into a separate existing file (index.html) in line 4
  #&lt;to-do&gt;
  </code></pre>