title: Append a file in the middle of another file in bash
body: <p>I need to append a file in a specific location of another file.
  I got the line number so, my file is:</p>
  <p><code>file1.txt</code>:</p>
  <pre><code>I
  am
  Cookie
  </code></pre>
  <p>While the second one is</p>
  <p><code>file2.txt</code>:</p>
  <pre><code>a
  black
  dog
  named
  </code></pre>
  <p>So, after the solution, <code>file1.txt</code> should be like</p>
  <pre><code>I
  am
  a
  black
  dog
  named
  Cookie
  </code></pre>
  <p>The solution should be compatible with the presence of characters like <code>&quot;</code> and <code>/</code> in both files.</p>
  <p>Any tool is ok as long as it's native (I mean, no new software installation).</p>