title: Shell script - sed doesn't work
body: <p>I have:
  test.txt:</p>
  
  <pre><code>version-1
  version-1
  </code></pre>
  
  <p>ori.sh:</p>
  
  <pre><code>old="version-1"
  new="version-2"
  sed -i .bak 's/${old}/${new}/g' test.txt
  </code></pre>
  
  <p>when running ori.sh, nothing happens. I would expect that test.txt would look like:</p>
  
  <p>test.txt*:</p>
  
  <pre><code>version-2
  version-2
  </code></pre>
  
  <p>Any ideas?</p>