How to append a newline after every match using xmlint --xpath

Question

I have the following HTML code:

1</script>
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                <span>2021-01-02 01:39</span>
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<p>Hello from the year 2020!</p>

<p>As of v2.9.9 of libxml, this behavior has been fixed in xmllint itself.</p>

<p>However, if you're using anything older than that, and don't want to build libxml from source just to get the fixed <code>xmllint</code>, you'll need one of the other workarounds here. As of this writing, the latest CentOS 8, for example, is still using a version of libxml (2.9.7) that behaves the way the OP describes.</p>

<p>As I gather from this SO answer, it's theoretically possible to feed a command into the <code>--shell</code> option of older (<2.9.9) versions of <code>xmllint</code>, and this will produce each node on a separate line. However, you end up having to post-process it with <code>sed</code> or <code>grep</code> to remove the visual detritus of shell mode's (human-oriented) output. It's not ideal.</p>

<hr>

<p>XMLStarlet, if available, offers another solution, but you do need to use <code>xmlstarlet fo</code> to format your HTML fragment into valid XML before using <code>xmlstarlet sel</code> to extract nodes:</p>

<pre><code>echo '
<textarea name="command" class="setting-input fixed-width"
 rows="9">1
2' \
  | xmlstarlet fo -H -R \
  | xmlstarlet sel -T -t -v '//textarea[@name="command"]' -n

If the Attempt to load network entity message from the second xmlstarlet invocation annoys you, just add 2>/dev/null at the very end to suppress it (at the risk of suppressing other messages printed to standard error).

The XMLStarlet options explained (see also the user's guide):

• fo -H -R — format the output, expecting HTML input, and recovering as much bad input as possible
  • this will add an root node, making the fragment in the OP's example valid XML
• sel -T -t -v //xpath -n — select nodes based on XPath //xpath
  • output plain text (-T) instead of XML
  • using the given template (-t) that returns the value (-v) of the node rather than the node itself (allowing you to forgo using text() in the XPath expression)
  • finally, add a newline (-n)

Edit(s): Removed half-implemented xmllint --shell solution because it was just bad. Added an XMLStarlet example that actually works with the OP's data.