Adding a child element in XSLT

I recently had to edit a big XML file, and add a child elemnt to every element within. To simplify matters, say I had something like this:

<?xml version="1.0" encoding="UTF-8"?>
<rooty xmlns:ppl="some_identifier">
  <ppl:person>
    <age>42</age>
    <name>bob</name>
  </ppl:person>
  <ppl:person>
    <age>53</age>
    <name>doggo</name>
  </ppl:person>
</rooty>

I wanted to add a <cow> element to every person, like so:

<?xml version="1.0" encoding="UTF-8"?>
<rooty xmlns:ppl="some_identifier">
  <ppl:person>
    <age>42</age>
    <name>bob</name>
    <cow>Bessy</cow>
  </ppl:person>
  <ppl:person>
    <age>53</age>
    <name>doggo</name>
    <cow>Bessy</cow>
  </ppl:person>
</rooty>

My first approach was to spend 10 minutes manually editing the file, but why do that when you can spend 30 minutes writing automation to do it for you?
I then spent some time getting pissed off on Python’s XML libraries, as they didn’t properly handle custom XML namespaces (the ppl part).
I decided then that the “right” way to do it was using XSLT.

The solution

I still don’t fully understand XSLT, so I won’t try and explain it.
After some help from StackOverflow, I got a working XSLT file, which I’m happy to share:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:ppl="some_identifier" version="1.0">
  <xsl:output omit-xml-declaration="no" indent="yes" encoding="UTF-8"/>
  <xsl:strip-space elements="*"/>
  <!-- Identity transform -->
  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
  </xsl:template>
  <!-- Add cow to a person -->
  <xsl:template match="ppl:person">
    <xsl:copy>
      <xsl:copy-of select="@*"/>
      <xsl:copy-of select="node()"/>
      <cow>Bessy</cow>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

I run it using xsltproc, which is widely available for Linux distros, and the command looks like xsltproc XSLT_PATH XML_PATH

The file, broken down, looks like this:

Formatting

  <xsl:output omit-xml-declaration="no" indent="yes" encoding="UTF-8"/>
  <xsl:strip-space elements="*"/>

The above part controls the formatting. Without it, the resulting XML might have weird linebreaks and spaces.

Identity transform

  <!-- Identity transform -->
  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
  </xsl:template>

This part makes every node appear in the transformed document.

Adding the child elemennt

  <!-- Add cow to a person -->
  <xsl:template match="ppl:person">
    <xsl:copy>
      <xsl:copy-of select="@*"/>
      <xsl:copy-of select="node()"/>
      <cow>Bessy</cow>
    </xsl:copy>
  </xsl:template>

This part matches only the elements we want to add children to.
We tell the transformer to copy the node, and include all of the original attributes (@*) and child elements (node()), and add our own new child element (the <cow> part).
Removing this part will effectively make the XSLT file a “reformatter”.

Have fun!