xmlstarlet is unable to add subnode when more than one xmlns attribute is present in the first element

I have an xml file test.gpx:

cat test.gpx

<?xml version="1.0" encoding="UTF-8"?>
<gpx xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.topografix.com/GPX/1/1">
  <trk>
    <name>Name 1</name>
    <desc>Description 1</desc>
  </trk>
  <trk>
    <name>Name 2</name>
    <desc>Description 2</desc>
  </trk>
</gpx>

xmlstartlet is not able to perform successfully when trying to insert a subnode in this file:

xmlstarlet edit --subnode "/gpx/trk" --type  elem -n "new" -v "Hello World!" test.gpx

<?xml version="1.0" encoding="UTF-8"?>
<gpx xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.topografix.com/GPX/1/1">
  <trk>
    <name>Name 1</name>
    <desc>Description 1</desc>
  </trk>
  <trk>
    <name>Name 2</name>
    <desc>Description 2</desc>
  </trk>
</gpx>

Whereas with only one attribute in the first gpx element, it works:

xmlstarlet edit --subnode "/gpx/trk" --type  elem -n "new" -v "Hello World!" test.gpx

<?xml version="1.0" encoding="UTF-8"?>
<gpx xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <trk>
    <name>Name 1</name>
    <desc>Description 1</desc>
    <new>Hello World!</new>
  </trk>
  <trk>
    <name>Name 2</name>
    <desc>Description 2</desc>
    <new>Hello World!</new>
  </trk>
</gpx>

Hence my question: how could I succeed with the original file, having more than one attribute in its first gpx element?

Version info:

xmlstarlet --version
1.6.1
compiled against libxml2 2.9.10, linked with 20913
compiled against libxslt 1.1.34, linked with 10134

On Ubuntu 22.04.

Solution

The attributes your are dealing with aren't just garden variety attributes - they are namespaces and have to be taken into account accordingly. Since you have a default namespace in your first (but not second) file, you can deal with it in one of two ways.

The long, explicit way:

xmlstarlet ed -N x=http://www.topografix.com/GPX/1/1 --subnode "/x:gpx/x:trk" --type  elem -n "new" -v "Hello World!" test.gpx

or the shorter:

xmlstarlet ed  --subnode "/_:gpx/_:trk" --type  elem -n "new" -v "Hello World!" test.gpx

Either one should give you your expected output.