How to declare XPath namespaces in xmlstarlet?

I am new to xmlstarlet so hoping this answer is a simple one.

I am writing a script to modify Inkscape SVG files from the command line. I chose the tool xmlstarlet.

After testing the command syntax on test files, I am having trouble on the real SVG files. I think the use of namespaces is throwing me off.

Example file:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!-- Created with Inkscape (http://www.inkscape.org/) -->    
<svg
   xmlns:osb="http://www.openswatchbook.org/uri/2009/osb"
   xmlns:dc="http://purl.org/dc/elements/1.1/"
   xmlns:cc="http://creativecommons.org/ns#"
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   xmlns:svg="http://www.w3.org/2000/svg"
   xmlns="http://www.w3.org/2000/svg"
   xmlns:xlink="http://www.w3.org/1999/xlink"
   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
   width="603"
   height="1000"
   viewBox="0 0 159.54375 264.58334"
   version="1.1"
   id="svg8"
   inkscape:version="0.92.1 r"
   sodipodi:docname="test.svg"
   inkscape:export-filename="/home/user/dev/inkscape/test/1.png"
   inkscape:export-xdpi="96"
   inkscape:export-ydpi="96">
  <defs
     id="defs2">
    <linearGradient
       inkscape:collect="always"
       id="linearGradient6204">
      <stop
         style="stop-color:#8f1a22;stop-opacity:1;"
         offset="0"
         id="stop6200" />
      <stop
         style="stop-color:#8f1a22;stop-opacity:0;"
         offset="1"
         id="stop6202" />
    </linearGradient>
  </defs>
</svg>

I want to change Gradient6204 to Gradient9999.

I wrote this command, which does not work (just returns the original file).

xmlstarlet ed -u "/svg/defs/linearGradient[@id='linearGradient6204']/@id" -v 'linearGradient9999' text.txt

I also tried again, adding the namespaces with -N but no luck. I found that if I delete the line:

xmlns="http://www.w3.org/2000/svg"

from the file then the command I wrote above works.

What is the proper syntax for updating the SVG file above in the way I described?

Explicit Namespace Declaration

Adding -N s=http://www.w3.org/2000/svg and then using the s: namespace prefix works:

xmlstarlet ed  -N s=http://www.w3.org/2000/svg -u "/s:svg/s:defs/s:linearGradient[@id='linearGradient6204']/@id" -v 'linearGradient9999' text.txt

Implicit Declaration of Default Namespace

Starting with XMLStarlet v1.2.1, an explicit command line definition for the default namespace (such as is the case with OP's SVG file) can be avoided via use of an automated binding of _ to the default namespace:

1.3. A More Convenient Solution

XML documents can also use different namespace prefixes, on any element in the document. In order to handle namespaces with greater ease, XMLStarlet (versions 1.2.1+) will use the namespace prefixes declared on the root element of the input document. The default namespace will be bound to the prefixes _ and DEFAULT (in versions 1.5.0+).

So, the above command line could be re-written as:

xmlstarlet ed -u "/_:svg/_:defs/_:linearGradient[@id='linearGradient6204']/@id" -v 'linearGradient9999' text.txt