For the xml file foo.xml
<?xml version="1.0" encoding="UTF-8" ?>
<record><header><identifier>oai:tib.eu:TIBKAT:010000011</identifier><datestamp>2020-10-12</datestamp><setSpec>tibkat</setSpec></header><metadata><marcxml:collection xmlns:marcxml="http://www.loc.gov/MARC21/slim">
<marcxml:record>
<marcxml:leader>Hello world</marcxml:leader>
</marcxml:record>
</marcxml:collection>
</metadata></record>
the XMLStarlet query
xmlstarlet sel -N xmlns="http://www.loc.gov/MARC21/slim" -t -v '//_:collection/_:record/_:leader[text()]' -nl foo.xml
results in an error message due to an undefined namespace prefix:
Undefined namespace prefix xmlXPathCompiledEval: evaluation failed runtime error: element with-param Failed to evaluate the expression of variable 'select'. no result for foo.xml
Why does the query not work and deliver "Hello world"?