This is about JSON
xmlstarlet supports XPath 1, but the W3C did not stop there. They made XPath 2 featuring variables, lists and regular expressions, XPath 3 featuring higher order function, and finally XPath 3.1 featuring JSON support.
For example,
echo '[{"a": 1}, {"a": 2, "b": 3}, {"c": 4}]' | xidel - -e '?*?a!(. * 100)'
will print 100 and 200.
Or the same with a verbose syntax:
echo '[{"a": 1}, {"a": 2, "b": 3}, {"c": 4}]' | xidel - -e 'for $obj in array:flatten(.) return map:get($obj, "a") * 100'
