Upsert an XML attribute

https://til.marcuse.info/xml/upsert-xml-attribute.html

In order to upsert an attribute in an XML file, you need to have both an insert and an update in the same command.

Example that adds a style attribute to the root >svg< element of every SVG file.

xmlstarlet edit \
	--inplace \
	--update '/_:svg/@style' -x 'string("background-color: white")' \
	--insert '/_:svg[not(@style)]' --type attr -n newattr --value 'background-color: white' \
	*.svg