I have a piece of XML like so:
<root>
<foo src=""/>
<foo src="bar"/>
<foo />
</root>
I want to know which elements have a src attribute, which are empty and which have values.
The furthest I have come is with
$ xmlstarlet sel -t -m '//foo' -v @src -n foo.xml
bar
Though that doesn't tell me the third foo is missing the attribute.
Answer
This will select the foos with no src attribute.
/root/foo[not(@src)]
For the other two tasks, I would use a mix of the expressions pointed out by @TOUDIdel and @Dimitre Novatchev:
/root/foo[@src and string-length(@src)=0] for foos with an empty src, and /root/foo[@src and string-length(@src)!=0] for foos with an src with content in it.
As an aside, I would avoid using the "anywhere" selector, // (not to mention the * wildcard), unless you're sure that this is specifically what you need. // is like making your very eager dog sniff a piece of cloth and telling it, "bring me everything that smells like this, wherever you find it". You won't believe the weird crap it can decide to bring back.