If your xmllint supports XPath (it should), I would do something like xmllint --xpath 'string(/*[local-name()="EntityDescriptor" and namespace-uri()="urn:oasis:names:tc:SAML:2.0:metadata"]/@entityID)' $@ to get the entityID rather than xmllint --format, grep, and sed.

I've replaced xmllint, grep and sed with xmlstarlet, leaving the previous use of line-based processing tools as a comment in line 20, for use on old systems.

Note that the invocation of xmlstarlet above will only work if the XML documents have set the SAML 2.0 Metadata URI as default namespace, which I ensure by curating the XML carefully:

<EntityDescriptor xmlns="urn:oasis:names:tc:SAML:2.0:metadata" ...

Otherwise the XPath used by xmlstarlet would have to be changed to match whatever is the XML namespace prefix used within the metadata document, such as md: (shown below) or ns0: or whatever:

-    $(eval ENTITY = $(shell xmlstarlet sel -t -v '//_:EntityDescriptor/@entityID' -n $@)) 
+    $(eval ENTITY = $(shell xmlstarlet sel -t -v '//md:EntityDescriptor/@entityID' -n $@)) 

OTOH the example invocation for xmllint provided by @icb should work in all cases, default namespace or not.