Is a copula a function word?

Most resources I have read state that verbs are content words (excluding helping verbs).

I was just wondering whether copulas are considered content or function words. To me, a copula seems more like a function word.

Also, would the same thing be true of linking verbs in general?

Solution 1:

When you read what somebody has written about grammar on the Web, remember:

• They always mean "most" or "usually" or "generally".
  They never mean "all" or "always" or "in every case".

That is, there are always exceptions. Most verbs are content words. But auxiliary verbs aren't.
Be, the most-cited "copula" in English, has no meaning content, and is always an auxiliary verb.
I.e, it's a function word. So are articles, conjunctions, complementizers, and most quantifiers.