It is a bit condescending... but that's not against the rules.

It comes as both part of an answer to the question, and in answer to a question from somebody starting out with Bash scripting. Suggesting a learner do some book learning is not a bad suggestion.

It could be improved with relevant links out to (I would assume) TLDP, or some other page that further explains the answer. But cutting the suggestion out completely that out doesn't improve the answer.

What should be the correct course of action here?

Vote, improve or ignore. There's nothing that needs to be done with it.

It is a bit condescending... but that's not against the rules.

It comes as both part of an answer to the question, and in answer to a question from somebody starting out with Bash scripting. Variable scope isn't entirely fundamental (you can womble along for a long time before it kicks you in the face) but it's something you need to eventually learn, along with a raft of other dry topics. So suggesting a learner do some book learning is not a bad suggestion.

It could be improved with relevant links out to (I would assume) TLDP, or some other page that further explains the answer. But cutting the suggestion out completely that out doesn't improve the answer.

What should be the correct course of action here?

Vote, improve or ignore. There's nothing that needs to be done with it.