See e.g. here. The answer does not mention anything the other answers didn't mention 9 hours before, except for a (indeed useful) link. Shouldn't this rather be a comment on one of the answers?
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If the answer adds new information - in this case a link - then there's nothing wrong with it. If the link is useful up-vote, if not down-vote (if you really feel strongly about it). A comment asking for the relevant information from the link to be quoted might be in order, but that depends on how much would have to be quoted.
If it's from a new user (one with less than 50 rep) then they can't comment, so in these cases I'd leave a comment but not do anything more. After all down-voting would just postpone the time at which they could use the site properly.
I've seen better examples for those oneliners. But if they`re bad enough, they get their downvotes or at least comments. The referenced example is instead fine to me. I'm not a unix crack and my first impression was, that this is somehow different answer to the question. And there's a link that might help the OP more than the other answers...
I've seen people leaving answers as comments but to my experience those are mainly members with high reputation who just don't want to get upvotes for simple google or tutorial links.