Is there an Apex Linter out there?

There's PMD Apex, which has a bunch of linting, plus various rules for SOQL inside loops, etc. That's the only one I'm aware of today.

sorry, this is an old post. But consider Clayton.io. It attaches to your source control repository (assuming you have one). I think it works on bitbucket and github at least.

I've used it for a month or two and it's great https://www.clayton.io/