More details are here: Thanks, pic.twitter. You must provide full disk access for /usr/bin/ruby to make #Emacs work correctly on #Mac #Catalina. The Stack Exchange link it contains is different from the one above so you should look at it too if you’re having problems. From the command line, a which emacs returned no results: which emacs Nothing.
Here’s a tweet that also describes the problem. emacs I made the dreadful mistake of upgrading to Mac OSX Catalina this week, while in the midst of some programming projects, and all of a sudden Emacs stopped working. Every report I saw about the problem said that the solution provided by Stack Exchange got things working again. If you do experience problems with accessing the file system, take a look at the above link. My conclusion is that there’s no reason not to upgrade as far as Emacs is concerned. I’m using the same binary (Emacs 26.3 compiled from source) as before so I haven’t tried recompiling Emacs yet but the old binary is working as it was previously without having to adjust permissions or anything else. In any event, after the upgrade Emacs worked just fine without me doing anything. When I checked the above Stack Exchange entry, though, it said the poster was seeing the problem in Emacs 26.1.
I hadn’t heard of the Ruby script before but assumed it was something new in Emacs 27. The TL DR is that Emacs was being loaded by a Ruby script so it was Ruby that needed to be given permissions. This time, there were reports of Emacs not being able to access (any of) the file system. That happened last time with the upgrade to Mojave because Apple stopped supporting the old display model and you couldn’t use Emacs in GUI mode until the Emacs devs generated a fix. I’d been holding off to see if there were reports of problems with Emacs on the new OS. Last night I upgraded my MacBook Pro to macOS Catalina (macOS 10.15).