OSX Homebrew error: uninitialized constant MACOS

The MACOS constant is set in globals.rb. It seems like you may have a borked installation of Homebrew.

Check the output of brew doctor for any suggestions.

If that doesn't help please update with the results of brew --config.

If all else fails you may want to try re-installing Homebrew.

---

UPDATE:

Since this was a previous user's machine make sure your user is the owner of /usr/local and everything within. Fix it with sudo chown -R $USER /usr/local.

I'm pretty sure the root cause of this is a failed upgrade attempt to Homebrew 0.9.5 from a much earlier version. Basically, if you run brew update as opposed to sudo brew update a portion of files are updated, while others are not. Here's what worked for me:

1. Edit /usr/local/bin/brew (it's just a Ruby file, not a compiled binary, so any text editor will do). You'll find a block like:

   if MACOS and MACOS_VERSION < 10.5
     abort <<-EOABORT.undent
       Homebrew requires Leopard or higher. For Tiger support, see:
       https://github.com/mistydemeo/tigerbrew
     EOABORT
   end
   

   Comment this out. Even if you don't know Ruby, you can probably intuit what this is doing—it's checking to see if you have a current version of OSX. Assuming that you do in fact have this version, this sanity check isn't necessary. Brew is still broken, but at least now it will load far enough to give error messages.

2. Run sudo brew update, spoiler alert: it fails, but this time with a meaningful error message:

    $ brew update
    error: Your local changes to the following files would be overwritten by merge:
    [giant list of files here]
   

   Well, today I learned that brew update is just a wrapper for git pull because anyone who has worked with git knows that error message. We can fix this too.

3. Switch into the homebrew git repository with cd /usr/local and give the command git reset --hard FETCH_HEAD. This piece found here.

4. Give the command sudo brew update. Homebrew should now update successfully and work properly!

Once the system is working again, you can actually kind of see why an error like this would have occurred. For one, usr/local/bin/brew has been completely rewriten and isn't even Ruby anymore, and most of its configuration has been moved into /usr/local/Library/brew.rb which no longer uses the constants MACOS or MACOS_VERSION constants, as they have been replaced by the more object oriented OS.mac and MacOS.version.