The Error
$ brew /usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory /usr/local/bin/brew: line 21: /usr/local/Library/brew.rb: Undefined error: 0 $ brew doctor /usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory /usr/local/bin/brew: line 21: /usr/local/Library/brew.rb: Undefined error: 0
The Fix
Make homebrew think /System/Library/Frameworks/Ruby.framework/Versions/1.8 still exists until you successfully update:
cd /System/Library/Frameworks/Ruby.framework/Versions sudo ln -s Current 1.8 brew update sudo rm 1.8