You have uncommitted modifications to Homebrew/homebrew-core. If this is a surprise to you, then you should stash these modifications. Stashing returns Homebrew to a pristine state but can be undone should you later need to do so for some reason. cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core && git stash -u && git clean -d -f Uncommitted files: M Formula/s/swiftlint.rb