% brew update
Error: homebrew-core is a shallow clone. To `brew update` first run:
git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow
This restriction has been made on GitHub's request because updating shallow
clones is an extremely expensive operation due to the tree layout and traffic of
Homebrew/homebrew-core. We don't do this for you automatically to avoid
repeatedly performing an expensive unshallow operation in CI systems (which
should instead be fixed to not use shallow clones). Sorry for the inconvenience!