Mac command line execution:

brew update 

The following error is reported:

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 command may take a few minutes to run due to the large size of the repository.
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 and Homebrew/homebrew-cask. 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!

The cause of this problem is usually the problem caused by the system update.

Solution: delete the homebrew core and then update it

cd /usr/local/Homebrew/Library/Taps/homebrew
rm -rf homebrew-core
brew upgrade