`brew upgrade` hangs on El Capitan

It could be downloading something large and taking a long time. Try running the following so you can see the download progress:

brew update --debug --verbose

Verbose mode does work for me:

brew update --verbose

• ctrl + c to exit the hanging upgrade.
• Run brew doctor. It prompted me to
• run brew cleanup to clean up false symlinks. Then I
• ran brew doctor again and it prompted me
• to install xcode CLT via the command sudo xcode-select --install.
• Finally brew update worked.