brew link

• brew doctor

  brew dr

  This will give you the:
  ‘Warning: You have unlinked kegs in your Cellar…’
  message

  underneath this message it will prompt which kegs are there with the potential problem. My personal warning was:

  Warning: You have unlinked kegs in your Cellar Leaving kegs unlinked can lead to build-trouble and cause brews that depend on those kegs to fail to run properly once built. Run `brew link` on these:
  	mongodb

• brew link

  read your options, so you understand what is happening with this command.

• brew link --dry-run <yourArgument>

  Homebrew will list all files which would be linked or which would be deleted by brew link –overwrite, but will not actually link or delete any files.

• brew link --overwrite <yourArgument>

  This will execute what the dry-run did, for real this time.

  My experience with this was:
  cli cmnd:brew link –overwrite mongodb
  cli response: Linking /usr/local/Cellar/mongodb/3.2.7… 0 symlinks created

• brew doctor Finally I ran this command and my response no longer had issues.

  cli response: Your system is ready to brew.