Cowardly refusing to `sudo brew update` or failure

First find out what is the current ownership of the installed package of brew (you will need this info in the last step)

ls -al `which brew`

Change the user and group of brew to root and wheel respectively:

sudo chown root:wheel `which brew`

Now execute your brew command as root, for eg. sudo brew link mcrypt. Lastly, revert back the ownership of brew to what you had found out in step 1 above

sudo chown : `chown brew`

brew
update failure while executing git checkout

$ cd `brew --prefix`
$ git remote add origin https://github.com/Homebrew/homebrew.git $ git fetch origin
$ git reset --hard origin/master
$ brew update
Already up-to-date.