I'd been having this issue whenever installing casks:
$ brew cask install macvim ==> This subcommand requires root privileges: bsexec Error: Empty plist input
This was a known bug, and has been addressed with a newer version.
$ brew update $ brew upgrade caskroom/cask/brew-cask
Be prepared for this to take a while.
==> Checking out tag v0.54.0 [..] 10M, built in 7.0 minutes
I've had mixed to positive success with Homebrew Cask, but it’s just useful enough to keep it around for installing Mac applications. XQuartz, Tunnelblick and a few others are still tricky, as they require admin access or wizard installs.