homebrew: list "unneccessary" installed packages (i.e. not needed by any other package)

brew-unnecessary.sh

#!/bin/bash

####
# list "unneccessary" installed packages (i.e. not needed by any other package)
# 
# save:  $ curl -Lo /usr/local/bin/brew-unnecessary https://git.io/brew-unnecessary
# chmod: $ chmod +x /usr/local/bin/brew-unnecessary
# run:   $ brew unnecessary
####

for pkg in $(brew list); do
	if [ -z "$(HOMEBREW_NO_AUTO_UPDATE=1 brew uses --installed --skip-build $pkg 2> /dev/null)" ]; then
		echo $pkg
	fi
done