https://frankl.in/code/update-script-for-homebrew

homebrew_update.sh

#!/bin/sh
if [ "$1" == "-h" ]; then
  echo "Colorful Homebrew update script"
  echo
  echo "USAGE: update [-y]"
  echo
  echo "   -y  skip questions"
  echo "   -h  display this help"
  echo
  exit 0
fi

echo "\033[93mFetching packages list:\033[0m"
brew update
brewsy=`brew outdated | wc -l | awk {'print $1'}`

if [ $brewsy != 0 ]; then
  echo "\033[93mOutdated packages:\033[0m" $brewsy
  echo
  brew outdated
  echo

  if [ "$1" != "-y" ]; then
    ask=$(echo "\033[93mUpdate these packages?\033[92m [yn] \033[0m")
    read -p "$ask" yn
  fi

  if [ "$yn" = "y" ]; then
    brew upgrade --all && brew cleanup
  else
    echo "\033[93mOK, not doing anything\033[0m\n"
  fi
else
  echo "\033[93mNothing to do\033[0m"
fi