Ever wondered which of your homebrew formulas are unused legacy dependencies? Run this!

brew-legacy.sh

#!/bin/bash
GREEN="\033[1;32m";
RESET="\033[m";

echo "Searching for standalone formulae...";
dependencies=();
for formula in $(brew list); do
  deps=(`brew uses $formula --installed | tr '\n' ' '`);
  # Add the formula if it's being used by another, installed one
  if [ ${#deps[@]} -lt 1 ]; then
    dependencies+=($formula);
  fi
done

if [ ${#dependencies[@]} -gt 1 ]; then
  echo -e "${GREEN}${#dependencies[@]} standalone formulae found${RESET}";
  for formula in "${dependencies[@]}"; do
    echo $formula;
  done
  echo "None of these formulae are used by any other formula, some of them might be legacy dependencies...";
else
  echo "No standalone formula found";
fi

Hey, just FYI there's a built-in command for this, which happens to be pretty quick since it analyzes the whole dependency tree at once:

$ brew leaves