brew/Library/Homebrew/config.rb

def cache
  if ENV['HOMEBREW_CACHE']
    Pathname.new(ENV['HOMEBREW_CACHE'])
  else
    # we do this for historic reasons, however the cache *should* be the same
    # directory whichever user is used and whatever instance of brew is executed
    home_cache = Pathname.new("~/Library/Caches/Homebrew").expand_path
    if home_cache.directory? and home_cache.writable_real?
      home_cache
    else
      Pathname.new("/Library/Caches/Homebrew").extend Module.new {
        def mkpath
          unless exist?
            super
            chmod 0775
          end
        end
      }
    end
  end
end

HOMEBREW_CACHE = cache
undef cache

# Where brews installed via URL are cached
HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE+"Formula"

if not defined? HOMEBREW_BREW_FILE
  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] || which('brew').to_s
end

# Where we link under
HOMEBREW_PREFIX = Pathname.new(HOMEBREW_BREW_FILE).dirname.parent

# Where .git is found
HOMEBREW_REPOSITORY = Pathname.new(HOMEBREW_BREW_FILE).realpath.dirname.parent

HOMEBREW_LIBRARY = HOMEBREW_REPOSITORY/"Library"
HOMEBREW_CONTRIB = HOMEBREW_REPOSITORY/"Library/Contributions"

# Where we store built products; /usr/local/Cellar if it exists,
# otherwise a Cellar relative to the Repository.
HOMEBREW_CELLAR = if (HOMEBREW_PREFIX+"Cellar").exist?
  HOMEBREW_PREFIX+"Cellar"
else
  HOMEBREW_REPOSITORY+"Cellar"
end

HOMEBREW_LOGS = Pathname.new(ENV['HOMEBREW_LOGS'] || '~/Library/Logs/Homebrew/').expand_path

HOMEBREW_TEMP = Pathname.new(ENV.fetch('HOMEBREW_TEMP', '/tmp'))

if not defined? HOMEBREW_LIBRARY_PATH
  HOMEBREW_LIBRARY_PATH = Pathname.new(__FILE__).realpath.parent.join("Homebrew")
end

HOMEBREW_LOAD_PATH = HOMEBREW_LIBRARY_PATH