2 files changed, 19 insertions, 0 deletions
diff --git a/Library/Homebrew/cmd/install.rb b/Library/Homebrew/cmd/install.rb
index 4b2711994..07942f64e 100644
--- a/Library/Homebrew/cmd/install.rb
+++ b/Library/Homebrew/cmd/install.rb
@@ -9,6 +9,12 @@ module Homebrew extend self
       raise "No available formula for #{name}\n#{msg}" if msg
     end unless ARGV.force?
 
+    ARGV.formulae.each do |f|
+      if File.directory? HOMEBREW_REPOSITORY/"Library/LinkedKegs/#{f.name}"
+        raise "#{f} already installed\nTry: brew upgrade #{f}"
+      end
+    end
+
     if Process.uid.zero? and not File.stat(HOMEBREW_BREW_FILE).uid.zero?
       # note we only abort if Homebrew is *not* installed as sudo and the user
       # calls brew as root. The fix is to chown brew to root.
diff --git a/Library/Homebrew/keg.rb b/Library/Homebrew/keg.rb
index ad0e97653..a0d028d08 100644
--- a/Library/Homebrew/keg.rb
+++ b/Library/Homebrew/keg.rb
@@ -33,10 +33,21 @@ class Keg < Pathname
       n+=1
       Find.prune if src.directory?
     end
+    linked_keg_record.unlink if linked_keg_record.exist?
     n
   end
 
+  def fname
+    parent.basename.to_s
+  end
+
+  def linked_keg_record
+    @linked_keg_record ||= HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname
+  end
+
   def link
+    raise "Cannot link #{fname}\nAnother version is already linked: #{linked_keg_record.realpath}" if linked_keg_record.directory?
+
     $n=0
     $d=0
 
@@ -68,6 +79,8 @@ class Keg < Pathname
       end
     end
 
+    (HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname).make_relative_symlink(self)
+
     return $n+$d
   end