`brew doctor`

Library/Homebrew/brew.h.rb

 FORMULA_META_FILES = %w[README README.md ChangeLog COPYING LICENSE LICENCE COPYRIGHT AUTHORS]
 PLEASE_REPORT_BUG = "#{Tty.white}Please report this bug at #{Tty.em}http://github.com/mxcl/homebrew/issues#{Tty.reset}"
+HOMEBREW_RECOMMENDED_GCC = 5577
 
 def check_for_blacklisted_formula names
   return if ARGV.force?
@@ -24,6 +25,55 @@ Mercurial can be install thusly:
   end
 end
 
+def brew_doctor
+  read, write = IO.pipe
+
+  if fork == nil
+    read.close
+    $stdout.reopen write
+    
+    bad_dylibs = Dir['/usr/local/lib/*.dylib'].select { |f| File.file? f and not File.symlink? f }
+    if bad_dylibs.count > 0
+      puts "You have unbrewed dylibs in /usr/local/lib. These could cause build problems"
+      puts "when building Homebrew formula. If you no longer need them, delete them:"
+      puts
+      puts *bad_dylibs.collect { |f| "    #{f}" }
+      puts
+    end
+
+    if gcc_build < HOMEBREW_RECOMMENDED_GCC
+      puts "Your GCC version is older than the recommended version. It may be advisable"
+      puts "to upgrade to the latest release of Xcode."
+      puts
+    end
+
+    if macports_or_fink_installed?
+      puts "You have Macports or Fink installed. This can cause trouble."
+      puts "You don't have to uninstall them, but you may like to try temporarily"
+      puts "moving them away, eg."
+      puts
+      puts "    sudo mv /opt/local ~/macports"
+      puts
+    end
+
+    unless File.exists? '/usr/X11/lib/libpng.dylib'
+      puts "You don't have X11 installed as part of your Xcode installation."
+      puts "This isn't required for all formula. But it is expected by some."
+    end
+
+    exit! 0
+  else
+    write.close
+
+    unless (out = read.read).chomp.empty?
+      puts out
+    else
+      puts "Your OS X is ripe for brewing. Any troubles you may be experiencing are"
+      puts "likely purely psychosomatic."
+    end
+  end
+end
+
 def __make url, name
   require 'formula'
   require 'digest'

bin/brew

@@ -365,6 +365,9 @@ begin
   when 'branch', 'checkout', 'pull', 'push', 'rebase', 'reset'
     onoe "Unknown command: #{arg} (did you mean 'git #{arg}'?)"
 
+  when 'doctor'
+    brew_doctor
+
   else
     onoe "Unknown command: #{arg}"
   end