diff --git a/lib/spack/spack/cmd/pkg.py b/lib/spack/spack/cmd/pkg.py
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+++ b/lib/spack/spack/cmd/pkg.py
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+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+import os
+
+from external import argparse
+import llnl.util.tty as tty
+from llnl.util.tty.colify import colify
+
+import spack
+from spack.util.executable import *
+
+description = "Query packages associated with particular git revisions in spack."
+
+def setup_parser(subparser):
+    sp = subparser.add_subparsers(
+        metavar='SUBCOMMAND', dest='pkg_command')
+
+    list_parser = sp.add_parser('list', help=pkg_list.__doc__)
+    list_parser.add_argument('rev', default='HEAD', nargs='?',
+                             help="Revision to list packages for.")
+
+    diff_parser = sp.add_parser('diff', help=pkg_diff.__doc__)
+    diff_parser.add_argument('rev1', nargs='?', default='HEAD^',
+                             help="Revision to compare against.")
+    diff_parser.add_argument('rev2', nargs='?', default='HEAD',
+                             help="Revision to compare to rev1 (default is HEAD).")
+
+    add_parser = sp.add_parser('added', help=pkg_added.__doc__)
+    add_parser.add_argument('rev1', nargs='?', default='HEAD^',
+                             help="Revision to compare against.")
+    add_parser.add_argument('rev2', nargs='?', default='HEAD',
+                            help="Revision to compare to rev1 (default is HEAD).")
+
+    rm_parser = sp.add_parser('removed', help=pkg_removed.__doc__)
+    rm_parser.add_argument('rev1', nargs='?', default='HEAD^',
+                           help="Revision to compare against.")
+    rm_parser.add_argument('rev2', nargs='?', default='HEAD',
+                           help="Revision to compare to rev1 (default is HEAD).")
+
+
+def get_git():
+    # cd to spack prefix to do git operations
+    os.chdir(spack.prefix)
+
+    # If this is a non-git version of spack, give up.
+    if not os.path.isdir('.git'):
+        tty.die("No git repo in %s. Can't use 'spack pkg'" % spack.prefix)
+
+    return which("git", required=True)
+
+
+def list_packages(rev):
+    git = get_git()
+    relpath = spack.packages_path[len(spack.prefix + os.path.sep):] + os.path.sep
+    output = git('ls-tree', '--full-tree', '--name-only', rev, relpath,
+                 return_output=True)
+    return sorted(line[len(relpath):] for line in output.split('\n') if line)
+
+
+def pkg_list(args):
+    """List packages associated with a particular spack git revision."""
+    colify(list_packages(args.rev))
+
+
+def diff_packages(rev1, rev2):
+    p1 = set(list_packages(rev1))
+    p2 = set(list_packages(rev2))
+    return p1.difference(p2), p2.difference(p1)
+
+
+def pkg_diff(args):
+    """Compare packages available in two different git revisions."""
+    u1, u2 = diff_packages(args.rev1, args.rev2)
+
+    if u1:
+        print "%s:" % args.rev1
+        colify(sorted(u1), indent=4)
+        if u1: print
+
+    if u2:
+        print "%s:" % args.rev2
+        colify(sorted(u2), indent=4)
+
+
+def pkg_removed(args):
+    """Show packages removed since a commit."""
+    u1, u2 = diff_packages(args.rev1, args.rev2)
+    if u1: colify(sorted(u1))
+
+
+def pkg_added(args):
+    """Show packages added since a commit."""
+    u1, u2 = diff_packages(args.rev1, args.rev2)
+    if u2: colify(sorted(u2))
+
+
+def pkg(parser, args):
+    action = { 'diff'    : pkg_diff,
+               'list'    : pkg_list,
+               'removed' : pkg_removed,
+               'added'   : pkg_added }
+    action[args.pkg_command](args)