* A garbage collector for installed packages. nix-collect-garbage

  doesn't actually delete any packages, it just prints their
  descriptor hashes.  So we can do

    nix info $(nix-collect-garbage)

  to print out the ids of the packages that would be deleted, and

    nix delpkg $(nix-collect-garbage)

  to actually delete them.

2 files changed, 23 insertions, 1 deletions

diff --git a/scripts/Makefile.am b/scripts/Makefile.am
index 4e2eada86b9d..4140cdf5b311 100644
--- a/scripts/Makefile.am
+++ b/scripts/Makefile.am
@@ -1,4 +1,4 @@
-bin_SCRIPTS = nix-generate-regscript nix-switch
+bin_SCRIPTS = nix-generate-regscript nix-switch nix-collect-garbage
 
 install-exec-local:
 	$(INSTALL) -d $(sysconfdir)/profile.d
diff --git a/scripts/nix-collect-garbage b/scripts/nix-collect-garbage
new file mode 100755
index 000000000000..adaba5b7c65b
--- /dev/null
+++ b/scripts/nix-collect-garbage
@@ -0,0 +1,22 @@
+#! /usr/bin/perl -w
+
+my $prefix = $ENV{"NIX"} || "/nix"; # !!! use prefix
+my $linkdir = "$prefix/var/nix/links";
+
+my %alive;
+
+open HASHES, "nix closure \$(cat $linkdir/*.hash) |";
+while (<HASHES>) {
+	chomp;
+	$alive{$_} = 1;
+}
+close HASHES;
+
+open HASHES, "nix listinst |";
+while (<HASHES>) {
+	chomp;
+	if (!$alive{$_}) {
+	    print "$_\n";
+	}
+}
+close HASHES;