nix-super/scripts/nix-collect-garbage

#! /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;