1 files changed, 42 insertions, 0 deletions
diff --git a/scripts/nix-switch b/scripts/nix-switch
new file mode 100755
index 000000000000..74bcef8566f9
--- /dev/null
+++ b/scripts/nix-switch
@@ -0,0 +1,42 @@
+#! /usr/bin/perl -w
+
+use strict;
+my $hash = $ARGV[0];
+$hash || die "no package hash specified";
+
+my $prefix = $ENV{"NIX"} || "/nix"; # !!! use prefix
+my $linkdir = "$prefix/var/nix/links";
+
+# Build the specified package, and all its dependencies.
+my $pkgdir = `nix getpkg $hash`;
+if ($?) { die "`nix getpkg' failed"; }
+chomp $pkgdir;
+
+my $id = `nix info $hash | cut -c 34-`;
+if ($?) { die "`nix info' failed"; }
+chomp $id;
+
+# Figure out a generation number.
+my $nr = 0;
+while (-e "$linkdir/$id-$nr") { $nr++; }
+my $link = "$linkdir/$id-$nr";
+print "$pkgdir\n";
+
+# Create a symlink from $link to $pkgdir.
+symlink($pkgdir, $link) or die "cannot create $link";
+
+# Make $link the current generation by pointing $linkdir/current to
+# it.  The rename() system call is supposed to be essentially atomic
+# on Unix.  That is, if we have links `current -> X' and `new_current
+# -> Y', and we rename new_current to current, a process accessing
+# current will see X or Y, but never a file-not-found or other error
+# condition.  This is sufficient to atomically switch the current link
+# tree.
+
+my $current = "$linkdir/current";
+
+print "switching $current to $link\n";
+
+my $tmplink = "$linkdir/new_current";
+symlink($link, $tmplink) or die "cannot create $tmplink";
+rename($tmplink, $current) or die "cannot rename $tmplink";