Index: trunk/tools/rundiffskycell.pl
===================================================================
--- trunk/tools/rundiffskycell.pl	(revision 29159)
+++ trunk/tools/rundiffskycell.pl	(revision 29163)
@@ -22,5 +22,5 @@
 use IO::File;
 
-my ($first, $last, $stage, $alt_workdir, $no_verbose);
+my ($first, $last, $stage, $alt_workdir, $no_verbose, $pretend);
 
 my $dbname = "gpc1";
@@ -33,4 +33,5 @@
     'redirect-output'   => \$redirect,
     'update'            => \$update,
+    'pretend'           => \$pretend,
     'dbname=s'          => \$dbname,
     'no-verbose'        => \$no_verbose, 
@@ -79,14 +80,22 @@
 if ($update) {
     my $revert_command = "difftool -revertdiffskyfile -diff_id $diff_id -skycell_id $skycell_id -dbname $dbname";
-    my $rc = system $revert_command;
-    if ($rc != 0) {
-        my $status = $rc >> 8;
-        print STDERR "$revert_command failed: $rc $status\n";
-        exit $status;
+    if ($pretend) {
+        print "skipping $revert_command\n";
+    } else {
+        my $rc = system $revert_command;
+        if ($rc != 0) {
+            my $status = $rc >> 8;
+            print STDERR "$revert_command failed: $rc $status\n";
+            exit $status;
+        }
     }
 }
 
+print "command to process this skycell\n";
+print "$command\n";
+
+exit 0 if $pretend;
+
 exit system $command;
-