mkdir a
mkdir b
ln -s a S
ln -s ../b S/S
touch b/foo
ln -s S/foo foo
ln -s S/foo S/foo

##</code><code>##

perl -e 'my $file = shift; while(defined(my $link = readlink $file)) { print "LINK: $file\n"; $file = $link; sleep 1; } print "NOT LINK: $file\n"' foo