Atomic symlink switcheroo test script for GNU mv

switcheroo.sh

#!/bin/bash

# Make a temporary directory for our little test.
mkdir linkage.tmp || exit $?;
cd linkage.tmp;

# Create two directories and a symlink to the first.
rm -rvf v1 v2 current;
mkdir v1 v2;
ln -s v1 current;
ls -AlF;

function toggle {
     # Create a symlink to the directory that is not symlinked.
     [ "$(readlink 'current')" = 'v1' ] && target=v2 || target=v1;
     ln -vs "$target" "$target.symlink";

     # Now atomically overwrite the "current" symlink with the new symlink.
     mv -Tvf "$target.symlink" current; # strace shows this indeed uses rename()                                                                                                                        
}

# Toggle twice, to show that everything works as expected.
toggle;
ls -AlF;
toggle;
ls -AlF;

# Clean up.
cd ..;
rm -rf linkage.tmp

Also see @pixelb's File Replacement on UNIX.