readlink.sh is a pure shell implementation that uses dirname, basename, readlink and pwd utils.
Note that you cannot rename it to just readlink as then the script will call itself instead of the system utility.
realpath script simply calls Python's os.path.realpath.
Python is provided in OS X and major Linux distributions.
You can use instead of the system utility by making a symlink: ln -s realpath readlink.
Another way is to install coreutils package via Homebrew or MacPorts and use greadlink.
The code is taken from the following page on StackOverflow: http://goo.gl/Yw9OY