realpath is a simple implementation of the realpath utility:
http://www.gnu.org/software/coreutils/manual/html_node/realpath-invocation.html
since Mac OS X does not have anything equivalent.

This version takes no options.

This is based on the default GNU/Linux implementation that allows the last
component to not exist. This is different than the Debian implementation that
requires the last component to exist. Supporting the GNU/Linux implementation
makes writing scripts that depend on realpath much easier, and is probably
closer to what Apple may actually ship as a realpath implementation in the
future.

realpath only builds/runs on Darwin.