hi coutts - on the bitbucket thread, i made sure to provide correct instructions for others and noted that a serverfault post is incorrect
truncate or gnu truncate is not included in osx/darwin (strange that it is on freebsd).
if the code used dd it may have just worked on osx, but it is easy enough to install
one way of which is via homebrew
brew install coreutils
which places gnu versions of gnu.org/coreutils in /usr/local/bin/ prefixed with a g
since there is no osx version of truncate, i found it simple to symlink
ln -s /usr/local/bin/gtruncate /usr/local/bin/truncate
hope that the macports method above works for you.