Trac + SVN: Why post commit hook doesn't work?
I assume the problem is permission-related.
Make sure both the post-commit script and the rac-post-commit-hook have the proper privileges. If it works for you using the command line, try to do it as the user that is running the svn daemon (sudo -u svnuser [cmd]).
I could fix a similar problem a year or so ago by adding "chmod o+x" both of the scripts.
let's 'set -x' in the script for troubleshooting.