From the hadoop issues list comes issue 3318: https://issues.apache.org/jira/browse/HADOOP-3318 "Hadoop streaming doesn't recognize "Darwin" as an OS but Soylatte (OpenJDK port to Mac) reports that rather than "Mac OS X"" I think we can assume that Ant has the same problem...we need to search for the OS string "Darwin" and assume that means mac+unix
svn rev 675623