Pass parent process environment when run mercurial Otherwise some commands don't work. E.g. 'hg identify' fails with the error 'Invalid Signature' somewhere inside random.pyc when trying to import module tempfiles, because of the lack of SystemRoot environment variable. More details here: http://mail.python.org/pipermail/python-bugs-list/2011-December/154012.html http://jpassing.com/2009/12/28/the-hidden-danger-of-forgetting-to-specify-systemroot-in-a-custom-environment-block/