def run(self,exe_name="pestpp",cwd=None):
        """run a command related to the pst instance. If
        write() has been called, then the filename passed to write
        is in the command, otherwise the original constructor
        filename is used

        exe_name : str
            the name of the executable to call.  Default is "pestpp"
        cwd : str
            the directory to execute the command in.  If None,
            os.path.split(self.filename) is used to find
            cwd.  Default is None


        """
        filename = self.filename
        if self.new_filename is not None:
            filename = self.new_filename
        cmd_line = "{0} {1}".format(exe_name,os.path.split(filename)[-1])
        if cwd is None:
            cwd = os.path.join(*os.path.split(filename)[:-1])
            if cwd == '':
                cwd = '.'
        print("executing {0} in dir {1}".format(cmd_line, cwd))
        pyemu.utils.os_utils.run(cmd_line,cwd=cwd)