<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _execute(self, command, stdin=None, stdout=subprocess.PIPE):
 """Executes the specified command relative to the repository root. Returns a tuple containing the return code and the process output. """

        process = subprocess.Popen(command, shell=True, cwd=self.root_path, stdin=stdin, stdout=stdout)
        return (process.wait(), None if stdout is not subprocess.PIPE else process.communicate()[0].decode('utf-8'))