Execute the command.

def run(self):
        """ Execute the command.

        """
        venv = "virtualenv {:s}"
        if self.python:
            venv += " -p {:s}"
        pip = "{0:s}/bin/pip install -r {2:s}" if self.requirements else None
        args = self.name, self.python, self.requirements
        try:
            check_call(venv.format(*args).split())
            if pip:
                log.info("installing requirements")
                check_call(pip.format(*args).split())
        except CalledProcessError:
            raise SystemExit(1)
        return