def __instructions(self):
        """String representation of the building block"""
        if hpccm.config.g_linux_distro == linux_distro.UBUNTU:
            if self.__apt:
                ospackages = self.__apt
            else:
                ospackages = self.__ospackages

            self += apt_get(_apt_key=self.__apt_key,
                            aptitude=self.__aptitude,
                            download=self.__download,
                            download_directory=self.__download_directory,
                            extra_opts=self.__extra_opts,
                            extract=self.__extract,
                            keys=self.__apt_keys,
                            ospackages=ospackages,
                            ppas=self.__apt_ppas,
                            repositories=self.__apt_repositories)
        elif hpccm.config.g_linux_distro == linux_distro.CENTOS:
            if self.__yum:
                ospackages = self.__yum
            else:
                ospackages = self.__ospackages

            self += yum(download=self.__download,
                        download_directory=self.__download_directory,
                        extra_opts=self.__extra_opts,
                        extract=self.__extract,
                        epel=self.__epel,
                        force_add_repo=self.__force_add_repo,
                        keys=self.__yum_keys,
                        ospackages=ospackages,
                        powertools=self.__powertools,
                        release_stream=self.__release_stream,
                        scl=self.__scl,
                        repositories=self.__yum_repositories,
                        yum4=self.__yum4)
        else:
            raise RuntimeError('Unknown Linux distribution')
String representation of the building block
hpccm/building_blocks/packages.py