def _build(self, args):
        '''
        Build a package
        '''
        if len(args) < 2:
            raise SPMInvocationError('A path to a formula must be specified')

        self.abspath = args[1].rstrip('/')
        comps = self.abspath.split('/')
        self.relpath = comps[-1]

        formula_path = '{0}/FORMULA'.format(self.abspath)
        if not os.path.exists(formula_path):
            raise SPMPackageError('Formula file {0} not found'.format(formula_path))
        with salt.utils.files.fopen(formula_path) as fp_:
            formula_conf = salt.utils.yaml.safe_load(fp_)

        for field in ('name', 'version', 'release', 'summary', 'description'):
            if field not in formula_conf:
                raise SPMPackageError('Invalid package: a {0} must be defined'.format(field))

        out_path = '{0}/{1}-{2}-{3}.spm'.format(
            self.opts['spm_build_dir'],
            formula_conf['name'],
            formula_conf['version'],
            formula_conf['release'],
        )

        if not os.path.exists(self.opts['spm_build_dir']):
            os.mkdir(self.opts['spm_build_dir'])

        self.formula_conf = formula_conf

        formula_tar = tarfile.open(out_path, 'w:bz2')

        if 'files' in formula_conf:
            # This allows files to be added to the SPM file in a specific order.
            # It also allows for files to be tagged as a certain type, as with
            # RPM files. This tag is ignored here, but is used when installing
            # the SPM file.
            if isinstance(formula_conf['files'], list):
                formula_dir = tarfile.TarInfo(formula_conf['name'])
                formula_dir.type = tarfile.DIRTYPE
                formula_tar.addfile(formula_dir)
                for file_ in formula_conf['files']:
                    for ftype in FILE_TYPES:
                        if file_.startswith('{0}|'.format(ftype)):
                            file_ = file_.lstrip('{0}|'.format(ftype))
                    formula_tar.add(
                        os.path.join(os.getcwd(), file_),
                        os.path.join(formula_conf['name'], file_),
                    )
        else:
            # If no files are specified, then the whole directory will be added.
            try:
                formula_tar.add(formula_path, formula_conf['name'], filter=self._exclude)
                formula_tar.add(self.abspath, formula_conf['name'], filter=self._exclude)
            except TypeError:
                formula_tar.add(formula_path, formula_conf['name'], exclude=self._exclude)
                formula_tar.add(self.abspath, formula_conf['name'], exclude=self._exclude)
        formula_tar.close()

        self.ui.status('Built package {0}'.format(out_path))