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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _start(self):
 """The actual logic of starting logcat."""

        self._enable_logpersist()
        logcat_file_path = self._configs.output_file_path
        if not logcat_file_path:
            f_name = 'adblog,%s,%s.txt' % (self._ad.model,
                                           self._ad._normalized_serial)
            logcat_file_path = os.path.join(self._ad.log_path, f_name)
        utils.create_dir(os.path.dirname(logcat_file_path))
        cmd = '"%s" -s %s logcat -v threadtime %s >> "%s"' % (
            adb.ADB, self._ad.serial, self._configs.logcat_params,
            logcat_file_path)
        process = utils.start_standing_subprocess(cmd, shell=True)
        self._adb_logcat_process = process
        self.adb_logcat_file_path = logcat_file_path