docs: def shell_command(cmd, dry_run=False):
      """
      Run a shell command.
      """
      if cmd[0] == '/usr/bin/rsync':
          if dry_run:
              cmd = [cmd[0], '-n'] + cmd[1:]
          LOGGER.info('run: %s', ' '.join(cmd))
          subprocess.check_call(cmd)
      else:
          LOGGER.info('run: %s', ' '.join(cmd))
          if not dry_run:
              subprocess.check_call(cmd)