目标：在启动时运行 crontab 将arp命令的输出记录到 txt 文件中。
> Chrontab:
>
> # daemon's notion of time and timezones.
> #
> # Output of the crontab jobs (including errors) is sent through
> # email to the user the crontab file belongs to (unless redirected).
> #
> # For example, you can run a backup of all your user accounts
> # at 5 a.m every week with:
> # 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/ …cron ×1