date是Linux系统里自带的一个系统命令，用来显示当前的系统时间，不过默认显示的结果里包括很多信息，特别是做为文件名输出时，不是很方便 好在date命令里包含格式化输出的选项。
$ date "+%Y-%m-%d"
2020-08-09
$ date "+%Y-%m-%d %H:%M:%S"
2020-08-09 17:51:17
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