modification time（mtime）：
当该文件的【内容数据】变更时，就会更新这个时间！内容数据指的是文件内容，而不是文件的属性或权限喔！

status time（ctime）：
当该文件的【状态（status）】改变时，就会更新这个时间，举例来说，像是权限与属性被更改了，都会更新这个时间啊。

access time（atime）：
当【该文件的内容被取用】时，就会更新这个读取时间（access）。举例来说，使用cat去读取/etc/man_db.conf，就会更新该文件的actime了。

在默认的情况下，ls显示出来的是该文件的mtime，也就是这个文件的内容上次被更动的时间。

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