https://www.gnu.org/software/coreutils/manual/coreutils.html#File-permissions
文件权限被包括在coreutils包中。
The file mode bits have two parts: the file permission bits, which control ordinary access to the file, and special mode bits, which affect only some files.
文件模式位有两部分：文件权限位，它控制着文件的普通访问，特殊模式位，它只影响一些文件。
限制删除标志（restricted deletion flag）也称为 sticky 标志
原文：http://www.cnblogs.com/createyuan/p/4493837.html