root用户无法修改文件权限(lsattr/chattr: i 和 a 属性含义)

　　今天想在实验室分配的服务器上添加一个普通用户, 所以用root身份登录服务器后执行useradd命令,却提示无法读写

/etc/shadow文件; ls -l /etc/shadow发现什么权限都没有,于是就用chmod更改我(root)对/etc/shadow文件的权限,但

让我万万没有想到的是,又提示无法更改该文件的权限,于是到网上查询之后,发现了这个帖子.问题解决了。

　　但，i和a属性是什么东西？

　　我们可以通过man chattr查看(仅摘录相关部分):

DESCRIPTION: "chattr" changes the file attributes on a Linux second extended file system.

A file with the 'a' attribute set can only be open in append mode for writing. Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.

A file with the 'i' attribute cannot be modified: it cannot be deleted or renamed, no link can be created to this file and no data can be  written to the file. Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.

 What do 'a' attribute and 'i' attribute mean?

append only (a), immutable (i).

补充：

linux三个特殊权限setuid、setgid和stick bit使用详解