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1.4.2  Users and Groups (用户和组)

  In a multiuser system, each user has a private space on the machine: typically,he owns some quota(配额)of the disk space to store files,receives private mail messages, and so on.
在多用户操作系统上,每一个用户在机器上都有自己的私有空间:典型的如,用户都有自己的一定磁盘空间,用于存储文件,接收私人的邮件等等.

The operating system must ensure that the private portion of a user space is visible only to its owner.
操作系统必须确保某个用户空间的私有部分只对它的拥有者可见.
In particular,it must ensure that no user can exploit a system application for the purpose of violating the private space of another user.
特别是必须保证没有人能够利用系统应用(程序)去暴力破坏别人的私有空间.

All users are identified by unique number called the User ID,or UID.Usually only a restricted number of persons are allowed to make use of a computer system.
所有的用户都通过被称为用户ID或UID的,独一无二的识别码来识别.通常只有( a restricted number of persons)才能访问计算机系统.

when one of these users starts a working session, the operating system asks for a login name and a password.
当其中某个用户启动工作任务时,操作系统要求其输入登陆名和密码.

If the user does not input a valid pair(成对),the system denies access.Since the password is assumed to be secret,the user's privacy is ensured.
如果用户输入的密码和用户名不相符.则系统拒绝访问,因为假设密码是保密的,这样用户的私有权受到了保护.

In order to selectively share material with other users, each user is a member of one or more groups,which are identified by a unique number called a Group ID,or GID.
为了与其他用户有选择性的共享资料,(当然)每个用户有可能属于一个组和多个组的成员,它们都是通过称之谓组ID或GID的,唯一的认证码来区分的.
Each file is also associated with exactly one group.每个文件也是跟某个确定的组相关联的.
For example ,access should be set so that the user owning the file has read and write privileges ,the group has read-only privileges, and other users on the system are denied access to the file.
比如,访问也许被设为:用户拥有某文件的读写权限,而组拥有该文件的只读权限,并且该文件拒绝这个系统上的其他用户访问.

Any Unix-Like operating system has a special user called root,superuser,or supervisor.
The system administrator must log in as root in order to handle user accounts,perform maintenance tasks like system backups and program upgrades, and so on .
任何类Unix操作系统都有一个特别的用户叫做,root(根),超级用户或超级监管人.系统管理员必须以根用户登陆,去处理用户帐号,升级程序,进行系统备份等等之类的任务维护.

The root user can do almost everything,since the operating system does not apply the usual protection mechanisms to her.
In particular, the root user can access every file on  the system and can interfere with the activity of every running user program.
根用户几乎无所不能,因为操作系统不对其使用平常的保户机制.特别是,根用户可以访问系统上的任何文件,并且可以干涉每个运行的用户程序的活动.


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