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.
The operating system must ensure that the private portion of a user space is visible only to its owner.
In particular,it must ensure that no user can exploit a system application for the purpose of violating the private space of another user.
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.
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.
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.
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.
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 .
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.