Objective 5: Use File Permissions to Control Access to Files

Filesystem security is a fundamental requirement for any multiuser operating system. The system's files, such as the kernel, configuration files, and programs, must be protected from accidents and tampering by unauthorized people. Users' files must be protected from modification by other users and sometimes must be kept completely private. In general, a form of access control must be implemented to allow secure operations.

Linux Access Control

Native Linux filesystem access control is implemented using a set of properties, maintained separately for each file. These properties are collectively called the access mode, or simply the mode, of the file. The mode is a part of the file's inode, the information retained in the filesystem that describes the file. A file's mode controls access by these three classes of users :


The user that owns the file.


The group that owns the file.


All other users on the system.

Like the mode, user and group ownership properties are a part of the inode, and both are assigned when a file is created. Usually, the owner is the user who created the file. The file's group is usually set to its creator's default group. Group ownership adds flexibility in situations in which a team shares files. The "other" users are those who aren't members of the file's group and are not the file's owner. For each of these three user classes, the access mode defines three types of permissions, ...

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