Operating System Questions Long
File permissions and access control are crucial aspects of an operating system that ensure the security and integrity of files and data. They determine who can access, modify, or execute files and directories within a system. The concept of file permissions and access control revolves around three main components: users, groups, and permissions.
Users are individuals who interact with the operating system, and each user is assigned a unique user identifier (UID). Groups, on the other hand, are collections of users with similar access requirements, and each group is assigned a unique group identifier (GID). The operating system uses these identifiers to manage file permissions and access control.
File permissions define the level of access that users and groups have to a file or directory. There are three types of permissions: read (r), write (w), and execute (x). The read permission allows users to view the contents of a file or directory, the write permission enables users to modify or delete the file, and the execute permission grants users the ability to run executable files or access directories.
File permissions are assigned to three categories of users: the owner, the group, and others. The owner is the user who created the file or directory and has the highest level of control over it. The group consists of users who share similar access requirements, and others refer to all remaining users on the system.
Each category of users can be assigned different permissions, represented by a three-digit number known as the permission mode. The first digit represents the owner's permissions, the second digit represents the group's permissions, and the third digit represents the permissions for others. Each digit is a sum of values assigned to read (4), write (2), and execute (1) permissions. For example, a permission mode of 755 means the owner has read, write, and execute permissions (4+2+1=7), while the group and others have only read and execute permissions (4+1=5).
Access control lists (ACLs) provide a more granular level of control over file permissions. ACLs allow administrators to define specific permissions for individual users or groups, overriding the default permissions. This enables more fine-grained access control, especially in complex systems with multiple users and groups.
In summary, file permissions and access control in an operating system ensure that only authorized users can access, modify, or execute files and directories. By assigning permissions to users and groups, the operating system maintains the security and integrity of the system, protecting sensitive data from unauthorized access or modification.