File System Permissions
File-System Permissions
File-system permissions are a set of rules that determine who can access and modify files and directories within a file system. These permissions are an essential aspect of security and resource management in operating systems.
Traditional Unix Permissions
In Unix-like systems, file-system permissions are typically divided into three types: read, write, and execute. These permissions can be assigned to three categories of users:
- Owner: The user who created the file or directory.
- Group: A set of users who have been designated to share resources.
- Others: All users who are not the owner or part of the group.
File permissions in Unix are displayed using symbolic notation (e.g., rwxr-xr-x) or octal numbers (e.g., 755). The command chmod is used to change the permissions of a file or directory.
Access Control Lists (ACLs)
In addition to traditional Unix permissions, many file systems support Access Control Lists (ACLs). ACLs provide a more fine-grained permission mechanism, allowing specific permissions to be set for individual users or groups beyond the simple owner/group/others model.
ACLs are especially useful in environments where complex security requirements necessitate more detailed access control than traditional Unix permissions can provide.
Extended File Systems
The Extended File System (ext) family, which includes ext2, ext3, and ext4, has become the standard file system for Linux distributions. These file systems have robust support for file-system permissions, including ACLs.
File-System Attributes
File-system permissions are closely related to file attributes. Attributes can include metadata that defines additional properties of files, such as whether a file is read-only, hidden, or a system file.
Umask
The umask is a default permission mask that determines the permissions set for newly created files and directories. It acts as a filter that strips away permissions that are not desired. Each bit set in the umask removes its corresponding permission from the default.
Historical Context
Historically, file-system permissions have evolved from early time-sharing systems, such as the Compatible Time-Sharing System (CTSS). These systems supported multiple users with distinct access needs, laying the groundwork for modern permission models.
Innovative File Systems
Modern file systems like HAMMER, developed for DragonFly BSD, continue to innovate in permission management. These systems provide high-availability features and advanced permission handling, crucial for managing resources in distributed and networked environments.