ACL
also: Access Control List
Access Control List (ACL) is a set of rules that defines which users or groups have permission to access a file or directory and what operations they can perform.
An ACL extends standard Unix file permissions (owner, group, other) by allowing fine-grained control over file access. While traditional permissions support only three categories, ACLs let you specify permissions for multiple users and groups individually.
For example, with standard permissions you can give read access to the owner and group, but with an ACL you can grant read-only access to user alice, read-write access to user bob, and execute access to group developers—all on the same file.
Use getfacl to view ACLs and setfacl to modify them. Example: setfacl -m u:alice:rw /var/data/file.txt grants alice read-write permissions on the file.