Access Control Lists allow more granular permission on files than the standard 9 byte system via RBAC (role-based access control). Usecase examples:

• granting file read to a user without creating a new group only for that

ACL is handled on the filesystem level and its availability depends on the specific filesystem and its mount options: normally acl/noacl. E.g. ACL is on by default on btrfs.

Usage

• ls -l shows + after the first column to indicate non-standard ACL.
• getfacl prints ACL.
• setfacl sets ACL.