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POSIX ACL

POSIX ACLs extend Unix's owner/group/other model with per-user and per-group entries stored as extended attributes. Managed with setfacl and getfacl, they are supported by ext4, XFS, Btrfs, and others, and shown by a trailing + in ls -l.

POSIX ACLs (Access Control Lists) extend the traditional owner/group/other permission model of Unix-like systems with per-user and per-group entries. The interface was defined by the POSIX.1e draft (which never became a full standard but was widely adopted) and is implemented on Linux for ext2/3/4, XFS, Btrfs, JFS, and others, on FreeBSD UFS, and on Solaris ZFS in compatibility mode. An ACL is stored as an extended attribute (xattr) named `system.posix_acl_access` for files and `system.posix_acl_default` for directories. Each ACL contains a list of entries, each naming a principal (a specific user, a specific group, the file owner, the owning group, the mask, or 'other') and a rwx triplet. The `mask` entry places an upper bound on the effective permissions of named users, named groups, and the owning group — chmod on the group bits updates the mask, which is the subtle interaction that confuses newcomers. Management uses two tools: `getfacl` prints an ACL and `setfacl` modifies one. Typical invocations include `setfacl -m u:alice:rw report.txt` (give alice read/write), `setfacl -x g:contractors file` (remove a named group), and `setfacl -d -m u:bob:rx project/` (set a default ACL on a directory so new entries inherit bob's access). The `+` suffix in `ls -l` output (e.g. `-rw-r--r--+`) signals that an ACL is attached. ACLs are filesystem- and tool-aware: many archivers (`tar`, `cp`, `rsync`) require explicit flags (`--acls`, `-A`, `-a`) to preserve them, and a `mount` without the `acl` option (default on modern ext4 and XFS) will silently ignore them. NFSv4 defines a richer, Windows-style ACL model that is not interchangeable with POSIX ACLs; bridging between them is a known source of permission surprises in mixed environments.

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June 05, 2026

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umask

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