acl

View the ACL of a file

$ getfacl [path/to/file]

$ setfacl -m u:[username]:rw [path/to/file]

$ setfacl -m g:[groupname]:rx [path/to/directory]

$ setfacl -x u:[username] [path/to/file]

$ setfacl -b [path/to/file]

$ setfacl -d -m u:[username]:rwx [path/to/directory]

$ setfacl -R -m u:[username]:rx [path/to/directory]

$ getfacl [source_file] | setfacl --set-file=- [target_file]

getfacl [options] file...
setfacl [options] aclspec file_...

Access Control Lists (ACLs) extend the traditional Unix permission model, allowing fine-grained permissions for multiple users and groups on a single file or directory. While standard permissions support only owner, group, and others, ACLs can specify distinct access rights for any number of named users and groups.
The getfacl command displays ACL entries, while setfacl modifies them. ACL entries follow the format type:name:permissions where type is u (user), g (group), o (other), or m (mask), name identifies the user or group, and permissions are combinations of r (read), w (write), and x (execute).
Default ACLs on directories determine the initial ACL for new files created within. The ACL mask entry limits the effective permissions granted to named users and groups.

-m aclspec_

Modify the ACL by adding or updating entries.

Remove specified ACL entries.

Remove all extended ACL entries.

Apply operations to the default ACL (for directories).

Apply recursively to directories and their contents.

Remove the default ACL.

Replace the entire ACL with the specified entries.

Read ACL entries from a file (use - for stdin).

Do not recalculate the effective rights mask.

Not all filesystems support ACLs; ext4, XFS, and Btrfs do when mounted with ACL support. Some backup and archive tools may not preserve ACLs. Files with ACLs show a + after the permission bits in ls -l output. NFSv4 uses a different ACL model than POSIX ACLs.

POSIX ACLs were developed in the 1990s as part of the POSIX 1003.1e draft standard, which was ultimately withdrawn but widely implemented. Linux ACL support was added to the kernel in version 2.6 (2003). The specification aimed to provide a portable ACL interface across Unix systems, though implementation details vary between operating systems.

chmod(1), chown(1), getfacl(1), setfacl(1), ls(1)