Learn the Linux Command 'setfacl'

Control access to your files and folder

Setfacl utility sets Access Control Lists (ACLs) of files and directories. On the command line, a sequence of commands is followed by a sequence of files (which in turn can be followed by another sequence of commands, etc.).

  • The options -m, and -x expect an ACL on the command line. Multiple ACL entries are separated by comma characters (`,'). The options -M, and -X read an ACL from a file or from standard input. The ACL entry format is described in Section ACL ENTRIES.
  • The --set and --set-file options set the ACL of a file or a directory. The previous ACL is replaced. ACL entries for this operation must include permissions.
  • The -m (--modify) and -M (--modify-file) options modify the ACL of a file or directory. ACL entries for this operation must include permissions.
  • The -x (--remove) and -X (--remove-file) options remove ACL entries. Only ACL entries without the perms field are accepted as parameters unless POSIXLY_CORRECT is defined.

When reading from files using the -M, and -X options, setfacl accepts the output getfacl produces. There is at most one ACL entry per line. After a Pound sign (`#'), everything up to the end of the line is treated as a comment.

If setfacl is used on a file system that does not support ACLs, setfacl operates on the file mode permission bits. If the ACL does not fit completely in the permission bits, setfacl modifies the file mode permission bits to reflect the ACL as closely as possible, writes an error message to standard error, and returns with an exit status greater than 0.

Using Setfacl

First, try setting up a basic list for a folder in your home directory. Since you're trying this out for the first time, make a new one to play with. This way, you won't risk locking yourself out of important documents.

mkdir ~/acl-test

Next, use setfacl to grant another user on your system access to the folder. Setfacl uses the -m flag to modify the access control lists for a file. This usually means adding a user or group.

setfacl -m u:username:rwx ~/acl-test/

The u before the first colon tells the command you're talking about a username. The rwx following the second one grants read, write, and execute permissions.

The same works for groups by specifying g instead of u before the first colon.

setfacl -m g:groupname:rwx ~/acl-test/

It works the same as the previous example, except that it grants permissions to the entire group.

The -x flag works the opposite of -m. It will revoke access to the specified user or group.

setfacl -x u:username:w ~/acl-test/

The command revokes write access to the directory to the user.

You can also directly modify the permissions for all current users. This includes the owner of the file and any additional users that were granted permissions. This is something of a shortcut, using the existing command but omitting the username.

setfacl -m u::rx ~/acl-test/

This grants all existing users read and execute permissions but removes write access.

The same works with the -x flag and removing permissions.

setfacl -x u::w ~/acl-test/

This is essentially the same as the previous example, inverted.

setfacl also lets you copy the access control lists from one file to another. Make sure that your user has full access to the test folder, and create two new files to work with.

touch ~/acl-test/file{1,2}.txt

Now, modify the permissions of the first file.

setfacl -m u:username:rx ~/acl-text/file1.txt

Now, you can pipe the results of getfacl, the command for retrieving the current ACL info, into setfacl.

getfacl ~/acl-test/file1.txt | setfacl --set-file=- ~/acl-text/file2.txt

The second fill will have the same permissions as the first.

Finally, you can remove all extended ACL entries created by the command. This will leave the file with the user and group permissions that it originally had. To do this, use the -b flag.

setfacl -b ~/acl-test/

Those are the most common uses of setfacl. If you want to dive deeper, the next portion of this guide is a complete technical breakdown of the command.

Technical Breakdown of Setfacl

setfacl [-bkndRLPvh] [{-m|-x} acl_spec] [{-M|-X} acl_file] file ...

setfacl --restore=file

Permissions

The file owner and processes capable of CAP_FOWNER are granted the right to modify ACLs of a file. This is analogous to the permissions required for accessing the file mode. (On current Linux systems, root is the only user with the CAP_FOWNER capability.)

Options

-b, --remove-all

  • Remove all extended ACL entries. The base ACL entries of the owner, group, and others are retained.

-k, --remove-default

  • Remove the Default ACL. If no Default ACL exists, no warnings are issued.

-n, --no-mask

  • Do not recalculate the effective rights mask. The default behavior of setfacl is to recalculate the ACL mask entry unless a mask entry was explicitly given. The mask entry is set to the union of all permissions of the owning group, and all named user and group entries. (These are exactly the entries affected by the mask entry).

--mask

  • Do recalculate the effective rights mask, even if an ACL mask entry was explicitly given. (See the -n option.)

-d, --default

  • All operations apply to the Default ACL. Regular ACL entries in the input set are promoted to Default ACL entries. Default ACL entries in the input set are discarded. (A warning is issued if that happens).

--restore=file

  • Restore a permission backup created by `getfacl -R' or similar. All permissions of a complete directory subtree are restored using this mechanism. If the input contains owner comments or group comments, and setfacl is run by root, the owner and owning group of all files are restored as well. This option cannot be mixed with other options except `--test'.

--test

  • Test mode. Instead of changing the ACLs of any files, the resulting ACLs are listed.

-R, --recursive

  • Apply operations to all files and directories recursively. This option cannot be mixed with `--restore'.

-L, --logical

  • Logical walk, follow symbolic links. The default behavior is to follow symbolic link arguments and to skip symbolic links encountered in subdirectories. This option cannot be mixed with `--restore'.

-P, --physical

  • Physical walk, skip all symbolic links. This also skips symbolic link arguments. This option cannot be mixed with `--restore'.

--version

  • Print the version of setfacl and exit.

--help

  • Print help explaining the command line options.

End of command line options. All remaining parameters are interpreted as file names, even if they start with a dash.

If the file name parameter is a single dash, setfacl reads a list of files from standard input.

ACL Entries

The setfacl utility recognizes the following ACL entry formats:

[d[efault]:] [u[ser]:]uid [:perms]

  • Permissions of a named user. Permissions of the file owner if uid is empty.

[d[efault]:] g[roup]:gid [:perms]

  • Permissions of a named group. Permissions of the owning group if gid is empty.

[d[efault]:] m[ask][:] [:perms]

  • Effective rights mask

[d[efault]:] o[ther][:] [:perms]

  • Permissions of others

Whitespace between delimiter characters and non-delimiter characters is ignored.

Proper ACL entries including permissions are used in modify and set operations. (options -m-M--set and --set-file). Entries without the perms field are used for deletion of entries (options -x and -X).

For uid and gid you can specify either a name or a number.

The perms field is a combination of characters that indicate the permissions: read (r), write (w), execute (x), execute only if the file is a directory or already has execute permission for some user(X). Alternatively, the perms field can be an octal digit (0-7).

Automatically Created Entries

Initially, files and directories contain only the three base ACL entries for the owner, the group, and others. There are some rules that need to be satisfied in order for an ACL to be valid:

  • The three base entries cannot be removed. There must be exactly one entry of each of these base entry types.
  • Whenever an ACL contains named user entries or named group objects, it must also contain an effective rights mask.
  • Whenever an ACL contains any Default ACL entries, the three Default ACL base entries (default owner, default group, and default others) must also exist.
  • Whenever a Default ACL contains named user entries or named group objects, it must also contain a default effective rights mask.

To help the user ensure these rules, setfacl creates entries from existing entries under the following conditions:

  • If an ACL contains named user or named group entries, and no mask entry exists, a mask entry containing the same permissions as the group entry is created. Unless the -n option is given, the permissions of the mask entry are further adjusted to include the union of all permissions affected by the mask entry. (See the -n option description).
  • If a Default ACL entry is created, and the Default ACL contains no owner, owning group, or others entry, a copy of the ACL owner, owning group, or others entry is added to the Default ACL.
  • If a Default ACL contains named user entries or named group entries, and no mask entry exists, a mask entry containing the same permissions as the default Default ACL's group entry is added. Unless the -n option is given, the permissions of the mask entry are further adjusted to include the union of all permissions affected by the mask entry. (See the -noption description).

Examples

  • Granting an additional user read access
  • setfacl -m u:lisa:r file
  • Revoking write access from all groups and all named users (using the effective rights mask)
  • setfacl -m m::rx file
  • Removing a named group entry from a file's ACL
  • setfacl -x g:staff file
  • Copying the ACL of one file to another
  • getfacl file1 | setfacl --set-file=- file2
  • Copying the access ACL into the Default ACL
  • getfacl -a dir | setfacl -d -M- dir

Conformance to Posix 1003.1e Draft Standard 17

If the environment variable POSIXLY_CORRECT is defined, the default behavior of setfacl changes as follows: All non-standard options are disabled. The ``default:'' prefix is disabled. The -x and -X options also accept permission fields (and ignore them).