From LQWiki

setfacl is a command that allows you to set the Access Control List information for a file or directory.

Access control lists are extended attributes added to most major file systems in the 2.6 kernel to improve ability to control the access of files. They allow permissions to be set for individual groups and users and not just the owning user, owning group, and all other users.

setfacl man page:

SETFACL(1) Access Control Lists SETFACL(1)

NAME

      setfacl - set file access control lists

SYNOPSIS

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

      setfacl --restore=file

DESCRIPTION

      This 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, ...).

      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 enries.  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 which 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.

  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  (blanks inserted for clarity):

      [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 inclu  de  the  union  of  all  permissions affected by the mask entry. (See the -n option 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 --access 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).

AUTHOR

      Andreas Gruenbacher, <a.gruenbacher@bestbits.at>.

      Please  send  your  bug reports, suggested features and comments to the above address.