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ACL(5) File Formats Manual ACL(5)
acl — Access Control Lists
This manual page describes POSIX Access Control Lists, which are
used to define more fine-grained discretionary access rights for
files and directories.
Every object can be thought of as having associated with it an ACL
that governs the discretionary access to that object; this ACL is
referred to as an access ACL. In addition, a directory may have an
associated ACL that governs the initial access ACL for objects
created within that directory; this ACL is referred to as a
default ACL.
An ACL consists of a set of ACL entries. An ACL entry specifies
the access permissions on the associated object for an individual
user or a group of users as a combination of read, write and
search/execute permissions.
An ACL entry contains an entry tag type, an optional entry tag
qualifier, and a set of permissions. We use the term qualifier to
denote the entry tag qualifier of an ACL entry.
The qualifier denotes the identifier of a user or a group, for
entries with tag types of ACL_USER or ACL_GROUP, respectively.
Entries with tag types other than ACL_USER or ACL_GROUP have no
defined qualifiers.
The following entry tag types are defined:
ACL_USER_OBJ The ACL_USER_OBJ entry denotes access rights
for the file owner.
ACL_USER ACL_USER entries denote access rights for
users identified by the entry's qualifier.
ACL_GROUP_OBJ The ACL_GROUP_OBJ entry denotes access
rights for the file group.
ACL_GROUP ACL_GROUP entries denote access rights for
groups identified by the entry's qualifier.
ACL_MASK The ACL_MASK entry denotes the maximum
access rights that can be granted by entries
of type ACL_USER, ACL_GROUP_OBJ, or
ACL_GROUP.
ACL_OTHER The ACL_OTHER entry denotes access rights
for processes that do not match any other
entry in the ACL.
When an access check is performed, the ACL_USER_OBJ and ACL_USER
entries are tested against the effective user ID. The effective
group ID, as well as all supplementary group IDs are tested
against the ACL_GROUP_OBJ and ACL_GROUP entries.
A valid ACL contains exactly one entry with each of the
ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER tag types. Entries with
ACL_USER and ACL_GROUP tag types may appear zero or more times in
an ACL. An ACL that contains entries of ACL_USER or ACL_GROUP tag
types must contain exactly one entry of the ACL_MASK tag type. If
an ACL contains no entries of ACL_USER or ACL_GROUP tag types, the
ACL_MASK entry is optional.
All user ID qualifiers must be unique among all entries of
ACL_USER tag type, and all group IDs must be unique among all
entries of ACL_GROUP tag type.
The acl_get_file() function returns an ACL with zero ACL entries
as the default ACL of a directory, if the directory is not
associated with a default ACL. The acl_set_file() function also
accepts an ACL with zero ACL entries as a valid default ACL for
directories, denoting that the directory shall not be associated
with a default ACL. This is equivalent to using the
acl_delete_def_file() function.
The permissions defined by ACLs are a superset of the permissions
specified by the file permission bits.
There is a correspondence between the file owner, group, and other
permissions and specific ACL entries: the owner permissions
correspond to the permissions of the ACL_USER_OBJ entry. If the
ACL has an ACL_MASK entry, the group permissions correspond to the
permissions of the ACL_MASK entry. Otherwise, if the ACL has no
ACL_MASK entry, the group permissions correspond to the
permissions of the ACL_GROUP_OBJ entry. The other permissions
correspond to the permissions of the ACL_OTHER entry.
The file owner, group, and other permissions always match the
permissions of the corresponding ACL entry. Modification of the
file permission bits results in the modification of the associated
ACL entries, and modification of these ACL entries results in the
modification of the file permission bits.
The access ACL of a file object is initialized when the object is
created with any of the creat(), mkdir(), mknod(), mkfifo(), or
open() functions. If a default ACL is associated with a directory,
the mode parameter to the functions creating file objects and the
default ACL of the directory are used to determine the ACL of the
new object:
1. The new object inherits the default ACL of the containing
directory as its access ACL.
2. The access ACL entries corresponding to the file permission
bits are modified so that they contain no permissions that
are not contained in the permissions specified by the mode
parameter.
If no default ACL is associated with a directory, the mode
parameter to the functions creating file objects and the file
creation mask (see umask(2)) are used to determine the ACL of the
new object:
1. The new object is assigned an access ACL containing entries
of tag types ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER. The
permissions of these entries are set to the permissions
specified by the file creation mask.
2. The access ACL entries corresponding to the file permission
bits are modified so that they contain no permissions that
are not contained in the permissions specified by the mode
parameter.
A process may request read, write, or execute/search access to a
file object protected by an ACL. The access check algorithm
determines whether access to the object will be granted.
1. If the effective user ID of the process matches the user ID
of the file object owner, then
if the ACL_USER_OBJ entry contains the requested
permissions, access is granted,
else access is denied.
2. else if the effective user ID of the process matches the
qualifier of any entry of type ACL_USER, then
if the matching ACL_USER entry and the ACL_MASK entry
contain the requested permissions, access is granted,
else access is denied.
3. else if the effective group ID or any of the supplementary
group IDs of the process match the file group or the
qualifier of any entry of type ACL_GROUP, then
if the ACL contains an ACL_MASK entry, then
if the ACL_MASK entry and any of the matching
ACL_GROUP_OBJ or ACL_GROUP entries contain the
requested permissions, access is granted,
else access is denied.
else (note that there can be no ACL_GROUP entries
without an ACL_MASK entry)
if the ACL_GROUP_OBJ entry contains the requested
permissions, access is granted,
else access is denied.
4. else if the ACL_OTHER entry contains the requested
permissions, access is granted.
5. else access is denied.
A long and a short text form for representing ACLs is defined. In
both forms, ACL entries are represented as three colon separated
fields: an ACL entry tag type, an ACL entry qualifier, and the
discretionary access permissions. The first field contains one of
the following entry tag type keywords:
user A user ACL entry specifies the access granted to
either the file owner (entry tag type ACL_USER_OBJ)
or a specified user (entry tag type ACL_USER).
group A group ACL entry specifies the access granted to
either the file group (entry tag type ACL_GROUP_OBJ)
or a specified group (entry tag type ACL_GROUP).
mask A mask ACL entry specifies the maximum access which
can be granted by any ACL entry except the user
entry for the file owner and the other entry (entry
tag type ACL_MASK).
other An other ACL entry specifies the access granted to
any process that does not match any user or group
ACL entries (entry tag type ACL_OTHER).
The second field contains the user or group identifier of the user
or group associated with the ACL entry for entries of entry tag
type ACL_USER or ACL_GROUP, and is empty for all other entries. A
user identifier can be a user name or a user ID number in decimal
form. A group identifier can be a group name or a group ID number
in decimal form.
The third field contains the discretionary access permissions. The
read, write and search/execute permissions are represented by the
r, w, and x characters, in this order. Each of these characters is
replaced by the - character to denote that a permission is absent
in the ACL entry. When converting from the text form to the
internal representation, permissions that are absent need not be
specified.
White space is permitted at the beginning and end of each ACL
entry, and immediately before and after a field separator (the
colon character).
LONG TEXT FORM
The long text form contains one ACL entry per line. In addition, a
number sign (#) may start a comment that extends until the end of
the line. If an ACL_USER, ACL_GROUP_OBJ or ACL_GROUP ACL entry
contains permissions that are not also contained in the ACL_MASK
entry, the entry is followed by a number sign, the string
“effective:”, and the effective access permissions defined by that
entry. This is an example of the long text form:
user::rw-
user:lisa:rw- #effective:r--
group::r--
group:toolies:rw- #effective:r--
mask::r--
other::r--
SHORT TEXT FORM
The short text form is a sequence of ACL entries separated by
commas, and is used for input. Comments are not supported. Entry
tag type keywords may either appear in their full unabbreviated
form, or in their single letter abbreviated form. The abbreviation
for user is u, the abbreviation for group is g, the abbreviation
for mask is m, and the abbreviation for other is o. The
permissions may contain at most one each of the following
characters in any order: r, w, x. These are examples of the short
text form:
u::rw-,u:lisa:rw-,g::r--,g:toolies:rw-,m::r--,o::r--
g:toolies:rw,u:lisa:rw,u::wr,g::r,o::r,m::r
IEEE 1003.1e draft 17 defines Access Control Lists that include
entries of tag type ACL_MASK, and defines a mapping between file
permission bits that is not constant. The standard working group
defined this relatively complex interface in order to ensure that
applications that are compliant with IEEE 1003.1 (“POSIX.1”) will
still function as expected on systems with ACLs. The IEEE 1003.1e
draft 17 contains the rationale for choosing this interface in
section B.23.
On a system that supports ACLs, the file utilities ls(1), cp(1),
and mv(1) change their behavior in the following way:
• For files that have a default ACL or an access ACL that
contains more than the three required ACL entries, the ls(1)
utility in the long form produced by ls -l displays a plus
sign (+) after the permission string.
• If the -p flag is specified, the cp(1) utility also preserves
ACLs. If this is not possible, a warning is produced.
• The mv(1) utility always preserves ACLs. If this is not
possible, a warning is produced.
The effect of the chmod(1) utility, and of the chmod(2) system
call, on the access ACL is described in “CORRESPONDENCE BETWEEN
ACL ENTRIES AND FILE PERMISSION BITS”.
The IEEE 1003.1e draft 17 (“POSIX.1e”) document describes several
security extensions to the IEEE 1003.1 standard. While the work on
1003.1e has been abandoned, many UNIX style systems implement
parts of POSIX.1e draft 17, or of earlier drafts.
Linux Access Control Lists implement the full set of functions and
utilities defined for Access Control Lists in POSIX.1e, and
several extensions. The implementation is fully compliant with
POSIX.1e draft 17; extensions are marked as such. The Access
Control List manipulation functions are defined in the ACL library
(libacl, -lacl). The POSIX compliant interfaces are declared in
the <sys/acl.h> header. Linux-specific extensions to these
functions are declared in the <acl/libacl.h> header.
DENIED PERMISSIONS AND LINUX USER NAMESPACES
While ACLs can be used to deny processes permissions based on the
groups they are in, this is considered bad practice. Privileged
helpers such as newuidmap(1) can give unprivileged processes
access to the setgroups(2) system call, which allows them to drop
supplementary group membership and render restrictions based on
that membership ineffective. For further details, see
user_namespaces(7).
chmod(1), creat(2), getfacl(1), ls(1), mkdir(2), mkfifo(2),
mknod(2), mount(8), open(2), setfacl(1), stat(2), umask(1)
POSIX 1003.1e DRAFT 17
https://wt.tuxomania.net/publications/posix.1e/download.html
POSIX 1003.1e FUNCTIONS BY CATEGORY
ACL storage management
acl_dup(3), acl_free(3), acl_init(3)
ACL entry manipulation
acl_copy_entry(3), acl_create_entry(3), acl_delete_entry(3),
acl_get_entry(3), acl_valid(3)
acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3),
acl_delete_perm(3), acl_get_permset(3), acl_set_permset(3)
acl_get_qualifier(3), acl_get_tag_type(3),
acl_set_qualifier(3), acl_set_tag_type(3)
ACL manipulation on an object
acl_delete_def_file(3), acl_get_fd(3), acl_get_file(3),
acl_set_fd(3), acl_set_file(3)
ACL format translation
acl_copy_entry(3), acl_copy_ext(3), acl_from_text(3),
acl_to_text(3), acl_size(3)
POSIX 1003.1e FUNCTIONS BY AVAILABILITY
The first group of functions is supported on most systems with
POSIX-like access control lists, while the second group is
supported on fewer systems. For applications that will be ported
the second group is best avoided.
acl_delete_def_file(3), acl_dup(3), acl_free(3), acl_from_text(3),
acl_get_fd(3), acl_get_file(3), acl_init(3), acl_set_fd(3),
acl_set_file(3), acl_to_text(3), acl_valid(3)
acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3),
acl_copy_entry(3), acl_copy_ext(3), acl_copy_int(3),
acl_create_entry(3), acl_delete_entry(3), acl_delete_perm(3),
acl_get_entry(3), acl_get_permset(3), acl_get_qualifier(3),
acl_get_tag_type(3), acl_set_permset(3), acl_set_qualifier(3),
acl_set_tag_type(3), acl_size(3)
LINUX EXTENSIONS
These non-portable extensions are available on Linux systems.
acl_check(3), acl_cmp(3), acl_entries(3), acl_equiv_mode(3),
acl_error(3), acl_extended_fd(3), acl_extended_file(3),
acl_extended_file_nofollow(3), acl_from_mode(3), acl_get_perm(3),
acl_to_any_text(3)
Andreas Gruenbacher, <[email protected]>
This page is part of the acl (manipulating access control lists)
project. Information about the project can be found at
http://savannah.nongnu.org/projects/acl. If you have a bug report
for this manual page, see
⟨http://savannah.nongnu.org/bugs/?group=acl⟩. This page was
obtained from the project's upstream Git repository
⟨git://git.savannah.nongnu.org/acl.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-05-12.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
Linux ACL March 23, 2002 ACL(5)
Pages that refer to this page: chacl(1), getfacl(1), setfacl(1), mkdir(2), mknod(2), open(2), umask(2), erofs(5), ext4(5), tmpfiles.d(5), xattr(7), mount(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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