Table 5.1, “ACL Entry Types” summarizes the six possible types of ACL entries, each defining permissions for a user or a group of users. The owner entry defines the permissions of the user owning the file or directory. The owning group entry defines the permissions of the file's owning group. The superuser can change the owner or owning group with chown or chgrp, in which case the owner and owning group entries refer to the new owner and owning group. Each named user entry defines the permissions of the user specified in the entry's qualifier field. Each named group entry defines the permissions of the group specified in the entry's qualifier field. Only the named user and named group entries have a qualifier field that is not empty. The other entry defines the permissions of all other users.
The mask entry further limits the permissions granted by named user, named group, and owning group entries by defining which of the permissions in those entries are effective and which are masked. If permissions exist in one of the mentioned entries as well as in the mask, they are effective. Permissions contained only in the mask or only in the actual entry are not effective—meaning the permissions are not granted. All permissions defined in the owner and owning group entries are always effective. The example in Table 5.2, “Masking Access Permissions” demonstrates this mechanism.
There are two basic classes of ACLs: A minimum ACL contains only the entries for the types owner, owning group, and other, which correspond to the conventional permission bits for files and directories. An extended ACL goes beyond this. It must contain a mask entry and may contain several entries of the named user and named group types.
Table 5.1. ACL Entry Types
Table 5.2. Masking Access Permissions
Figure 5.1, “Minimum ACL: ACL Entries Compared to Permission Bits” and Figure 5.2, “Extended ACL: ACL Entries Compared to Permission Bits” illustrate the two cases of a minimum
ACL and an extended ACL. The figures are structured in three
blocks—the left block shows the type specifications of the
ACL entries, the center block displays an example ACL, and the right
block shows the respective permission bits according to the
conventional permission concept, for example, as displayed by
-l. In both cases, the owner
class permissions are mapped to the ACL entry owner.
Other class permissions are mapped to the
respective ACL entry. However, the mapping of the group
class permissions is different in the two cases.
In the case of a minimum ACL—without mask—the group class permissions are mapped to the ACL entry owning group. This is shown in Figure 5.1, “Minimum ACL: ACL Entries Compared to Permission Bits”. In the case of an extended ACL—with mask—the group class permissions are mapped to the mask entry. This is shown in Figure 5.2, “Extended ACL: ACL Entries Compared to Permission Bits”.
This mapping approach ensures the smooth interaction of applications, regardless of whether they have ACL support. The access permissions that were assigned by means of the permission bits represent the upper limit for all other “fine adjustments” made with an ACL. Changes made to the permission bits are reflected by the ACL and vice versa.
With getfacl and setfacl on the command line, you can access ACLs. The usage of these commands is demonstrated in the following example.
Before creating the directory, use the
umask command to define which access permissions
should be masked each time a file object is created. The command
027 sets the default permissions by giving the
owner the full range of permissions (
the group write access (
2), and giving other
users no permissions at all (
umask actually masks the corresponding permission
bits or turns them off. For details, consult the
umask man page.
mkdir mydir creates the
mydir directory with the default permissions as
set by umask. Use ls
mydir to check whether all permissions were assigned
The output for this example is:
drwxr-x--- ... tux project3 ... mydir
mydir, check the initial state of the ACL. This gives
# file: mydir # owner: tux # group: project3 user::rwx group::r-x other::---
The first three output lines display the name, owner, and owning group of the directory. The next three lines contain the three ACL entries owner, owning group, and other. In fact, in the case of this minimum ACL, the getfacl command does not produce any information you could not have obtained with ls.
Modify the ACL to assign read, write, and execute permissions
to an additional user
geeko and an additional
setfacl -m user:geeko:rwx,group:mascots:rwx mydir
setfacl to modify the existing ACL. The following
argument indicates the ACL entries to modify (multiple entries are
separated by commas). The final part specifies the name of the
directory to which these modifications should be applied. Use the
getfacl command to take a look at the resulting
# file: mydir # owner: tux # group: project3 user::rwx user:geeko:rwx group::r-x group:mascots:rwx mask::rwx other::---
In addition to the entries initiated for the user
geeko and the group
a mask entry has been generated. This mask entry is set
automatically so that all permissions are effective.
setfacl automatically adapts existing mask
entries to the settings modified, unless you deactivate this feature
-n. mask defines the maximum effective
access permissions for all entries in the group class. This includes
named user, named group, and owning group. The group class
permission bits displayed by ls
-dl mydir now correspond to the
drwxrwx---+ ... tux project3 ... mydir
The first column of the output contains an additional
+ to indicate that there is an
extended ACL for this item.
According to the output of the ls command,
the permissions for the mask entry include write access.
Traditionally, such permission bits would mean that the owning group
project3) also has write access to the
mydir. However, the effective access
permissions for the owning group correspond to the overlapping
portion of the permissions defined for the owning group and for the
r-x in our example (see
Table 5.2, “Masking Access Permissions”). As far as the effective permissions of
the owning group in this example are concerned, nothing has changed
even after the addition of the ACL entries.
Edit the mask entry with setfacl or
chmod. For example, use chmod
g-w mydir. ls
-dl mydir then shows:
drwxr-x---+ ... tux project3 ... mydir
mydir provides the following output:
# file: mydir # owner: tux # group: project3 user::rwx user:geeko:rwx # effective: r-x group::r-x group:mascots:rwx # effective: r-x mask::r-x other::---
After executing the chmod command to remove
the write permission from the group class bits, the output of the
ls command is sufficient to see that the mask
bits must have changed accordingly: write permission is again
limited to the owner of
mydir. The output of
the getfacl confirms this. This output includes a
comment for all those entries in which the effective permission bits
do not correspond to the original permissions, because they are
filtered according to the mask entry. The original permissions can
be restored at any time with chmod
Directories can have a default ACL, which is a special kind of ACL defining the access permissions that objects in the directory inherit when they are created. A default ACL affects both subdirectories and files.
There are two ways in which the permissions of a directory's default ACL are passed to the files and subdirectories:
A subdirectory inherits the default ACL of the parent directory both as its default ACL and as an access ACL.
A file inherits the default ACL as its access ACL.
All system calls that create file system objects use a
mode parameter that defines the access
permissions for the newly created file system object. If the parent
directory does not have a default ACL, the permission bits as
defined by the
umask are subtracted from the
permissions as passed by the
with the result being assigned to the new object. If a default ACL
exists for the parent directory, the permission bits assigned to
the new object correspond to the overlapping portion of the
permissions of the
mode parameter and those that
are defined in the default ACL. The
disregarded in this case.
The following three examples show the main operations for directories and default ACLs:
Add a default ACL to the existing directory
setfacl -d -m group:mascots:r-x mydir
-d of the
setfacl command prompts
setfacl to perform the following modifications
-m) in the default ACL.
Take a closer look at the result of this command:
getfacl mydir # file: mydir # owner: tux # group: project3 user::rwx user:geeko:rwx group::r-x group:mascots:rwx mask::rwx other::--- default:user::rwx default:group::r-x default:group:mascots:r-x default:mask::r-x default:other::---
getfacl returns both the access ACL and the
default ACL. The default ACL is formed by all lines that start
default. Although you merely executed the
setfacl command with an entry for the
mascots group for the default ACL,
setfacl automatically copied all other
entries from the access ACL to create a valid default ACL.
Default ACLs do not have an immediate effect on access
permissions. They only come into play when file system objects
are created. These new objects inherit permissions only from the
default ACL of their parent directory.
In the next example, use mkdir to create
a subdirectory in
mydir, which inherits the
mkdir mydir/mysubdir getfacl mydir/mysubdir # file: mydir/mysubdir # owner: tux # group: project3 user::rwx group::r-x group:mascots:r-x mask::r-x other::--- default:user::rwx default:group::r-x default:group:mascots:r-x default:mask::r-x default:other::---
As expected, the newly-created subdirectory
mysubdir has the permissions from the
default ACL of the parent directory. The access ACL of
mysubdir is an exact reflection of the
default ACL of
mydir. The default ACL that
this directory will hand down to its subordinate objects is also
Use touch to create a file in the
mydir directory, for example,
-l mydir/myfile then shows:
-rw-r-----+ ... tux project3 ... mydir/myfile
The output of getfacl
# file: mydir/myfile # owner: tux # group: project3 user::rw- group::r-x # effective:r-- group:mascots:r-x # effective:r-- mask::r-- other::---
touch uses a
mode with the
0666 when creating new files, which
means that the files are created with read and write permissions
for all user classes, provided no other restrictions exist in
umask or in the default ACL (see Section 188.8.131.52, “Effects of a Default ACL”). In effect, this means
that all access permissions not contained in the
mode value are removed from the respective ACL
entries. Although no permissions were removed from the ACL entry
of the group class, the mask entry was modified to mask
permissions not set in
This approach ensures the smooth interaction of applications, such as compilers, with ACLs. You can create files with restricted access permissions and subsequently mark them as executable. The mask mechanism guarantees that the right users and groups can execute them as desired.
A check algorithm is applied before any process or application is granted access to an ACL-protected file system object. As a basic rule, the ACL entries are examined in the following sequence: owner, named user, owning group or named group, and other. The access is handled in accordance with the entry that best suits the process. Permissions do not accumulate.
Things are more complicated if a process belongs to more than one group and would potentially suit several group entries. An entry is randomly selected from the suitable entries with the required permissions. It is irrelevant which of the entries triggers the final result “access granted”. Likewise, if none of the suitable group entries contains the required permissions, a randomly selected entry triggers the final result “access denied”.