POSIX Access Control Lists
Access Control Lists (ACLs) are a way of modifying file permissions in a way that is far more granular and flexible than standard Unix file permissions. Unfortunately, ACLs are also more complex and can lead to very confusing situations if you're not careful. Consequently, we don't recommend using ACLs unless you find yourself in a situation that cannot work with the standard permission model. (When in doubt, you can always email firstname.lastname@example.org for help with our systems.)
We used to use POSIX ACLs on our systems, but we now use NFSv4 Access Control Lists exclusively. Please read the NFSv4 page for more information.
The standard Unix permissions allow you to specify permissions for exactly one account (which is almost always your account), exactly one group, and "everyone else". Sometimes you might want more flexibility than that. You might, for example, want members of one group to be able to change a file, members of another group to be able to read it, and no access for everyone else. The standard Unix file permissions can't do that.
ACLs are things you attach to files and directories that basically just list additional accounts and groups with permissions specific to each account and group. You look at ACLs with the getfacl command and set them with the setfacl command. If a file or directory has ACLs, it will show a "+" symbol at the end of the ls -l permission listing, like the README file here:
-rw-r-----+ 1 account users 4153 Apr 16 2013 README
ACLs (unlike standard Unix permissions) have only one format, which is used for both setting and displaying them:
"permission-set" is one of "user", "group", and "other". (When setting ACLs, you can abbreviate those to "u", "g", and "o".) "account-or-group" is the account name for "user" ACLs and the group name for "group" ACLs. If you leave the "account-or-group" section empty, you'll affect the equivalent standard Unix permission set. (So setfacl -m group::rw file is the same as chmod g=rw file.) "permissions" is some combination of "r", "w", and "x", for read, write, and execute permissions, or a "-" to signify no permissions. ACLs don't have the extra properties for set UID, set GID, and sticky bit.
The output of getfacl will look something like this:
# file: README # owner: account # group: users user::rw- user:bob:r-- group::--- mask::r-- other::---
That shows that the "bob" account has read access to the file, even though no one (aside from the owner) has any permissions for it. (We'll cover the "mask" line shortly.)
Setting and Changing ACLs
To add an ACL to a file (or to change the permissions on an existing ACL), use setfacl -m :
setfacl -m u:bob:rw file
To remove an ACL, use setfacl -x, without the permission section of the ACL:
setfacl -x u:bob file
One nonintuitive thing that comes up with ACLs is the ACL mask. As soon as you add an ACL to a file, you will see a "mask" line show up in the getfacl output in addition to the ACL you added. The works a little like the symbolic representation of a umask: it gives the maximum permissions allowed to any account or group with its own ACL. It is also what's shown in the "group" section of the output of ls -l.
To repeat that for emphasis: Once you add an ACL to an object, ls -l will show the ACL mask in its middle set of permissions, not the permissions corresponding to the group that owns the object. The README example above shows this: the users group has no permissions for the file even though ls -l shows "-rw-r-----+".
Normally, setfacl will set the mask appropriately for the ACLs on the file. In some circumstances, especially if you use chmod on a file with ACLs, the mask may get out of sync with the other ACLs, in which case getfacl will show things like this:
user:bob:rw- #effective:r-- mask::r--
That means, roughly, "bob has an ACL that should give him read and write permissions, but because of the mask, he'll really only have read permissions."
To reset the mask, you can run:
setfacl --mask -m m::- file
Finally, directories can have "default ACLs". They have the same format as regular ACLs, but with "default:" at the beginning. (You can shorten that to "d:" if you want.) When a file is created in a directory with default ACLs, it automatically gets ACLs based on the defaults. When a directory is created in a directory with default ACLs, it gets ACLs based on the defaults (just like a file) and also gets its own copies of the default ACLs. Here's an example:
$ mkdir example $ setfacl -m default:user:bob:rw example $ touch example/foo $ getfacl -c example/foo user::rw- user:bob:rw- group::--- mask::rw- other::---
In summary, ACLs are useful, but can be potentially confusing (especially if you set them and then forget about them later). We don't really recommend using them unless you absolutely need to do so.
Also, just because this should be mentioned somewhere, but it doesn't really have any bearing on using the ACLs: There's technically no such thing as "POSIX" ACLs. "POSIX" comprises a set of Unix-related standards approved by the IEEE. The IEEE has never actually approved a standard for Access Control Lists. The ACLs described above are based on a proposed, but never approved, IEEE standard. Despite never being formally approved, the proposal has been widely implemented, so it's become a de-facto standard. No one's come up with a better name for them than "POSIX ACLs", so that's the name everyone uses.