UNIX/Linux : Access control lists (ACLs) basics

Why we need ACLs ? Every file on any UNIX file will have a owner/group and set of permissions. Imagine a case when multiple users need access to the same file and the users are from different groups. The file access control lists (FACLs) or simply ACLs are the list of additional user/groups and their permission to the file.

How to know when a file has ACL attached to it It is very easy to know when a file has a attached ACL to it. ls -l command would produce a output as show below.

# ls -l
-rw-r--r-+ 1   root   root  0  Sep  19  14:41  file

Note the + sign at the end of the permissions. This confirms that the file has an ACL attached to it.

# getfacl /tmp/test

# file: test
# owner: root
# group: root
user::rw-
user:john:rw-
user:sam:rwx
group::r--
mask::rwx
other:---

Notice the 3 different user: lines. The first line lists the standard file permissions of the owner of the file. The 2 other user permissions are the individual permission for the user john and sam. The mask field here only applies to the additional permissions we have given to the user and groups. If the mask is set to rwx the read, write and execute permissions will be granted to additional user/groups. If the mask is set to r-x, the write permission will not be granted to additional user/groups.In general, DO NOT set mask to anything other than rwx. The mask value does not affect the standard UNIX user/group/others permissions.

# getfacl test

# file: test
# owner: root
# group: root
user::rw-
group::r--
other::r--

The setfacl command is used to set ACL on the given file. To give a rw access to user john on the file /tmp/test :

# setfacl -m u:john:rw /tmp/test

• The -m option tells setfacl to modify ACLs on the file(s) mentioned in command line. Instead of user john we can have a group to have a specific permission on the file :

  # setfacl -m g:accounts:rw /tmp/test

• FACLs for multiple user and groups can also be set with single command :

  # setfacl -m u:john:rw,g:accounts:rwx /tmp/test

Default ACLs are only created on directories. When you set default ACLs on directories, any files created within that directory will also have that default FACL assigned automatically.

To create a default FACL on a directory:

# setfacl -m default:u:john:rw /accounts

# getfacl accounts/

# file: accounts/
# owner: root
# group: root
user::rwx
group::r-x
other::r-x
default:user::rwx
default:user:john:rw-
default:group::r-x
default:mask::rwx
default:other::r-x

Now create a new file in the accounts directory and list the FACL on the file :

# touch /accounts/test

# getfacl test
# file: test
# owner: root
# group: root
user::rw-
user:john:rw-
group::r-x			#effective:r--
mask::rw-
other::r--

To remove FACL, use the setfacl command with -x option :

# setfacl -x u:john /tmp/test

The above command removes the ACL for the user john on the file /tmp/test. The ACLs for other user/groups if any remains unaffected.

To remove all ACLs associated to a file use the -b option with setfacl :

# setfacl -b /tmp/test

Many a times, the backup software may not copy the metadata related to the FACL on the files. In that case you may want to backup the FACL information on the files. Now, the FACL on all the files in a directory (including all sub directories) can be copied in a single file.

# cd /accounts
# getfacl -R * > accounts_facl        ( -R -> recursive )

When you restore the files in /accounts directory, you would have to restore the FACLs associated with the files in that direcotry. TO do that use the FACL backup file accounts_facl along with the –restore option :

# setfacl --restore=accounts_facl

Quelle: https://www.thegeekdiary.com/unix-linux-access-control-lists-acls-basics/