In Linux, permissions are an important mechanism to govern who has access to files. If a file doesn’t grant permission to a user or a group, that user and group cannot access that file. There are three types of permissions: read (r), write (w), and execute (x). They are distributed across user, group, and “other” identities.

You can view the permissions assigned to a file using the ls command and the -l option:

$ ls -l sample
drwxrwxr-x.  2    user1    group1         6 Nov 11 20:16 sample

What is an ACL?

An access control list (ACL) lets you assign permissions for each unique user or group. Suppose you have user1, user2, and user3 on a system.

Users user2 and user3 are part of a demo group. You must assign this permission scheme to the sample directory:

• user1: read and write permission
• user2: read permission

You can use the setfacl and getfacl command utilities to assign and verify the ACL of a file or directory. To set an ACL, use the setfacl command with the --modify (-m for short) option:

$ setfacl --modify user1:rw sample
$ setfactl –modify user2:r sample

To verify an ACL, use getfacl:

$ getfacl sample
# file: sample
# owner: tux
# group: tux
user::rwx
user:user2:r--
user:user1:rw-
group::rwx
mask::rwx
other::r-x

Looks good!

Try an exercise

Suppose you have users called user1, user2, and user3, and the groups demo1 and demo2.

The demo1 group contains user1 and user2, and demo2 contains user2 and user3.

You need to set these permissions for a shared directory:

• user1: read and write
• user2: read
• user3: read and write and execute
• demo1: read and write
• demo2: read and write and execute
• other: read

To create this ACL, use setfacl:

$ setfacl -m u:user1:rw \
-m u:user2:r \
-m u:user3:rwx \
-m g:demo1:rw \
-m g:demo2:rwx \
-m o:r sample

Check your work

View the ACLs using the getfacl command:

$ getfacl sample
# file: sample
# owner: tux
# group: tux
user::rwx
user:user1:rw-
user:user2:r--
user:user3:rwx
group::r-x
group:demo1:rw-
group:demo2:rwx
mask::rwx
other::r--

All user and group permissions are listed and set correctly.

Delete an ACL

Use the -x option to delete an ACL:

$ setfacl -x u:user1 sample