Access Control Lists (ACLs) are a flexible permission mechanism in Linux that allows you to set granular permissions on a per-user and per-group basis. They are an extension to the standard Unix permissions model and are particularly useful when more than one user or group needs access to a file or directory.
Understanding ACLs
In Linux, each file and directory has an associated ACL. An ACL consists of entries, each of which defines the permissions for a user or a group. There are three types of ACL entries:
- User entries: Define permissions for a specific user.
- Group entries: Define permissions for a specific group.
- Other entries: Define permissions for users not matched by the user and group entries.
Each entry has a set of permissions associated with it, similar to the standard read (r
), write (w
), and execute (x
) permissions.
Viewing ACLs
You can view the ACL of a file or directory using the getfacl
command. For example:
$ getfacl myfile.txt
# file: myfile.txt
# owner: alice
# group: staff
user::rw-
user:bob:r--
group::r--
mask::r--
other::---
In this example, the file myfile.txt
is owned by alice
and the staff
group. alice
has read and write permissions, bob
has read-only permissions, and all other users have no permissions.
Modifying ACLs
You can modify the ACL of a file or directory using the setfacl
command. For example, to give bob
read and write permissions to myfile.txt
, you would use:
$ setfacl -m u:bob:rw myfile.txt
You can also remove an ACL entry using the -x
option. For example, to remove bob
's permissions, you would use:
$ setfacl -x u:bob myfile.txt
Default ACLs
In addition to access ACLs, directories can also have default ACLs. These are used as a template for the ACL of new files and directories created within the directory.
You can set default ACLs using the d:
prefix. For example, to set a default ACL giving bob
read and write permissions, you would use:
$ setfacl -m d:u:bob:rw mydir
In conclusion, Linux ACLs provide a powerful and flexible mechanism for managing file and directory permissions. They allow you to set permissions on a per-user and per-group basis, providing more granular control than the standard Unix permissions model.
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