Linux ACL Permissions

Access Control Lists (ACLs) in Linux provide a more flexible permission mechanism for file systems. They allow you to define permissions for specific users or groups beyond the standard owner, group, and others model.

What is ACL?

ACLs allow you to grant or deny access to files and directories for specific users or groups. This is particularly useful in multi-user environments where the traditional permission model is insufficient.

Enabling ACL

To use ACLs, the file system must be mounted with ACL support. Most modern Linux distributions enable this by default. If not, you can remount the file system with the acl option:

sudo mount -o remount,acl /mount/point

Basic Commands

Set ACL

Use the setfacl command to set ACLs. For example:

setfacl -m u:username:rw file.txt

This grants read and write permissions to the user username for the file file.txt.

setfacl -m g:username:rw file.txt

This grants read and write permissions to the group username for the file file.txt.

setfacl -m o:username:rw file.txt

This grants read and write permissions to others for the file file.txt.

View ACL

Use the getfacl command to view ACLs:

getfacl file.txt

Remove ACL

To remove an ACL entry, use:

setfacl -x u:username file.txt

This removes the ACL entry for the user username from the file file.txt.

To remove all ACL entries, use:

setfacl -b file.txt

This removes all ACL entries from the file file.txt.

Example

Suppose you have a file example.txt and you want to grant read and write permissions to a user named abhishek without changing the file's group or owner permissions.

Set the ACL:
```
setfacl -m u:abhishek:rw example.txt
```
Verify the ACL:
```
getfacl example.txt
```

The output will show an additional entry for abhishek with the specified permissions.

Default ACL

Default ACLs can be set for directories so that new files and subdirectories inherit the ACL. For example:

setfacl -m d:u:username:rw /directory