Extended Access Control Lists (ACLs) are a tool that allows for more flexible management of file and directory permissions in Linux operating systems. Unlike traditional methods where each file or directory can have permissions set only for the owner, group, and others, extended ACLs enable defining more detailed and specific access rules for different users and groups.
Tools for Working with Extended ACL
Key tools for working with extended ACL in Linux are setfacl
and getfacl
. These tools allow users to set and retrieve information about extended ACLs on files and directories.
Setfacl: Setting Extended ACL
The setfacl
tool is used to add, modify, or remove ACLs on files and directories. It enables defining permissions for specific users and groups beyond basic permissions. The syntax of the command is as follows:
setfacl -m u:user:permissions file
Where -m
indicates modification, u
denotes a user (for groups, g
is used), user
is the username, permissions
are the permissions (e.g., rwx), and file
is the path to the file or directory.
Example usage:
setfacl -m u:john:rw document.txt
This command sets read and write permissions for the user john
on the file document.txt
.
Getfacl: Retrieving Information about Extended ACL
On the other hand, getfacl
is a tool used to display a list of ACLs set on files and directories. With this command, users can easily find out what specific permissions have been assigned to a file or directory. The syntax of the command is:
getfacl file
Example usage:
getfacl document.txt
This command displays a list of all ACLs applied to document.txt
, including permissions for specific users and groups.
Practical Examples and Tips
For effective management of extended ACLs, it is important to understand the capabilities offered by setfacl
and getfacl
. Here are some tips and examples for common tasks:
- Adding permissions for multiple users: Multiple
-m
options can be used in a singlesetfacl
command to set permissions for multiple users at once. - Removing specific ACL: Using the
-x
option withsetfacl
allows you to remove specific permissions for users or groups. For example, removing permissions for the userjohn
fromdocument.txt
can be done with the command:setfacl -x u:john document.txt
- Setting default ACL for directories: Using the
-d
option, you can define default ACLs for all newly created files and subdirectories within a directory. This is useful for maintaining consistent permission settings within a directory structure.setfacl -d -m u:john:rw directory
-
This command ensures that all new files created in
directory
will automatically be assigned read and write permissions for the userjohn
. - Copying ACL between files: The
getfacl
command can be combined withsetfacl
to copy ACL from one file to another. First, you retrieve the ACL from the source file usinggetfacl
and then apply it to the target file usingsetfacl
:getfacl source_file | setfacl --set-file=- target_file
- Using mask to limit permissions: The mask in ACL limits effective permissions for all users and groups. It is important to understand the impact of the mask on the set permissions, as it can lead to unintended access restrictions. Setting the mask can be done using:
setfacl -m m:rwx file
This command sets the mask to
rwx
(read, write, execute), allowing maximum permissions defined by other ACLs.
Managing extended ACLs in Linux offers significant flexibility in defining access permissions to files and directories. The tools setfacl
and getfacl
are powerful aids for customizing permissions according to users' and groups' needs. While their usage may seem complex at first glance, practice and understanding of basic principles enable efficient utilization of extended ACLs for system security and management. It is important to note that incorrect settings can lead to unintended security risks, so it is advisable to familiarize oneself with recommended practices and thoroughly test configurations in a secure environment before implementation.