File Permissions and ACLs

Applies to CentOS Stream 9 & 10 / AlmaLinux 9.x & 10.x / Rocky Linux 9.x & 10.x

Linux file permissions are the cornerstone of system security. Every file and directory has a set of permission rules that control who can read, write, and execute them. When basic permissions are insufficient, ACLs (Access Control Lists) provide finer-grained control.

What You Will Learn

• Understand the rwx permission model
• Use chmod to modify permissions (symbolic and octal modes)
• Use chown and chgrp to change ownership and group
• SUID, SGID, and Sticky Bit special permissions
• Use ACLs for fine-grained permission control
• Understand and configure umask

Prerequisites

• A system with EL 9.x or 10.x installed (the commands here work on both)
• A user account with sudo privileges
• Familiarity with the basics of User and Group Management

Basic Permission Model

Viewing File Permissions

View file permission details

$ ls -l /etc/nginx/nginx.conf
-rw-r--r--. 1 root root 2488 Mar 24 10:00 /etc/nginx/nginx.conf

Breakdown of the output:

-rw-r--r--. 1 root root 2488 Mar 24 10:00 nginx.conf
│├──┤├──┤├──┤ │ │    │    │     │          │
│ │   │   │  │ │    │    │     │          └─ Filename
│ │   │   │  │ │    │    │     └─ Modification time
│ │   │   │  │ │    │    └─ File size
│ │   │   │  │ │    └─ Group
│ │   │   │  │ └─ Owner
│ │   │   │  └─ Hard link count
│ │   │   └─ Other users' permissions (other)
│ │   └─ Group permissions (group)
│ └─ Owner permissions (owner)
└─ File type (- regular file, d directory, l symbolic link)

rwx Permission Meanings

Permission	Character	Meaning for Files	Meaning for Directories
Read	r	Read file contents	List directory contents
Write	w	Modify file contents	Create/delete files in the directory
Execute	x	Execute the file (script/program)	Enter the directory (cd)

Note

The x permission on directories is often overlooked. Without x permission, you cannot cd into a directory, and even with r permission you cannot properly list detailed directory contents.

chmod: Modifying Permissions

Symbolic Mode

Symbolic mode uses u (owner), g (group), o (other), a (all) combined with + (add), - (remove), = (set) to modify permissions.

Add execute permission for the owner

$ chmod u+x script.sh

Remove write permission for others

$ chmod o-w file.txt

Add read and write permissions for the group

$ chmod g+rw shared.doc

Set owner to rwx, group and others to read-only

$ chmod u=rwx,g=r,o=r script.sh

Add execute permission for all users

$ chmod a+x script.sh

Octal Mode

Each permission bit corresponds to a numeric value:

Permission	Value
r	4
w	2
x	1
-	0

Add up the permission values for owner, group, and others respectively:

Number	Permission	Description
7	rwx	Read + write + execute
6	rw-	Read + write
5	r-x	Read + execute
4	r--	Read-only
3	-wx	Write + execute
2	-w-	Write-only
1	--x	Execute-only
0	---	No permissions

Set permissions to 755 (owner rwx, group r-x, others r-x)

$ chmod 755 script.sh

Set permissions to 644 (owner rw-, group r--, others r--)

$ chmod 644 config.conf

Set permissions to 700 (owner rwx, no permissions for others)

$ chmod 700 private-dir/

Recursive Permission Changes

Recursively change permissions on a directory and its contents

$ chmod -R 755 /var/www/html/

Caution

Using chmod 755 recursively adds execute permission to all files. A better approach is to set permissions separately for directories and files.

Set directories to 755 and files to 644 (recommended approach)

$ find /var/www/html/ -type d -exec chmod 755 {} +
$ find /var/www/html/ -type f -exec chmod 644 {} +

chown and chgrp: Changing Ownership and Group

Changing the Owner

Change the owner of a file

$ sudo chown webuser /var/www/html/index.html

Change both owner and group simultaneously

$ sudo chown webuser:webgroup /var/www/html/index.html

Recursively change the owner and group of a directory

$ sudo chown -R webuser:webgroup /var/www/html/

Changing Only the Group

Change the group of a file

$ sudo chgrp developers project-file.txt

Recursively change the group of a directory

$ sudo chgrp -R developers /opt/project/

Special Permission Bits

In addition to the basic rwx permissions, Linux has three special permission bits.

SUID (Set User ID)

When an executable file has the SUID bit set, any user who runs the file will execute it as the file’s owner (rather than as themselves).

A classic SUID example: the passwd command

$ ls -l /usr/bin/passwd
-rwsr-xr-x. 1 root root 32648 ... /usr/bin/passwd

The s in the owner permission is the SUID flag. When a regular user runs passwd, the process runs as root, allowing it to modify /etc/shadow.

Set SUID (octal prefix 4)

$ sudo chmod u+s /path/to/program
$ sudo chmod 4755 /path/to/program

Caution

SUID is a powerful but dangerous mechanism. If a SUID program has a vulnerability, an attacker can exploit it to gain root privileges. Only set SUID when truly necessary, and periodically audit SUID files on your system.

Find all files with SUID set on the system

$ sudo find / -perm -4000 -type f 2>/dev/null

SGID (Set Group ID)

For files: The process runs with the file’s group identity.

For directories: New files and subdirectories created within the directory automatically inherit the directory’s group instead of the creator’s primary group. This is very useful for team collaboration directories.

Set SGID (octal prefix 2)

$ sudo chmod g+s /opt/shared-project/
$ sudo chmod 2775 /opt/shared-project/

Verify the SGID effect

$ ls -ld /opt/shared-project/
drwxrwsr-x. 2 root developers 4096 ... /opt/shared-project/

The s in the group permission is the SGID flag.

Sticky Bit

When the Sticky Bit is set on a directory, files within it can only be deleted by the file’s owner or root, even if other users have write permission on the directory. The most common example is /tmp.

View the Sticky Bit on /tmp

$ ls -ld /tmp
drwxrwxrwt. 15 root root 4096 ... /tmp

The t in the other users’ permission is the Sticky Bit flag.

Set the Sticky Bit (octal prefix 1)

$ sudo chmod +t /opt/shared-uploads/
$ sudo chmod 1777 /opt/shared-uploads/

Special Permissions Summary

Permission	Octal	Symbol	Effect on Files	Effect on Directories
SUID	4	u+s	Execute as the owner	(No special effect)
SGID	2	g+s	Execute as the group	New files inherit the directory’s group
Sticky	1	+t	(No special effect)	Only the owner can delete files

Practical Example: Creating a Team Shared Directory

1. Create the shared directory

   Create the directory

   $ sudo mkdir /opt/team-share

2. Set the owner and group

   Set ownership to the developers group

   $ sudo chown root:developers /opt/team-share

3. Set permissions and SGID

   Set SGID to ensure new files inherit the group

   $ sudo chmod 2775 /opt/team-share

4. Verify

   Confirm permissions are set correctly

   $ ls -ld /opt/team-share
   drwxrwsr-x. 2 root developers 4096 ... /opt/team-share

   Now any member of the developers group who creates files in this directory will have those files automatically belong to the developers group.

5. Test

   Create a file as a developers group member

   $ touch /opt/team-share/test.txt
   $ ls -l /opt/team-share/test.txt
   -rw-rw-r--. 1 zhangsan developers 0 ... test.txt

umask: Default Permission Mask

umask determines the default permissions for newly created files and directories. The permissions of a new file = base permissions - umask value.

• Base permissions for files: 666 (no execute bit)
• Base permissions for directories: 777

View the current umask

$ umask
0022

View in symbolic form

$ umask -S
u=rwx,g=rx,o=rx

Effect of umask 0022:

	Base Permissions	umask	Actual Permissions
File	666	022	644 (rw-r—r—)
Directory	777	022	755 (rwxr-xr-x)

Changing umask

Temporarily change umask (current session only)

$ umask 027

Effect of umask 027:

	Base Permissions	umask	Actual Permissions
File	666	027	640 (rw-r-----)
Directory	777	027	750 (rwxr-x---)

Permanently change a user's umask

$ echo "umask 027" >> ~/.bashrc

Tip

For server environments, it is recommended to set umask to 027 or 077 to restrict default access to new files for “other” users.

ACLs (Access Control Lists)

When the basic owner/group/other permission model is insufficient, ACLs allow you to set independent permissions for specific users or groups.

Viewing ACLs

View a file's ACL

$ getfacl /opt/team-share/config.txt

Example output (no additional ACL):

opt/team-share/config.txt

# owner: zhangsan
# group: developers
user::rw-
group::rw-
other::r--

Setting ACLs

Grant read-write permission to a specific user

$ sudo setfacl -m u:lisi:rw /opt/team-share/config.txt

Grant read-only permission to a specific group

$ sudo setfacl -m g:qa:r /opt/team-share/config.txt

View the ACL after changes

$ getfacl /opt/team-share/config.txt

Example output:

opt/team-share/config.txt

# owner: zhangsan
# group: developers
user::rw-
user:lisi:rw-
group::rw-
group:qa:r--
mask::rw-
other::r--

Note

When a file has ACLs set, the ls -l output will show a + after the permissions, such as -rw-rw-r--+, indicating that the file has additional ACL rules.

ACL Operations in Detail

Set a user ACL

$ sudo setfacl -m u:username:permissions filepath

Set a group ACL

$ sudo setfacl -m g:groupname:permissions filepath

Remove a specific user's ACL

$ sudo setfacl -x u:lisi /opt/team-share/config.txt

Remove a specific group's ACL

$ sudo setfacl -x g:qa /opt/team-share/config.txt

Remove all ACLs from a file

$ sudo setfacl -b /opt/team-share/config.txt

Recursively set ACLs on a directory

$ sudo setfacl -R -m u:lisi:rwx /opt/team-share/

Default ACLs

Default ACLs cause newly created files within a directory to automatically inherit ACL rules:

Set default ACLs

$ sudo setfacl -d -m u:lisi:rw /opt/team-share/
$ sudo setfacl -d -m g:qa:r /opt/team-share/

View default ACLs

$ getfacl /opt/team-share/

Example output:

# file: opt/team-share/
# owner: root
# group: developers
# flags: -s-
user::rwx
group::rwx
other::r-x
default:user::rwx
default:user:lisi:rw-
default:group::rwx
default:group:qa:r--
default:mask::rwx
default:other::r-x

Now new files created in this directory will automatically include ACL rules for lisi and the qa group.

The mask in ACLs

The mask defines the maximum permissions that users and groups can have in the ACL. Even if you set a user’s permissions to rwx, if the mask is r--, the user’s effective permissions will only be r--.

Set the mask

$ sudo setfacl -m m::rx /opt/team-share/config.txt

Tip

When you use chmod to modify the group permissions of a file that has ACLs, you are actually modifying the mask value, not the real group permissions. This can lead to unexpected permission changes, so be careful.

Practical Example: Fine-Grained Project Permission Control

Suppose you have the following requirements:

• The project directory /opt/webapp belongs to the webdev group
• Members of the webdev group have full read-write access
• The qa group can only read, not modify
• User deployer has full access for deployments
• Newly created files automatically inherit these rules

1. Create the directory and groups

   Create the directory and groups

   $ sudo mkdir -p /opt/webapp
   $ sudo groupadd webdev
   $ sudo groupadd qa

2. Set basic permissions

   Set owner, group, and SGID

   $ sudo chown root:webdev /opt/webapp
   $ sudo chmod 2770 /opt/webapp

3. Set ACLs

   Read-only permissions for the qa group

   $ sudo setfacl -m g:qa:rx /opt/webapp

   Full permissions for the deployer user

   $ sudo setfacl -m u:deployer:rwx /opt/webapp

4. Set default ACLs (applied to newly created files)

   Set default ACLs

   $ sudo setfacl -d -m g:webdev:rwx /opt/webapp
   $ sudo setfacl -d -m g:qa:rx /opt/webapp
   $ sudo setfacl -d -m u:deployer:rwx /opt/webapp

5. Verify

   View the complete ACL settings

   $ getfacl /opt/webapp

Common Issues

Which Takes Priority, chmod or ACL

ACLs are an extension of basic permissions. When both exist:

• The file owner’s permissions are still controlled by user::
• Other users’ permissions are jointly affected by ACL rules and the mask
• chmod on group permissions modifies the mask value when ACLs are present

The File Owner is root, but Regular Users Can Still Read It

Because the other (other users) permission allows reading. For example, permissions of 644 mean all users can read the file.

How to Determine Why a User Cannot Access a File

Check file permissions and ACL

$ ls -la /path/to/file
$ getfacl /path/to/file

Check permissions at every level of the directory path

$ namei -l /path/to/file

namei -l displays the permissions of each directory level in the path, helping you identify which level is blocking access.

How to Back Up and Restore ACLs

Back up ACLs to a file

$ getfacl -R /opt/webapp > acl-backup.txt

Restore ACLs from a backup

$ sudo setfacl --restore=acl-backup.txt

Further Reading

• User and Group Management — Create and manage users and groups
• sudo Configuration — Administrator privilege configuration
• SELinux Introduction — Mandatory access control
• man chmod / man chown / man setfacl / man getfacl