Linux File Permissions Explained | Complete Guide with Octal Cheatsheet and Commands

Table of Contents

• What Are Linux File Permissions?
• File Permission Structure
• Octal (Numeric) Representation of Permissions
• Special File Permissions: SUID, SGID, Sticky Bit
• Changing Permissions Using chmod
• Viewing Permissions with ls -l
•  Best Practices for Managing Permissions
• Common Permission Use Cases
•  Conclusion
•  Frequently Asked Questions (FAQs)

Understanding Linux file permissions is essential for system administrators, ethical hackers, developers, and cybersecurity professionals. Proper file permissions protect sensitive data, prevent unauthorized access, and define who can read, write, or execute files or directories. In this blog, we’ll break down everything from permission notations to special bits like SUID, SGID, and the Sticky Bit, using real-world examples and terminal commands.

What Are Linux File Permissions?

Linux file permissions determine the level of access users have to a particular file or directory. These permissions are assigned to three categories of users:

• Owner: The user who owns the file

• Group: Users who belong to the file's group

• Others: Everyone else

Each of these categories can be assigned three types of permissions:

 File Permission Structure

A typical Linux permission string looks like:

-rwxr-xr--

Here’s how to read it:

• First character: - indicates a regular file; d is for directories.

• Next 3: rwx — Owner permissions (read, write, execute)

• Next 3: r-x — Group permissions (read, execute)

• Last 3: r-- — Other permissions (read only)

 Octal (Numeric) Representation of Permissions

Permissions can also be represented in binary or octal:

So a permission string like rwxr-xr-- is 754 in octal.

 Special File Permissions: SUID, SGID, Sticky Bit

Linux offers three special bits that can alter how files are accessed and executed:

 SUID (Set User ID)

Allows users to run an executable with the permissions of the file owner.

• Symbol: s in the owner execute field

• Example: -rwsr-xr-x

• Command:

  chmod u+s filename
  

 SGID (Set Group ID)

Files: Similar to SUID, but applies to group permissions.
Directories: Newly created files inherit the group of the directory.

• Symbol: s in the group execute field

• Example: -rwxr-sr-x

• Command:

  chmod g+s directory
  

Sticky Bit

Applied to directories to restrict file deletion within them to file owners.

• Symbol: t in the other execute field

• Example: drwxrwxrwt

• Command:

  chmod +t directory
  

 Changing Permissions Using chmod

Linux provides the chmod command to change permissions in both symbolic and octal format.

✅ Symbolic Example:

chmod u+x script.sh   # Add execute to user
chmod g-w script.sh   # Remove write from group
chmod o=r file.txt    # Set read-only for others

✅ Octal Example:

chmod 755 filename     # rwxr-xr-x
chmod 700 filename     # rwx------
chmod 644 file.txt     # rw-r--r--

 Viewing Permissions with ls -l

ls -l

Sample Output:

-rwxr-xr-- 1 user group  2048 Jun 19 12:30 example.sh

Breakdown:

• -rwxr-xr--: Permissions

• 1: Number of hard links

• user: File owner

• group: File’s group

• 2048: File size in bytes

• Jun 19 12:30: Last modified

• example.sh: Filename

 Best Practices for Managing Permissions

• Always follow the principle of least privilege.

• Avoid using 777 unless absolutely necessary.

• Use SUID/SGID/Sticky Bit carefully to avoid privilege escalation.

• Use groups to manage multiple users with similar permissions.

✅ Common Permission Use Cases

 Conclusion

Linux file permissions are one of the foundational elements of system security. With a deep understanding of permission notations, octal values, and special bits like SUID, SGID, and the Sticky Bit, you'll be equipped to secure any Linux environment effectively. Use the cheatsheet provided to reinforce your knowledge and apply best practices for file access control.