Understanding Linux File Ownership and Permissions: A Beginner’s Guide

In the previous article we explored the Linux filesystem hierarchy and got familiar with its structure.
Now, let’s unlock a powerful concept: who can access what in Linux.

Mastering ownership and permissions is key to becoming confident in Linux — it’s the foundation of security and system administration.

Why Permissions Matter?

In Linux, everything is a file — programs, devices, directories, even your keyboard input.
Permissions are how Linux controls who can read, write, or execute these files.
This ensures:

• Users don’t accidentally break system files.

• Sensitive data is protected.

• Multiple users can safely share one machine.

Understanding Ownership

Every file and directory in Linux has two owners:

1. User (Owner): The user account that owns the file.

2. Group: A collection of users who share permissions.

Let’s see ownership in action:

ls -l

Understanding Permissions

Permissions are shown in three sets of three characters:

-rw-r--r--

Let’s break this down:

Each permission has a letter:

• r = read

• w = write

• x = execute (run as a program)

The first - means it’s a file (it would be d for directory).

Changing Permissions

Using chmod

chmod lets you modify permissions.

Grant execute permission to the user:

chmod u+x script.sh

In the previous example, we used the command on the notes.txt file. You can see that the file name changed color to green, indicating it is now executable. It's recommended to use this command when running your bash script that ends with .sh.

Add write permission for group:

chmod g-w notes.txt

You can see how the write permission is removed from the group in the second photo.

Numeric (Octal) Mode

Permissions can also be set with numbers:

• r = 4, w = 2, x = 1

• Add them up for each set.

Example:

chmod 755 script.sh

Breakdown:

Changing Ownership

Use chown to change the owner:

sudo chown alice notes.txt

Change both user and group:

sudo chown alice:devs notes.txt

Quick Reference Table

Visualizing Permissions

[ File Type ] [ User ] [ Group ] [ Others ]
     -         rw-       r--       r--

Understanding umask (Default Permissions)

So far, you’ve learned how to view and modify permissions, but did you know new files and directories don’t start with random permissions?
Linux uses a setting called umask (user file-creation mode mask) to determine default permissions.

🔹 How umask works:

• New files start with 666 (read and write for everyone) minus the umask.

• New directories start with 777 (read, write, execute for everyone) minus the umask.

Example:

You can view an example of umask in the shell screenshot, which illustrates how default permissions are applied to new files and directories based on the umask value.

Trivia: Why Files Start at 666 and Directories at 777

Linux starts with full permissions:

• Files: 666 → everyone can read/write

• Directories: 777 → everyone can read/write/enter

Then umask subtracts permissions:

# With umask 022
Files: 666 - 022 = 644  (owner can write, others read-only)
Dirs:  777 - 022 = 755  (owner can write, others can read & enter)

You may wonder why:

• Files don’t get x (execute) by default because not all files are programs.

• Directories need x to let you cd into them.

Practice Challenge

1. Create a new directory to experiment in.

2. Inside it, create a sample text file.

3. Check its permissions using ls -l.

4. Change the file permissions to allow only the owner to read and write.

5. Change the permissions again to make it readable and executable by everyone.

6. Use ls -l after each change to see the effect.

7. Change the file’s ownership to your user and group.

8. Reflect on how permissions and ownership control access to files.

Tips to Remember

• Always use ls -l to check permissions before editing files.

• Use sudo carefully; it bypasses normal permission checks.

• Scripts must be executable (chmod +x) to run directly.

What’s Next in Your Linux Journey

The goal here is to get comfortable navigating your system and start recognizing patterns.
Don’t worry about understanding every permission combination just yet — we’ll revisit and build on this knowledge as you use Linux more.

You’ve now unlocked a key concept of Linux security by learning file permissions and ownership.

Up Next: Mastering Linux Package Management

Now that you understand permissions and ownership, you’re ready to take control of your system’s software.

In the next article, we’ll cover:

• How Linux installs and updates software

• Using yum (CentOS) and apt (Ubuntu)

• Tips to manage packages like a pro

By the end of the next article, you’ll be able to customize and manage your Linux environment with confidence.

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