🔐 Linux/Unix: File Permissions & ls -l Explained – Read, Write & Execute Control

🧲 Introduction – Why Learn Linux/Unix File Permissions?

Every file and directory in Linux/Unix has a set of permissions that determine who can read, write, or execute them. Understanding file permissions is critical for system security, collaboration, and automation. The ls -l command gives a clear view of permissions, ownership, and more—all in one place.

🎯 In this guide, you’ll learn:

• How to read and understand file permission strings using ls -l
• What read, write, and execute mean for users, groups, and others
• Permission breakdowns for files vs. directories
• Real examples of permission control

📂 Understanding ls -l Output

🧪 Example:

ls -l

💡 Output:

-rw-r--r-- 1 user group  1024 Jun 15 12:00 notes.txt

Breakdown:

🔐 Permission Types

For Directories:

• r → List contents
• w → Create/delete files
• x → Enter the directory

👥 User Classes

🔢 Numeric (Octal) Representation of Permissions

🧪 Example:

-rwxr-xr--  →  Owner=7, Group=5, Others=4  →  754

🛠️ Modify Permissions with chmod

✅ Syntax:

chmod [permissions] filename

🧪 Examples:

chmod 755 script.sh           # rwxr-xr-x
chmod u+x file.sh             # Add execute to owner
chmod go-w file.txt           # Remove write from group and others

👑 View and Change Ownership

View Ownership:

ls -l

Change Owner:

sudo chown user file.txt

Change Group:

sudo chgrp groupname file.txt

📌 Summary – Recap & Next Steps

Mastering Linux file permissions helps protect sensitive data, control access levels, and prevent accidental changes. Using ls -l gives you full visibility into permissions, ownership, and file metadata.

🔍 Key Takeaways:

• Use ls -l to check file types, permissions, owners, and size.
• Permissions are divided among user, group, and others with r, w, and x.
• Numeric (chmod 755) and symbolic (chmod u+x) modes both work.
• Use chown and chgrp to manage file ownerships.

❓ FAQs

❓ What does drwxr-xr-x mean?
✅ It’s a directory (d) with permissions: owner (read/write/execute), group (read/execute), others (read/execute).

❓ How do I make a file executable?
✅ Use:

chmod +x filename.sh

❓ How can I change file permissions recursively?
✅ Use:

chmod -R 755 myfolder/

❓ What’s the safest permission for scripts?
✅ Use chmod 755 to allow execute by all, but write only by the owner.

❓ Can a group member modify a file?
✅ Only if the group has write permission (w), and the user belongs to that group.