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Linux Tutorial — Part 7: File Permissions & Ownership

File permissions are the backbone of Linux security. If you misunderstand this topic, you will break systems — or expose them.

1. Why Linux Permissions Exist

Linux is a multi-user operating system. Not every user should be able to read, modify, or execute every file.

Permissions answer three questions:

• Who owns the file?
• Who belongs to the file’s group?
• What can others do?

2. Viewing Permissions

ls -l

Example output:

-rw-r--r-- 1 user group file.txt

3. Understanding Permission Symbols

The permission string has three sections:

• User (owner)
• Group
• Others

Each section contains:

• r → read
• w → write
• x → execute

4. chmod — Changing Permissions

chmod controls what actions are allowed.

chmod u+x script.sh

Adds execute permission for the owner.

5. Numeric Permission System

Linux also uses numbers:

• r = 4
• w = 2
• x = 1

Example:

chmod 755 script.sh

Meaning:

• User: read, write, execute
• Group: read, execute
• Others: read, execute

6. chown — Changing Ownership

Ownership controls responsibility.

sudo chown user file.txt

Change owner and group:

sudo chown user:group file.txt

7. Directories vs Files

Permissions behave differently on directories.

• r → list files
• w → create/delete files
• x → access directory

8. Recursive Permission Changes

chmod -R 755 myfolder

Applies permissions to all files and subfolders.

⚠️ Critical Warning

Never blindly use:

chmod -R 777 /

This destroys system security.

Real-World Mindset

Permissions are not about convenience. They are about control, accountability, and safety.

In the next part, we will learn about users, groups, and sudo — who is allowed to do what in Linux.