🔐 Day 3: File Permissions & Security

In Linux, every file is owned by someone. Understanding who can touch your files is the first line of defense.

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👥 The Three Identities

Who are we talking about when we say "Permissions"?

Key	Identity	Description
u	User (Owner)	The creator of the file. Usually has the most power.
g	Group	A collection of users (e.g., developers, admins).
o	Others	Everyone else on the system (The Public).

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🚦 The Three Permissions

What can they do?

Symbol	Action	Description	Numeric Value
r	Read	View file contents / List directory contents.	4
w	Write	Modify file / Create or Delete files in directory.	2
x	Execute	Run file as program / Enter directory.	1

💡 Quick Math:

• rwx = 4 + 2 + 1 = 7 (Full Permissions)
• rw- = 4 + 2 = 6 (Read & Write)
• r-x = 4 + 1 = 5 (Read & Execute)

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🛠️ Changing Permissions: chmod

The command chmod (Change Mode) is used to set these rules.

📝 Method 1: Symbolic Mode

Use + to add, - to remove.

chmod u+rwx filename
# Gives User (u) Read, Write, and Execute permissions.

chmod g-wx filename
# Removes Write and Execute permissions from Group (g).

🔢 Method 2: Numeric Mode (The Pro Way)

Set all permissions at once using the sum of values for User, Group, and Others.

chmod 755 filename
# User (7) = r+w+x (4+2+1)
# Group (5) = r+x   (4+1)
# Others (5)= r+x   (4+1)

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📂 Directories vs. Files

Permissions behave slightly differently for folders.

Permission	For a File 📄	For a Directory 📁
Read (r)	See the content text.	List files inside (ls).
Write (w)	Edit the content text.	Create or Delete files inside.
Execute (x)	Run as a script/program.	Enter the directory (cd).

⚠️ The "Bucket" Analogy

Why do I need x to list files?

Think of a directory as a Bucket.

• x (Execute): Allows you to look inside or step into the bucket.
• w (Write): Allows you to put things in or take things out.

Crucially: You cannot put things in (w) if you cannot even reach inside (x).
Without x, w is useless for a directory.

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