Understanding Permissions

You try to run a script: "Permission denied." You try to edit a config: "Permission denied." You try to access a directory: "Permission denied."

This chapter fixes that. Let's understand how Linux permissions work.

Why Permissions Exist

Linux is a multi-user system. Even on your personal laptop, there's:

Your user account
The root superuser
System service accounts (nginx, mysql, etc.)

Permissions control who can do what. Without them, any user could read your private files, delete system configs, or crash services.

The Three Permissions

Every file has three basic permissions:

Permission	Letter	Meaning
Read	r	View file contents
Write	w	Modify file contents
Execute	x	Run the file as a program

For directories, they mean slightly different things:

Permission	For Files	For Directories
Read (r)	View contents	List files
Write (w)	Modify contents	Create/delete files
Execute (x)	Run as program	Enter the directory

Directory Execute - The Most Confusing Part

You need x (execute) permission on a directory to cd into it or access files inside. This is the #1 source of permission confusion:

Read (r) on a directory = can list filenames
Execute (x) on a directory = can enter and access files
Without x, you can see names but can't access anything inside

The Three User Classes

Permissions apply to three classes of users:

Class	Symbol	Description
Owner	u	The user who owns the file
Group	g	Users in the file's group
Others	o	Everyone else

Every file has:

One owner (a user)
One group (a collection of users)

Permissions are checked in order:

If you're the owner → owner permissions apply
Else if you're in the group → group permissions apply
Else → others permissions apply

Visualizing Permissions

-rwxr-xr--
│└┬┘└┬┘└┬┘
│ │  │  └── Others: read only
│ │  └───── Group: read + execute
│ └──────── Owner: read + write + execute
└────────── File type (- = file, d = directory)

This is what you see with ls -l.

Permission Examples

Permission	Meaning
`rwx------`	Owner has full access, nobody else
`rw-r--r--`	Owner can edit, others can read
`rwxr-xr-x`	Owner can edit+run, others can run
`rw-rw----`	Owner and group can edit
`---------`	Nobody can do anything

Terminal

$ls -l

-rw-r--r-- 1 user user 1024 Jan 14 notes.txt -rwxr-xr-x 1 user user 4096 Jan 14 script.sh drwxr-xr-x 2 user user 4096 Jan 14 projects/

The Numeric System (Preview)

Each permission has a number:

r = 4
w = 2
x = 1

Add them up for each class:

rwx = 4+2+1 = 7
r-x = 4+0+1 = 5
r-- = 4+0+0 = 4

So rwxr-xr-- = 754

We'll cover this in detail later.

Knowledge Check

What does it mean if you have 'r' but not 'x' permission on a directory?

Key Takeaways

Three permissions: read (r), write (w), execute (x)
Three user classes: owner (u), group (g), others (o)
Directories need execute (x) permission to enter
Permissions are checked in order: owner → group → others
ls -l shows permissions in the format rwxrwxrwx

Next: how to read and interpret permission strings from ls -l.