Linux - permissions in Linux

What Are Permissions in Linux?

Linux is a multi-user system, meaning many users can share one machine. To keep things safe and organized, Linux assigns permissions to every file and folder.

These permissions determine:

• Who can read the file

• Who can write/edit the file

• Who can execute/run the file (if it’s a script or program)

Three Types of Users

Each file/folder has permissions for three categories of users:

1. Owner (u) – The person who created the file

2. Group (g) – A group the owner belongs to

3. Others (o) – Everyone else

Three Types of Permissions

Viewing Permissions

Use the ls -l command:

ls -l

Example output:

-rwxr-xr-- 1 user group 1234 Aug 6  file.sh

Breakdown:

• First character: - = file, d = directory

• Next 3: rwx (owner)

• Next 3: r-x (group)

• Last 3: r-- (others)

So here:

• Owner can read, write, execute

• Group can read, execute

• Others can read only

Changing Permissions with chmod

The chmod command changes permissions.

Symbolic Mode (letters)

chmod u+x file.sh   # Give execute permission to the owner
chmod g-w file.txt  # Remove write from group
chmod o+r file.txt  # Add read to others

Numeric Mode (numbers)

Each permission is a number:

• r = 4

• w = 2

• x = 1

Add the values to get the full permission.

So chmod 755 file.sh means:

• Owner: 7 = rwx

• Group: 5 = r-x

• Others: 5 = r-x

Changing Ownership with chown

You can change the owner and group of a file:

sudo chown user:group file.txt

Checking Effective Permissions

You can check your access to a file using:

ls -l file.txt

Or:

namei -l /path/to/file

Example Use Cases