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

Symbol Permission Meaning
r read Can view the file or list directory contents
w write Can edit the file or add/delete files in a directory
x execute Can run the file (if it's a script or program)

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.

Permission Number
rwx 7
rw- 6
r-- 4
--x 1
--- 0

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

Task Required Permission
Read a text file r (read)
Edit a file w (write)
Run a script or program x (execute)
Enter a directory x (execute on the directory)
List files in a directory r (read on the directory)