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Lesson 02 - File Permissions and Ownership

Estimated time: 60–75 minutes

Module: Module 02 – Linux Filesystem Administration

Prerequisites: - Module 00 – Introduction to Linux - Module 01 – Command Line Fundamentals - Lesson 01 – Linux Filesystem Hierarchy

Difficulty: Beginner to Intermediate

Overview

Linux is a multi-user operating system, which means multiple users can access the same system at the same time. To keep the system secure, Linux uses a permissions model that controls who can read, modify, or execute files and directories.

Every file and directory has an owner, belongs to a group, and has a set of permissions that determine what actions can be performed.

Understanding file permissions is essential for managing Linux systems, securing sensitive information, and troubleshooting access issues.

Learning Objectives

By the end of this lesson you will be able to:

  • Explain Linux ownership and permissions.
  • Identify the owner and group of a file.
  • Interpret the output of ls -l.
  • Understand read, write and execute permissions.
  • Modify permissions using chmod.
  • Change ownership using chown.
  • Change group ownership using chgrp.
  • Apply permission changes safely using best practices.

Linux Ownership

Every file and directory has:

  • An owner (the user who owns the file)
  • A group (a collection of users)
  • A set of permissions

Display a long listing:

ls -l

Example output:

-rw-r--r-- 1 james developers 245 Jul 22 notes.txt

Breaking it down:

-rw-r--r-- 1 james developers 245 Jul 22 notes.txt
│─────────│ │     │
│         │ │     └── Group
│         │ └──────── Owner
│         └────────── Permissions

Understanding Permissions

The first column represents the file type and permissions.

-rw-r--r--

Split into sections:

- rw- r-- r--
│ │   │   │
│ │   │   └── Others
│ │   └────── Group
│ └────────── Owner
└──────────── File type

File Types

The first character identifies the file type.

Symbol Type
- Regular file
d Directory
l Symbolic link
c Character device
b Block device

Example:

drwxr-xr-x

The d indicates a directory.

Permission Types

There are three basic permissions.

Symbol Meaning
r Read
w Write
x Execute

Read (r)

Allows a file to be viewed.

Example:

cat notes.txt

Write (w)

Allows the file to be modified.

Examples include:

  • Editing
  • Saving
  • Deleting (depending on directory permissions)

Execute (x)

Allows a file to be run as a program or script.

Example:

./backup.sh

Permission Groups

Permissions are applied to three groups.

Group Description
Owner User who owns the file
Group Members of the assigned group
Others Everyone else

Example:

-rwxr-x---

Owner:

rwx

Group:

r-x

Others:

---

Numeric Permissions

Permissions can also be represented using numbers.

Permission Value
Read 4
Write 2
Execute 1

Examples:

Number Meaning
7 rwx
6 rw-
5 r-x
4 r--
0 ---

Common combinations:

Numeric Symbolic
755 rwxr-xr-x
644 rw-r--r--
700 rwx------
600 rw-------

Changing Permissions with chmod

Symbolic example:

chmod u+x backup.sh

Adds execute permission for the owner.

Remove write permission:

chmod g-w notes.txt

Numeric example:

chmod 644 notes.txt

Make a script executable:

chmod 755 backup.sh

Changing Ownership

Display ownership:

ls -l

Change owner:

sudo chown james notes.txt

Change owner and group:

sudo chown james:developers notes.txt

Changing Group Ownership

Use:

sudo chgrp developers notes.txt

Verify:

ls -l

Directory Permissions

Permissions behave slightly differently on directories.

Permission Effect
Read List contents
Write Create, delete or rename files
Execute Enter the directory using cd

Example:

chmod 755 projects

Best Practices

✔ Give users only the permissions they need.

✔ Avoid using 777.

✔ Use groups instead of assigning permissions to individual users wherever possible.

✔ Verify changes using ls -l.

✔ Use sudo only when necessary.

✔ Be especially careful when changing permissions in /etc, /usr or /var.

Practical Exercise

Task 1 – View Permissions

Run:

ls -l

Identify:

  • Owner
  • Group
  • Permission string

Task 2 – Create a File

touch training.txt
ls -l

Record its permissions.

Task 3 – Make a Script Executable

Create a script:

touch backup.sh

Add execute permission:

chmod +x backup.sh

Verify:

ls -l backup.sh

Task 4 – Change Numeric Permissions

Set:

chmod 644 training.txt

Then:

chmod 600 training.txt

Observe the changes.

Task 5 – Change Group Ownership

If permitted:

sudo chgrp developers training.txt

Verify using:

ls -l

Knowledge Check

  1. What does the first character in ls -l output represent?

  2. What do the letters r, w and x mean?

  3. Which command changes file permissions?

  4. Which command changes file ownership?

  5. What numeric permission corresponds to rwxr-xr-x?

  6. Why should 777 permissions generally be avoided?

  7. Why are groups useful when managing permissions?

Summary

In this lesson you learned:

  • Every file has an owner and a group.
  • Linux permissions control access to files and directories.
  • Permissions are represented symbolically and numerically.
  • chmod changes permissions.
  • chown changes ownership.
  • chgrp changes group ownership.
  • Correct permissions are essential for both security and system administration.

Next Lesson

In Lesson 03 – Finding Files and Links, you'll learn how to locate files efficiently and how hard links and symbolic links work in Linux.