Lesson 2 — Linux Fundamentals
H
Network Engineer with 3+ years of experience in infrastructure, system administration, and enterprise networking, currently transitioning into DevOps and Cloud Engineering.
Hands-on experience with Fortinet firewalls, routers, switches, Windows/Linux servers, and Hyper-V virtualization.
Currently building and learning with Docker, Kubernetes, AWS, Linux, CI/CD, GitHub Actions, and automation tools while leveraging MERN stack knowledge to understand the complete application lifecycle from development to deployment.
Passionate about cloud infrastructure, automation, containers, and modern DevOps practices. Regularly sharing my learning journey, projects, and hands-on experiments.
File Permissions + Users + Ownership
This is one of the MOST important Linux topics.
Without this:
Docker permissions become confusing
SSH issues happen
deployment fails
scripts break
services cannot access files
1. Users in Linux
Linux is multi-user OS.
Every file belongs to:
a user
a group
check current user.
whoami
# Output: root
2. Understand ls -l
ls -l
# Output: -rw-r--r-- 1 hatim developers 120 May 22 test.txt
# Breakdown: This is permissions.
# -rw-r--r--
3. Permission Structure
-rwxr-xr--
Split:
- rwx r-x r--
| Section | Meaning |
|---|---|
| first | type |
| rwx owner | permissions |
| r-x group | permissions |
| r-- others | permissions |
4. Meaning of Permissions
| Symbol | Meaning |
|---|---|
| r | read |
| w | write |
| x | execute |
5. File Types
| Symbol | Type |
|---|---|
| - | file |
| d | directory |
| l | symbolic link |
Example:
drwxr-xr-x
# Means directory.
6. Permission Numbers
| Permission | Value |
|---|---|
| r | 4 |
| w | 2 |
| x | 1 |
Example:
| Value | Meaning |
|---|---|
| 7 | rwx |
| 6 | rw- |
| 5 | r-x |
| 4 | r-- |
7. chmod
Change permissions.
chmod 755 file.sh
Meaning:
Owner -> rwx (ALL)
Group -> r-x (READ-WRITE)
Others -> r-x (READ-EXECUTE)
Example 2:
chmod 644 notes.txt
Meaning:
Owner -> rw-
Group -> r--
Others -> r--
8. Make Script Executable
# Create a file.
touch app.sh
# Add
vi touch
# Content
#!/bin/bash
echo "Hello World"
# Run
./app.sh
Error: Permission denied
Fix
chmod +x app.sh
# Now run:
./app.sh
9. Symbolic chmod
Instead of numbers.
| Symbol | Meaning |
|---|---|
| u | user |
| g | group |
| o | others |
| a | all |
Example: Add Execute
chomod u+x file.sh
Example: Remove Write
chmod g-w file.sh
Give all read:
chmod a+r file.sh
10. Ownership
Check ownership:
ls -l
# Output: hatim developers
# hatim → owner
# developers → group
11. chown
Change owner.
# Syntax: sudo chown <user> file.txt
sudo chown hatim notes.txt
12. Change Group
# Syntax: sudo chgrp <group> notes.txt
sudo chgrp developer notes.txt
13. sudo
Run command as administrator.
sudo apt update
# Important: Linux protects system files using permissions.
14. Important Directory Permission Rule
Directories behave differently.
| Permission | Directory Meaning |
|---|---|
| r | list contents |
| w | create/delete files |
| x | enter directory |
15. Root User
Linux superuser:
root
# Has full permissions.
Switch:
sudo su - <user>
exit
Use to logout
exit
# Be VERY careful as root.
16. Useful Commands
# Check current user
whoami
# User ID
id
# Groups
groups
17. Real World Examples
Docker Permission Issue
permission denied while accessing docker.sock
Usually fixed with:
sudo usermod -aG docker $USER
Script Not Running
chmod +x file.txt
SSH Key Permission
SSH requires:
chmod 600 ~/.ssh/id_rsa
