Linux Documentation (Beginner to Advanced)

How to Use This Guide

This page is written like a tutorial handbook. Each tutorial includes:

• What you should learn
• Commands you can run right now
• Exercises (practice tasks)
• “What to check when it fails” notes

If you are new to Linux, do Tutorials 1 to 4 first. If you are already comfortable with the terminal, jump to Tutorials 6 to 10 and the Advanced Topics.

Quick Setup

Use any Linux machine:

• Your laptop (Linux)
• A VM (VirtualBox, VMware)
• Cloud VM (Azure, AWS, GCP)
• WSL on Windows

Create a safe practice folder:

mkdir -p ~/linux-labs/{files,logs,scripts}
cd ~/linux-labs

If you break something, delete the lab folder and start fresh:

rm -rf ~/linux-labs

Tutorial 1: Navigating the Filesystem

What you learn

• Where you are (pwd)
• What is around you (ls)
• How to move (cd)
• How paths work (. and ..)

Commands

Show your current directory:

pwd

List files:

ls
ls -la
ls -lh

Move around:

cd ~
cd /tmp
cd -

Concepts (in simple words)

• A path like /var/log is an absolute path.
• A path like ../logs is a relative path.
• . means “this directory”.
• .. means “parent directory”.

Exercises

1. Create a folder ~/linux-labs/files/tutorial-1.
2. Create another folder inside it called backups.
3. Move into backups, then return back to your home directory without typing the full path.

Tutorial 2: Creating, Viewing, and Editing Files

What you learn

• Create files quickly
• Read files fast
• Search in files
• Redirection and pipes (the Linux superpower)

Commands

Create files and directories:

cd ~/linux-labs/files
mkdir -p tutorial-2
cd tutorial-2
touch notes.txt

Write to a file:

echo "hello linux" > notes.txt
echo "second line" >> notes.txt

Read a file:

cat notes.txt
nl -ba notes.txt

Preview a large file:

less /etc/services

Search inside files:

grep -n "ssh" /etc/services | head

Pipes:

ps aux | head
ps aux | grep -i ssh

Exercises

1. Create a file todo.txt with 10 lines.
2. Find lines that contain the word learn (case-insensitive).
3. Count how many lines are in the file.

Useful commands:

wc -l todo.txt
grep -ni "learn" todo.txt

Tutorial 3: Permissions and Ownership

What you learn

• Read (r), write (w), execute (x)
• User / group / others
• Why “permission denied” happens

Commands

See permissions:

ls -l

Change permissions:

chmod 644 notes.txt
chmod 755 .

Change ownership (requires sudo):

sudo chown $USER:$USER notes.txt

Quick mental model

• Files need r to read, w to edit, and x to run as a program.
• Folders need x to enter (cd) and access files inside.

Exercises

1. Create a script called hello.sh.
2. Try to run it and observe the error.
3. Make it executable and run it again.

cat > hello.sh <<'SH'
#!/usr/bin/env bash
echo "hello from a script"
SH

./hello.sh
chmod +x hello.sh
./hello.sh

Tutorial 4: Processes and Signals

What you learn

• What a process is
• How to inspect CPU/RAM usage
• How to stop a program safely

Commands

List processes:

ps aux | head

Interactive view:

top

Find a process:

ps aux | grep -i node

Send a signal:

kill -TERM <pid>
kill -KILL <pid>

Notes

• TERM is “please stop gracefully”.
• KILL is “stop now” (use only when needed).

Exercise

1. Run sleep 9999 in one terminal.
2. Find its PID.
3. Stop it using kill -TERM.

Tutorial 5: Packages and Updates

Different Linux distributions use different package managers.

Debian/Ubuntu (apt)

sudo apt update
sudo apt install -y curl jq
sudo apt upgrade -y

RHEL/CentOS/Fedora (dnf/yum)

sudo dnf install -y curl jq

Why this matters in DevOps

• Your CI/CD runners install dependencies.
• Your servers need security updates.
• You must know how to install troubleshooting tools fast.

Tutorial 6: Networking Fundamentals

What you learn

• IP and routes (basic)
• DNS and name resolution
• Ports and listening services

Commands

Check network interfaces:

ip a

Check routes:

ip route

DNS lookup:

getent hosts devopslabx.com

Check listening ports:

ss -lntp | head

Test HTTP:

curl -I https://devopslabx.com

Exercise

1. Find which process is listening on port 3000 (if your Next.js app is running).
2. Confirm you can reach it locally.

ss -lntp | grep ':3000' || true
curl -I http://localhost:3000 || true

Tutorial 7: Logs and Troubleshooting

What you learn

• Where logs live (/var/log)
• How to follow logs live
• How to filter noise fast

Commands

Common system logs:

ls -la /var/log | head

Follow a log:

tail -f /var/log/syslog 2>/dev/null || true
tail -f /var/log/messages 2>/dev/null || true

Systemd journal:

journalctl -n 100 --no-pager
journalctl -u ssh --since "1 hour ago" --no-pager

Troubleshooting checklist

• What changed last?
• What errors appear in logs around the time of failure?
• Is disk full? Is memory exhausted?
• Is the service listening on the expected port?
• Is DNS resolving correctly?

Tutorial 8: Users, Groups, and SSH

Commands

Who am I:

whoami
id
groups

Create a user (admin-only):

sudo useradd -m -s /bin/bash demo-user
sudo passwd demo-user

SSH basics:

ssh user@server-ip

Generate an SSH key:

ssh-keygen -t ed25519 -C "devopslabx"

Tutorial 9: Services with systemd

What you learn

• Start/stop services
• Enable on boot
• Read service logs

Commands

systemctl status ssh
sudo systemctl restart ssh
sudo systemctl enable ssh
journalctl -u ssh --no-pager -n 200

Exercise (safe)

Create a simple service that prints a timestamp to a log every 10 seconds:

cat > ~/linux-labs/scripts/tick.sh <<'SH'
#!/usr/bin/env bash
while true; do
  date -Is
  sleep 10
done
SH
chmod +x ~/linux-labs/scripts/tick.sh

You can later run it under a service unit, but only do that after you understand permissions and paths.

Tutorial 10: Shell Scripting

What you learn

• Variables, loops, functions
• Defensive scripting
• Writing scripts you can reuse in DevOps work

A practical script: “is my port open?”

cat > ~/linux-labs/scripts/check_port.sh <<'SH'
#!/usr/bin/env bash
set -euo pipefail

host="${1:-localhost}"
port="${2:-3000}"

if command -v nc >/dev/null 2>&1; then
  nc -zv "$host" "$port"
else
  echo "nc not installed. Try: sudo apt install -y netcat-openbsd"
  exit 2
fi
SH

chmod +x ~/linux-labs/scripts/check_port.sh
~/linux-labs/scripts/check_port.sh localhost 3000

Scripting rules (production-friendly)

• Always use set -euo pipefail for safer scripts.
• Validate inputs.
• Print meaningful errors.
• Prefer predictable output (easy for CI/CD to parse).

Tutorial 11: Storage, Disks, and Mounts

What you learn

• How to read disk usage and avoid “disk full” outages
• What devices and partitions look like
• What a mount is and why it matters

Commands

Disk usage (what is full?):

df -h

Big folders (what is consuming space?):

du -sh /var/* 2>/dev/null | sort -h | tail

List block devices:

lsblk

See mounts:

mount | head

Exercises

1. Find your top 5 biggest folders in /var.
2. Explain the difference between “disk full” and “inode full”.

Useful inode command:

df -ih

Tutorial 12: Security Baseline (SSH + Firewall)

What you learn

• How attackers typically enter: weak SSH and open ports
• How to set safe defaults without locking yourself out

SSH checks

sudo sshd -T | head
sudo cat /etc/ssh/sshd_config | sed -n '1,120p'

Firewall basics (Ubuntu: ufw)

sudo ufw status verbose || true

Allow SSH first (important), then enable:

sudo ufw allow OpenSSH
sudo ufw enable

Exercises

1. List which ports are currently listening.
2. Decide which ports should be public and which should be private.

ss -lntp

Tutorial 13: Scheduling with cron

What you learn

• How scheduled tasks work (automation, backups, cleanup)
• How to avoid cron scripts that fail silently

Commands

Edit your user crontab:

crontab -e

List your cron jobs:

crontab -l

Example: run every day at 2:15 AM (log output):

15 2 * * * /home/<user>/linux-labs/scripts/backup.sh >> /home/<user>/linux-labs/logs/backup.log 2>&1

Exercises

1. Create a cron job that writes a timestamp to a log every minute.
2. Confirm the log grows and does not fill disk.

Tutorial 14: Troubleshooting Playbooks

Playbook A: “Service is down”

1. Is it running?

ps aux | grep -i <service-name> | head

1. Is it listening on the expected port?

ss -lntp | grep ':<port>' || true

1. What do logs say?

journalctl -u <service-name> --no-pager -n 200

1. Is disk full or memory exhausted?

df -h
free -m

Playbook B: “Server is slow”

top
uptime
free -m
df -h

Interpretation tips:

• Load average very high + CPU low can mean IO wait.
• Free memory low is fine if cache is high, but OOM events are not.
• Disk at 100% will break logs, deployments, and databases.

Tutorial 15: Linux Internals for DevOps (Namespaces and cgroups)

This is the mental model behind containers.

Namespaces (isolation)

• PID namespace: processes look different inside/outside
• NET namespace: network interfaces differ
• MNT namespace: filesystem mounts differ

cgroups (resource limits)

• CPU limits
• memory limits
• IO limits

Why this matters

When a container “works locally” but fails in production, the real cause is often:

• wrong filesystem path or missing mount
• wrong network/DNS route
• resource limits causing OOM kill

pwd
ls -la
cd /path
du -sh *
df -h

grep -Rni "pattern" .
find . -type f -name "*.log"

ps aux | head
top
kill -TERM <pid>

ip a
ip route
ss -lntp
curl -I https://example.com

systemctl status <service>
journalctl -u <service> --no-pager -n 200

ss -lntp | grep ':3000' || true
ps aux | grep -i node | head
df -h

cd ~/linux-labs/files
echo "secret" > locked.txt
chmod 444 locked.txt
echo "try" >> locked.txt || true
chmod 644 locked.txt
echo "fixed" >> locked.txt

mount | head
cat /etc/fstab