Linux Guide: Basics to Advanced

1. What is Linux?

Linux is the engine that powers millions of servers, phones (Android), and computers worldwide.

Core Parts

Kernel: The brain that talks to your hardware (CPU, RAM, disk)
Distributions (Distros): Complete packages like Ubuntu, Fedora, or Arch that bundle the kernel with tools
Shell: The command-line interface (Bash/Zsh) where you type commands
Package Manager: App store for installing software (apt, dnf, pacman)

Why Learn Linux?

Powers 90%+ of web servers and cloud infrastructure
Required for DevOps, system administration, and backend development
Complete control over your system
Free and open source

2. Getting Started

Choose a Distribution

Distro	Best For	Package Manager
Ubuntu/Debian	Beginners, stability	`apt`
Fedora/RHEL	Enterprise, latest features	`dnf`
Arch Linux	Advanced users, customization	`pacman`
Alpine	Containers, minimal systems	`apk`

Essential Commands

pwd                    # Show current directory
ls -la                 # List files (including hidden)
cd /path/to/folder     # Change directory
cd -                   # Go back to previous directory
cd ~                   # Go to home directory

Understanding the Filesystem

Important Directories


/  (Root directory — everything starts here)
├── /home         # User home directories (e.g., /home/john)
├── /etc          # System-wide configuration files
├── /var          # Variable data (logs, cache, databases, mail)
│   └── /var/log  # System and application log files
├── /usr          # User programs and read-only utilities
├── /opt          # Optional / third-party software packages
├── /tmp          # Temporary files (usually cleared on reboot)
├── /dev          # Device files (hard drives, USB, etc.)
├── /proc         # Virtual filesystem for process & system info
└── /sys          # Kernel and hardware information (sysfs)

Disk Usage

# Check disk space
df -h                  # Show disk usage for all partitions
du -sh *               # Size of each item in current directory
du -sh /var/log        # Total size of a directory

# List all disks and partitions
lsblk                  # Tree view of disks
lsblk -f               # Include filesystem types

# Show mounted filesystems
mount                  # List all mounts

Symbolic Links (Shortcuts)

# Create a shortcut to a file or folder
ln -s /path/to/original /path/to/shortcut

# Example: create shortcut to nginx config
ln -s /etc/nginx/sites-available/mysite /etc/nginx/sites-enabled/mysite

# Vi. Users and Permissions

### User Management

```bash
# Who am I?
whoami                 # Current username

Installing Software

Ubuntu/Debian (apt)

# Update package list
sudo apt update

# Install software
sudo apt install nginx
sudo apt install nginx mysql-server  # Multiple packages

# Remove software
sudo apt remove nginx                # Keep config files
sudo apt purge nginx                 # Delete everything

# Upgrade all packages
sudo apt upgrade                     # Safe upgrades
sudo apt full-upgrade                # Handle dependencies

# Search for packages
apt search nginx
apt show nginx                       # Package details

# Clean up
sudo apt autoremove                  # Remove unused packages
sudo apt clean                       # Clear download cache

Fedora/RHEL/CentOS (dnf)

sudo dnf install nginx
sudo dnf remove nginx
sudo dnf upgrade                     # Update all packages
sudo dnf search nginx
sudo dnf info nginx

Arch Linux (pacman)

sudo pacman -S nginx                 # Install
sudo pacman -R nginx                 # Remove
sudo pacman -Rns nginx               # Remove with dependencies
sudo pacman -Syu                     # Update system
sudo pacman -Ss nginx                # Search

Snap Packages (Universal)

sudo snap install code --classic     # VS Code
sudo snap remove code
snap find discord

View all users

# List all users

cat /etc/passwd

Groups

# See your groups
groups                 # Your groups
groups alice           # Alice's groups

# Create group
sudo groupadd developers

# Add user to group
sudo usermod -aG developers alice     # -a appends, don't remove existing
sudo usermod -aG sudo alice           # Give sudo access

# Remove user from group
sudo gpasswd -d alice developers

The `sudo` Command

sudo = "superuser do" - run commands as admin/root

# Run command as admin
sudo apt update
sudo systemctl restart nginx

# Edit system files
sudo nano /etc/hosts

# Become root user (not recommended)
sudo -i                # Login shell as root
sudo su                # Switch to root

# See what you can sudo
sudo -l

Important: Only users in the sudo group (Debian/Ubuntu) or wheel group (RHEL/Fedora) can use sudo

head -n 20 file.txt # First 20 lines
tail -f /var/log/syslog # Follow log file in real-time

Creating and editing

mkdir myfolder # Create directory
mkdir -p path/to/deep/folder # Create nested directories
touch newfile.txt # Create empty file
nano file.txt # Simple text editor
vim file.txt # Advanced text editor

Managing Processes

View Running Processes

# List all processes
ps aux                           # All processes, detailed
ps aux | grep nginx              # Find specific process

Managing Services (systemd)
Systemd controls background services like web servers, databases, SSH, etc.

Basic Service Control

Check service status

syst. Networking Basics

Check Network Status

# Show network interfaces and IP addresses
ip a                             # Modern way
ifconfig                         # Older way (may need net-tools package)

# Show routing table
ip route
route -n                         # Older way

Storage Management

View Disks and Partitions

# List all disks and partitions
lsblk                            # Tree view
lsblk -f                         # Show filesystems
sudo fdisk -l                    # Detailed partition info
sudo parted -l                   # Alternative tool

# Show UUIDs (unique disk identifiers)
sudo blkid

Create and Format a Partition

# ⚠️ WARNING: These commands can erase data!

# 1. Create partition (interactive)
sudo fdisk /dev/sdb              # Replace sdb with your disk
# Commands: n (new), p (primary), w (write)

# 2. Format partition
sudo mkfs.ext4 /dev/sdb1         # ext4 filesystem
sudo mkfs.xfs /dev/sdb1          # XFS filesystem

# 3. Create mount point and mount
sudo mkdir /mnt/mydisk
sudo mount /dev/sdb1 /mnt/mydisk

# 4. Auto-mount on boot (add to /etc/fstab)
# /dev/sdb1  /mnt/mydisk  ext4  defaults  0  2

Check and Repair Filesystem

# ⚠️ IMPORTANT: Unmount first!
sudo umount /dev/sdb1

# Check and repair
sudo fsck /dev/sdb1              # Check filesystem
sudo fsck -y /dev/sdb1           # Auto-fix errors

Add Swap Space

Monitoring and Logs

System Resource Usage

# Memory usage
free -h                          # RAM and swap
free . Security Best Practices

Essential Security Steps

# 1. Keep system updated
sudo apt update && sudo apt upgrade -y
sudo apt install unattended-upgrades    # Auto security updates

# 2. Disable root SSH login
sudo nano /etc/ssh/sshd_config
# Change: PermitRootLogin no
# Change: PasswordAuthentication no
sudo systemctl restart sshd

# 3. Use SSH keys instead of passwords
ssh-keygen -t ed25519
ssh-copy-id user@server

# 4. Enable firewall
sudo ufw enable
sudo ufw allow 22              # Only allow what you need
sudo ufw allow 80
sudo ufw allow 443

# 5. Remove unused services
systemctl list-units --type=service
sudo systemctl disable apache2  # If not using

File Permission Security

# Bad - anyone can read/write/execute
chmod 777 file.txt              # ❌ NEVER DO THIS

# Good - only owner can write
chmod 644 file.txt              # ✅ rw-r--r--
chmod 600 ~/.ssh/id_ed25519     # ✅ SSH keys should be private

# Scripts
chmod 755 script.sh             # ✅ Owner can run, others can read

# Sensitive files
chmod 600 /etc/app/secrets.conf # ✅ Only owner can read

# See who tried to log in
sudo journalctl -u ssh --since today | grep -i failed
sudo tail -f /var/log/auth.log | grep -i failed

# Install fail2ban (blocks brute force)
sudo apt install fail2ban
sudo systemctl enable --now fail2ban
sudo fail2ban-client status sshd

Security Checklist

Automation with Scripts and Cron

Basic Shell Script

Create a file backup.sh:

#!/bin/bash
# Simple backup script
. Web Server Setup (Nginx)

Install Nginx

# Ubuntu/Debian
sudo apt update
sudo apt install nginx

# Sta. Docker Basics

### Install Docker

```bash
# Ubuntu/Debian
curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh

# Add your user to docker group (no sudo needed)
sudo usermod -aG docker $USER
# Log out and back in for this to take effect

# Verify installation
docker --version
docker run hello-world

Working with Images

# Search for images
docker search nginx

# Download image
docker pull nginx
docker pull ubuntu:22.04         # Specific version

# List images
docker images

# Remove image
docker rmi nginx
docker rmi ubuntu:22.04

Running Containers

# Run container (interactive)
docker run -it ubuntu bash       # Run Ubuntu and get a shell
# Inside container, type 'exit' to leave

# Run container (background)
docker run -d --name web nginx   # -d = detached mode
docker run -d --name web -p 8080:80 nginx  # Map port 8080 -> 80

# List running containers
docker ps
docker ps -a                     # Include stopped containers

# Stop/Start containers
docker stop web
docker start web
docker restart web

# Remove container
docker Troubleshooting Common Issues

System is Slow

# 1. Check CPU usage
top                              # Press '1' to see per-core
htop                             # Better UI

# 2. Check memory
free -h                          # Is swap being used heavily?

3. . Backups

Simple Backup with rsync

# Backup directory to external drive
rsync -av --delete /home/john/ /mnt/backup/john/
# -a = archive mode (preserves permissions, times, etc.)
# -v = verbose
# --delete = remove files in destination that don't exist in source

# Backup over network
rsync -avz /home/john/ user@server:/backup/john/
# -z = compress during transfer

# Dry run (see what would change)
rsync -avn --delete /home/john/ /mnt/backup/john/

Compress and Archive

# Create compressed backup
tar -czf backup-$(date +%Y%m%d).tar.gz /home/john

# Extract backup
tar -xzf backup-20240115.tar.gz

# List contents without extracting
tar -tzf backup.tar.gz

Find large files

System Information

uname -a                         # Kernel version
lsb_release -a                   # OS version (Ubuntu/Debian)
hostnamectl                      # System details
uptime                           # Uptime and load

File Operations

ls -la                           # List all files
cp file.txt backup.txt           # Copy
mv old.txt new.txt               # Move/rename
rm file.txt                      # Delete
find . -name "*.log"             # Find files

Processes

ps aux                           # List all processes
top                              # Monitor processes
kill <pid>                       # Stop process
pkill nginx                      # Kill by name

Networking

ip a                             # IP addresses
ping google.com                  # Test connectivity
ss -tulpen                       # Open ports
curl https://example.com         # HTTP request

Services

systemctl status nginx           # Check service
systemctl start nginx            # Start service
systemctl enable nginx           # Enable on boot
journalctl -u nginx              # View logs

Storage

df -h                            # Disk space
du -sh *                         # Directory sizes
lsblk                            # List disks

19. Additional Resources

Getting Help

man ls                           # Manual for 'ls' command
ls --help                        # Quick help
info ls                          # Detailed info
tldr ls                          # Simple examples (install tldr first)

Online Resources

Ubuntu Documentation: https://help.ubuntu.com
Arch Wiki: https://wiki.archlinux.org (works for all distros)
DigitalOcean Tutorials: https://digitalocean.com/community/tutorials
Linux Journey: https://linuxjourney.com (beginner-friendly)

1. Check firewall

sudo ufw status
sudo ufw allow 22

2. Check if port is listening

sudo ss -tulpen | grep :22

3. Test from server itself

ssh localhost

4. Check logs

journalctl -u sshd -f
tail -f /var/log/auth.log

Permission Denied Errors

# Check file ownership and permissions
ls -la file.txt

# Fix ownership
sudo chown user:user file.txt

# Fix permissions
chmod 644 file.txt               # For regular files
chmod 755 directory/             # For directories
chmod +x script.sh               # For scripts

# Add user to group
sudo usermod -aG groupname username
# Log out and back in

Trace System Calls (Advanced)

# See what a process is doing
strace -p <pid>                  # Attach to running process
strace command                   # Trace command from start

# Example: why is ls slow?
strace ls /var/log 2>&1 | grep open

Execute command in running container

docker exec -it web bash # Get shell inside

container

docker exec web ls /etc/nginx # Run single command

View container details

docker inspect web

docker stats # Live resource usage

Docker Compose (Multi-container apps)

Create docker-compose.yml:

version: '3'
services:
  web:
    image: nginx
    ports:
      - "8080:80"
  db:
    image: postgres:14
    environment:
      POSTGRES_PASSWORD: secret
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Commands:

# Start all services
docker compose up -d

# View logs
docker compose logs -f

# Stop all services
docker compose down

# Rebuild and start
docker compose up -d --build

Cleanup

# Remove stopped containers
docker container prune

# Remove unused images
docker image prune

# Remove everything unused
docker system prune -a

# Remove specific container + image
docker rm web
docker rmi nginx

Basic Configuration

# Config files location
/etc/nginx/nginx.conf            # Main config
/etc/nginx/sites-available/      # Site configs
/etc/nginx/sites-enabled/        # Active sites (symlinks)

# Create new site
sudo nano /etc/nginx/sites-available/mysite

# Add this:
server {
    listen 80;
    server_name example.com www.example.com;
    root /var/www/mysite;
    index index.html;

    location / {
        try_files $uri $uri/ =404;
    }
}

# Enable site
sudo ln -s /etc/nginx/sites-available/mysite /etc/nginx/sites-enabled/

# Test config
sudo nginx -t

# Reload nginx
sudo systemctl reload nginx

SSL/HTTPS with Let's Encrypt

# Install Certbot
sudo apt install certbot python3-certbot-nginx

# Get free SSL certificate
sudo certbot --nginx -d example.com -d www.example.com

# Auto-renewal (runs automatically)
sudo certbot renew --dry-run

Reverse Proxy (for Node.js/Python apps)

# /etc/nginx/sites-available/api
server {
    listen 80;
    server_name api.example.com;

    location / {
        proxy_pass http://localhost:3000;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_cache_bypass $http_upgrade;
    }
}

Useful Nginx Commands

# Test config for errors
sudo nginx -t

# Reload config (no downtime)
sudo systemctl reload nginx

# Restart nginx
sudo systemctl restart nginx

# View logs
sudo tail -f /var/log/nginx/access.log
sudo tail -f /var/log/nginx/error.log

# Check which sites are enabled
ls -l /etc/nginx/sites-enabled/

Create backup directory

mkdir -p "$BACKUP"

Copy files

echo "Starting backup..."
cp -r "$SOURCE" "$BACKUP"
echo "Backup complete: $BACKUP"

Make it executable and run:

chmod +x backup.sh
./backup.sh

Script with Arguments

#!/bin/bash
# greet.sh - Say hello

if [ $# -eq 0 ]; then
    echo "Usage: $0 <name>"
    exit 1
fi

NAME=$1
echo "Hello, $NAME!"

Usage:

./greet.sh John
# Output: Hello, John!

Scheduled Tasks with Cron

Cron runs commands automatically at scheduled times.

# Edit your crontab
crontab -e

# Crontab format:
# ┌─ minute (0-59)
# │ ┌─ hour (0-23)
# │ │ ┌─ day of month (1-31)
# │ │ │ ┌─ month (1-12)
# │ │ │ │ ┌─ day of week (0-6, 0=Sunday)
# │ │ │ │ │
# * * * * * command to execute

Common examples:

# Run every day at 2 AM
0 2 * * * /home/john/backup.sh

# Run every hour
0 * * * * /usr/local/bin/cleanup.sh

# Run every Monday at 9 AM
0 9 * * 1 /home/john/weekly-report.sh

# Run every 15 minutes
*/15 * * * * /home/john/check-status.sh

# Run on reboot
@reboot /home/john/start-services.sh

# View your scheduled tasks
crontab -l

Useful Script Examples

Check if service is running:

#!/bin/bash
if systemctl is-active --quiet nginx; then
    echo "nginx is running"
else
    echo "nginx is NOT running"
    sudo systemctl start nginx
fi

Loop through files:

#!/bin/bash
for file in /var/log/*.log; do
    echo "Processing: $file"
    # Do something with $file
done

Check if command exists:

#!/bin/bash
if command -v docker &> /dev/null; then
    echo "Docker is installed"
else
    echo "Docker is NOT installed"
fi

Use the principle of least privilege
top # Basic monitor (press q to quit)
htop # Better UI (install with apt/dnf)
uptime # System uptime and load average

Real-time resource monitor

vmstat 1 # Update every second
iostat -x 1 # Disk I/O stats (install sysstat)

System Logs

# Modern (systemd) logs
journalctl -xe                   # Recent errors
journalctl -f                    # Follow live logs
journalctl --since today         # Today's logs
journalctl --since "1 hour ago"  # Last hour
journalctl -u nginx              # Specific service
journalctl -k                    # Kernel messages
journalctl -p err                # Only errors

# Traditional log files (in /var/log)
sudo tail -f /var/log/syslog     # System log
sudo tail -f /var/log/auth.log   # Authentication attempts
sudo tail -f /var/log/kern.log   # Kernel messages
sudo less /var/log/apache2/error.log  # Apache errors

# Follow any log file
tail -f /var/log/myapp.log       # Live updates
less +F /var/log/myapp.log       # Less with follow mode

Kernel Messages

dmesg                            # Boot and hardware messages
dmesg | grep -i error            # Find errors
dmesg -w                         # Follow mode
dmesg --level=err,warn           # Only errors and warnings

Quick Health Check

# Check if anything is wrong
systemctl --failed               # Failed services
journalctl -p err --since today  # Today's errors
df -h | grep -v tmpfs           # Disk space issues
free -h                          # Memory issues
uptime                           # Load average (if > CPU count, system is busy)

sudo chmod 600 /swapfile # Security
sudo mkswap /swapfile
sudo swapon /swapfile

Make permanent (add to /etc/fstab)

/swapfile none swap sw 0 0

Check swap

free -h
swapon --show

DNS lookup

dig google.com                   # Detailed DNS info
nslookup google.com             # Simple DNS lookup
host google.com                 # Quick DNS check

Check open ports

sudo ss -tulpen                  # List listening ports (modern)
sudo netstat -tulpen            # Older way

Firewall (UFW - Ubuntu/Debian)

# Enable firewall
sudo ufw enable
sudo ufw status                  # Check status

# Allow ports
sudo ufw allow 22/tcp            # SSH
sudo ufw allow 80/tcp            # HTTP
sudo ufw allow 443/tcp           # HTTPS
sudo ufw allow 22,80,443/tcp     # Multiple ports

# Allow from specific IP
sudo ufw allow from 192.168.1.100

# Delete rules
sudo ufw status numbered         # Show rule numbers
sudo ufw delete 3                # Delete rule #3

# Disable firewall
sudo ufw disable

SSH (Remote Access)

# Connect to remote server
ssh user@192.168.1.100
ssh user@example.com

# Generate SSH key (do this on your local machine)
ssh-keygen -t ed25519 -C "your_email@example.com"
# Keys saved to: ~/.ssh/id_ed25519 (private) and ~/.ssh/id_ed25519.pub (public)

# Copy key to server (enables passwordless login)
ssh-copy-id user@server-ip

# SSH config file (~/.ssh/config)
Host myserver
    HostName 192.168.1.100
    User john
    Port 22
    IdentityFile ~/.ssh/id_ed25519

# Now you can connect with:
ssh myserver

Test Web Services

# Download file
curl -O https://example.com/file.zip

# Test HTTP endpoint
curl http://localhost:8080               # GET request
curl -I https://example.com              # Show headers only
curl -v https://api.example.com          # Verbose output

# POST request with JSON
curl -X POST http://localhost:3000/api \
  -H "Content-Type: application/json" \
  -d '{"name":"John"}'

# Test if port is open
nc -zv localhost 80                      # netcat port check
telnet localhost 80                      # telnet (old school)

Download Files

# Using curl
curl -O https://example.com/file.zip     # Save with original name
curl -o myfile.zip https://example.com/file.zip  # Custom name

# Using wget
wget https://example.com/file.zip
wget -c https://example.com/bigfile.zip  # Resume download

sudo systemctl enable nginx # Start on boot
sudo systemctl disable nginx # Don't start on boot
sudo systemctl enable --now nginx # Enable AND start immediately

View All Services

systemctl list-units --type=service          # Running services
systemctl list-unit-files --type=service     # All services
systemctl list-units --failed                # Failed services

Service Logs

# View service logs
journalctl -u nginx                          # All nginx logs
journalctl -u nginx --since "1 hour ago"     # Last hour
journalctl -u nginx --since today            # Today's logs
journalctl -fu nginx                         # Follow live (-f)
journalctl -u nginx -n 50                    # Last 50 lines

# System-wide logs
journalctl -xe                               # Recent errors
journalctl -k                                # Kernel messages
journalctl --since "2024-01-15"              # Specific date

Example: Set up a Web Server

# Install nginx
sudo apt install nginx

# Start and enable on boot
sudo systemctl enable --now nginx

# Check status
systemctl status nginx

# View logs
journalctl -fu nginx

# Restart after config change
sudo systemctl restart nginx


pidof nginx # Same as pgrep
pstree # Tree view of processes

Stop/Kill Processes

# Gentle stop (gives process time to cleanup)
kill 1234                        # Send SIGTERM to PID 1234
killall nginx                    # Kill all nginx processes
pkill -f python                  # Kill by name pattern

# Force kill (last resort - data may be lost!)
kill -9 1234                     # Send SIGKILL
killall -9 nginx

Kill Signal Cheat Sheet:

SIGTERM (15): Polite "please exit" - default
SIGKILL (9): Force kill - use when SIGTERM fails
SIGHUP (1): Reload config without restart

Background Jobs

# Run in background
command &                        # Start in background
./long-script.sh &

# Manage jobs
jobs                             # List background jobs
fg                               # Bring to foreground
bg                               # Resume in background
Ctrl+Z                           # Pause current process
bg                               # Continue paused process in background

# Example workflow
./server.sh                      # Start server
Ctrl+Z                           # Pause it
bg                               # Run in background
jobs                             # See it running
fg                               # Bring back to foreground

Resource Control

# Start process with lower priority (nice = less CPU)
nice -n 10 ./cpu-heavy-script.sh      # Lower priority
nice -n -5 ./important-task.sh        # Higher priority (requires sudo)

# Change priority of running process
renice 10 -p 1234                     # Make PID 1234 lower priority

File Permissions

# Read permissions
ls -l file.txt         # Shows: -rw-r--r-- (owner, group, others)

# Change permissions
chmod 644 file.txt     # rw-r--r-- (owner: read+write, others: read)
chmod +x script.sh     # Make executable
chmod 755 script.sh    # rwxr-xr-x (common for scripts)

# Change owner
sudo chown user:group file.txt

Permission Numbers:

7 = read + write + execute (rwx)
6 = read + write (rw-)
5 = read + execute (r-x)
4 = read only (r--)

Searching Files

# Find files by name
find . -name "*.log"              # Find all .log files
find /var -type d -name "nginx"   # Find directories named nginx

# Search inside files
grep "error" logfile.txt          # Find "error" in file
grep -r "TODO" .                  # Search all files recursively
grep -i "warning" file.txt        # Case-insensitive search

Text Processing

# Count lines/words
wc -l file.txt                    # Count lines
wc -w file.txt                    # Count words

# Sort and unique
sort names.txt                    # Sort alphabetically
sort -n numbers.txt               # Sort numerically
sort file.txt | uniq              # Remove duplicates

# Pipes - chain commands together
cat file.txt | grep "error" | wc -l    # Count error lines
ls -l | grep "\.txt$"                  # List only .txt files

Command History

history                # View command history
!123                   # Run command #123 from history
!!                     # Repeat last command
!$                     # Use last argument from previous command
Ctrl+R                 # Search command history (interactive)

Environment Variables

# View environment
echo $HOME             # Your home directory
echo $PATH             # Where Linux looks for commands
env                    # Show all environment variables

# Set variables
export MY_VAR="value"  # Set for current session

# Add to PATH (in ~/.bashrc or ~/.zshrc)
export PATH="$HOME/bin:$PATH"

sudo usermod -aG sudo john # Give admin privileges

3. Set your timezone

timedatectl set-timezone America/New_York

4. Enable firewall

sudo ufw enable # Ubuntu/Debian
sudo systemctl enable firewalld # Fedora/RHEL

For Remote Servers (SSH Setup)

# Install SSH server
sudo apt install openssh-server

# Generate SSH key on your local machine
ssh-keygen -t ed25519

# Copy key to server
ssh-copy-id user@server-ip

# Secure SSH: edit /etc/ssh/sshd_config
PermitRootLogin no
PasswordAuthentication no

Shell Essentials (Bash/Zsh)

Navigation: pwd, ls -la, cd - (previous dir), pushd/popd.
File ops: cat, less, head, tail -f, cp -r, mv, rm -rf, mkdir -p.
Permissions: chmod 644 file, chmod +x script.sh, chown user:group file, umask.
Searching: grep -R "pattern" ., rg (ripgrep), find . -type f -name "*.log".
Text processing: cut, awk, sed, sort, uniq, wc -l, xargs, pipes |.
History & reuse: history, !n, !!, !$, Ctrl+R reverse search.
Shell config: .bashrc, .bash_profile, .zshrc; PATH edits: export PATH="$HOME/bin:$PATH".

Linux Guide: Basics to Advanced

Linux Guide: Basics to Advanced

1. What is Linux?

Core Parts

Why Learn Linux?

2. Getting Started

Choose a Distribution

Essential Commands

Navigation

Understanding the Filesystem

Important Directories

Disk Usage

Symbolic Links (Shortcuts)

Installing Software

Ubuntu/Debian (apt)

Fedora/RHEL/CentOS (dnf)

Arch Linux (pacman)

Snap Packages (Universal)

View all users

Groups

The sudo Command

Creating and editing

Managing Processes

View Running Processes

Basic Service Control

Check service status

Check Network Status

Storage Management

View Disks and Partitions

Create and Format a Partition

Check and Repair Filesystem

Add Swap Space

System Resource Usage

Essential Security Steps

File Permission Security

Monitor Failed Login Attempts

Security Checklist

Basic Shell Script

Install Nginx

Working with Images

Running Containers

System is Slow

3. . Backups

Simple Backup with rsync

Compress and Archive

Find large files

System Information

File Operations

Processes

Networking

Services

Storage

19. Additional Resources

Getting Help

Online Resources

1. Check firewall

2. Check if port is listening

3. Test from server itself

4. Check logs

Permission Denied Errors

Trace System Calls (Advanced)

Execute command in running container

View container details

Docker Compose (Multi-container apps)

Cleanup

Basic Configuration

SSL/HTTPS with Let's Encrypt

Reverse Proxy (for Node.js/Python apps)

Useful Nginx Commands

Create backup directory

Copy files

Script with Arguments

Scheduled Tasks with Cron

Useful Script Examples

Real-time resource monitor

System Logs

Kernel Messages

Quick Health Check

Make permanent (add to /etc/fstab)

/swapfile none swap sw 0 0

The `sudo` Command