# SSH basics
ssh user@server-ip                     # Password auth
ssh -i ~/.ssh/mykey.pem user@server   # Key-based auth
ssh -p 2222 user@server               # Custom port

# SSH config (add to ~/.ssh/config)
Host myserver
    HostName 43.135.172.134
    User root
    Port 22
    IdentityFile ~/.ssh/mykey.pem
    ServerAliveInterval 60            # Keep connection alive

# Now just: ssh myserver

# Copy files
scp file.txt user@server:/path/       # Upload
scp user@server:/path/file.txt ./     # Download
scp -r folder/ user@server:/path/     # Upload directory
rsync -avz ./local/ user@server:/remote/  # Sync (faster for updates)

# Navigation
pwd                    # Where am I?
cd /path/to/dir       # Go to directory
cd ~                  # Home directory
cd -                  # Previous directory

# Listing
ls                    # List files
ls -la                # List ALL with details (permissions, size, owner)
ls -lh                # Human-readable sizes (KB, MB, GB)
ls -lt                # Sort by modification time (newest first)
ls -R                 # Recursive listing

# File operations
touch newfile.txt     # Create empty file
mkdir newdir          # Create directory
mkdir -p a/b/c        # Create nested directories
cp file.txt backup/   # Copy file
cp -r folder backup/  # Copy directory
mv old.txt new.txt     # Move/rename
rm file.txt           # Delete file
rm -r folder/          # Delete directory (recursive!)
# NEVER: rm -rf /     # Destroys everything!

# File info
file mystery-file      # Detect file type
wc -l file.txt        # Count lines
du -sh directory/     # Directory size (human-readable)
df -h                 # Disk space (human-readable)
stat file.txt         # Detailed file metadata

# Finding files
find / -name "*.log" 2>/dev/null          # Find by name
find . -type f -mtime -1                  # Files modified in last 24h
find . -size +100M                       # Files larger than 100MB
find . -name "node_modules" -type d -prune -exec rm -rf {} +  # Delete all node_modules!
grep -r "password" --include="*.js" .    # Search inside files

# Quick tip: locate is faster but uses a database
sudo updatedb        # Update the database first
locate nginx.conf    # Instant lookup

# Quick viewing
cat file.txt          # Print entire file
head -20 file.txt     # First 20 lines
tail -20 file.txt     # Last 20 lines
tail -f /var/log/app.log  # Follow log in real-time!
less file.txt         # Pager (scroll, search, quit with q)

# Searching inside files
grep "error" log.txt                # Find lines containing "error"
grep -i "error" log.txt             # Case insensitive
grep -n "error" log.txt             # With line numbers
grep -A 5 "error" log.txt           # 5 lines after match
grep -B 5 "error" log.txt           # 5 lines before match
grep -r "TODO" src/                 # Recursive search
grep -r --include="*.ts" "import" . # Search only TypeScript files

# Inline editing with sed
sed -i 's/old/new/g' config.txt    # Replace all occurrences
sed -i 's/old/new/2' config.txt    # Replace 2nd occurrence only
sed -i '3d' config.txt              # Delete line 3

# AWK for column processing
awk '{print $1, $3}' data.csv        # Print columns 1 and 3
awk -F',' '{sum += $2} END {print sum}' data.csv  # Sum column 2

# Editor choices:
# nano - Simple, always available
# vim  - Powerful, worth learning basics
# For remote editing: use your local IDE with SFTP/SSH

# Permission model: owner | group | others
# rwxr-xr-- = owner(rwx) group(r-x) others(r--)
# r = read (4), w = write (2), x = execute (1)

# View permissions
ls -la
# drwxr-xr-x 2 root root 4096 May 29 14:00 dir/
# -rw-r--r-- 1 root root  123 May 29 14:00 file.txt

# Change permissions
chmod 755 script.sh     # rwxr-xr-x (executable by all)
chmod 644 file.txt      # rw-r--r-- (readable by all, writable by owner)
chmod +x script.sh      # Add execute permission
chmod -w file.txt       # Remove write permission

# Change ownership
chown user:group file.txt
chown -R www-data:www-data /var/www  # Recursive

# Sudo (run as root)
sudo command          # Run as root
sudo -u www-data whoami  # Run as specific user
sudo visudo          # Safely edit sudoers file

# Process management
ps aux                # All running processes
ps aux | grep node    # Find Node.js processes
top                   # Interactive process viewer
htop                  # Better top (if installed)

# System resources
free -h               # Memory usage
uptime                # Uptime and load average
cat /proc/cpuinfo     # CPU info
nproc                 # Number of CPU cores
lscpu                 # Detailed CPU info

# Disk
df -h                 # Disk space by filesystem
du -sh /var/log/*     # Size of log files
du -sh * | sort -rh | head -10  # Top 10 largest items

# Network
ip addr               # IP addresses
ss -tlnp              # Listening ports (replaces netstat)
curl -I https://example.com  # HTTP headers
wget -qO- https://example.com # Fetch URL content

# Killing processes
kill 12345            # Graceful kill (SIGTERM)
kill -9 12345         # Force kill (SIGKILL)
killall node          # Kill all processes named "node"
pkill -f "node server"  # Kill by command pattern

# Background processes
nohup node server.js &   # Run in background, survives logout
disown                    # Detach from current shell
# Or use: npm run start &>> output.log &

# apt (Debian/Ubuntu)
sudo apt update                  # Update package lists
sudo apt upgrade                  # Upgrade all packages
sudo apt install nginx            # Install package
sudo apt remove package           # Remove package
sudo apt autoremove                # Remove unused dependencies
apt search keyword                # Search packages
apt show package                  # Package info

# Snap (alternative)
snap install package
snap list                         # Installed snaps

# Node.js version management
curl -fsSL https://fnm.vercel.app/install | bash
fnm install 20                    # Install Node 20
fnm use 20                       # Use Node 20
fnm default 20                   # Set default

# npm global packages
npm list -g --depth=0             # List global packages
npm install -g pnpm               # Install globally

# Service commands (replace "nginx" with your service name)
sudo systemctl start nginx         # Start service
sudo systemctl stop nginx          # Stop service
sudo systemctl restart nginx       # Restart
sudo systemctl reload nginx        # Reload config (no downtime)
sudo systemctl status nginx        # Check status
sudo systemctl enable nginx        # Start on boot
sudo systemctl disable nginx       # Don't start on boot

# Journal logs
sudo journalctl -u nginx           # Service-specific logs
sudo journalctl -u nginx -f        # Follow logs
sudo journalctl --since "1 hour ago"  # Recent logs
sudo journalctl --since today      # Today's logs

# Creating a custom service
# /etc/systemd/system/myapp.service
# [Unit]
# Description=My Node.js App
# After=network.target
#
# [Service]
# Type=simple
# User=root
# WorkingDirectory=/root/myapp
# ExecStart=/root/.nvm/versions/node/v20/bin/node server.js
# Restart=always
# RestartSec=5
# Environment=NODE_ENV=production
#
# [Install]
# WantedBy=multi-user.target
#
# Then: sudo systemctl enable myapp && sudo systemctl start myapp

# UFW (Ubuntu Firewall)
sudo ufw status                  # Check firewall status
sudo ufw enable                  # Enable firewall
sudo ufw allow 22/tcp            # Allow SSH
sudo ufw allow 80/tcp            # Allow HTTP
sudo ufw allow 443/tcp           # Allow HTTPS
sudo ufw allow 3000/tcp          # Allow custom port
sudo ufw delete allow 3000/tcp   # Remove rule
sudo ufw deny from 1.2.3.4       # Block specific IP

# Fail2ban (auto-ban attackers)
sudo apt install fail2ban
sudo systemctl enable fail2ban
sudo systemctl start fail2ban
sudo fail2ban-client status       # Check banned IPs
sudo fail2ban-client status sshd  # SSH-specific bans

# SSH hardening
# Edit /etc/ssh/sshd_config:
# Port 22                    # Or change to non-standard
# PermitRootLogin no         # Disable root login
# PasswordAuthentication no  # Key-only auth
# MaxAuthTries 3             # Limit login attempts
# AllowUsers deploy          # Only specific users

# Edit crontab
crontab -e                     # Edit your crontab
crontab -l                     # List scheduled tasks

# Cron format: MIN HOUR DOM MON DOW COMMAND
# * * * * * = every minute
# */5 * * * * = every 5 minutes
# 0 * * * * = every hour
# 0 0 * * * = midnight daily
# 0 9 * * 1 = Monday 9 AM

# Examples:
*/5 * * * * /root/scripts/health-check.sh >> /var/log/health.log 2>&1
0 3 * * * /root/scripts/backup.sh                         # Daily at 3 AM
0 */6 * * * /root/scripts/sync.sh                          # Every 6 hours

# System-wide crontab: /etc/crontab
# User-specific: crontab -e
# Cron logs: grep CRON /var/log/syslog

# Port already in use?
ss -tlnp | grep :3000           # What's on port 3000?
lsof -i :3000                   # Same thing

# Can't connect to a service?
curl -v http://localhost:3000   # Verbose connection test
nc -zv localhost 3000            # Port check

# Out of disk space?
df -h                            # Check filesystems
du -sh /var/log                  # Log file sizes
journalctl --disk-usage          # Journal size
sudo journalctl --vacuum-size=100M  # Trim journal to 100MB
sudo apt autoremove              # Remove unused packages

# High memory?
free -h                          # Overview
ps aux --sort=-%mem | head -10   # Top memory consumers
pmap 12345                       # Process memory map

# High CPU?
ps aux --sort=-%cpu | head -10   # Top CPU consumers
top -o %CPU                      # Interactive view

# Can't bind to port 80/443?
sudo netstat -tlnp | grep ':80'  # Check what's using it
sudo fuser -k 80/tcp              # Kill process on port 80

# SSH connection issues?
ssh -vvv user@server              # Debug connection
systemctl status sshd              # Check SSH service
tail -f /var/log/auth.log         # SSH auth logs

# Node.js won't start?
node --version                    # Check Node is installed
which node                         # Check PATH
npm ls --depth=0                   # Check dependencies
NODE_DEBUG=request node app.js    # Debug mode