🐧 Linux Fundamentals

Why Linux for AWS?

Over 90% of cloud workloads run on Linux. EC2 instances, ECS containers, Lambda environments — all Linux. Understanding Linux means you can:

• Debug EC2 instances via SSH
• Write UserData bootstrap scripts
• Create and manage Docker containers
• Answer scenario questions on SOA-C02 & DVA-C02 exams

Linux File System

File Permissions

-rwxr-xr-- 1 ubuntu aws-team 1234 Jul 10 12:00 deploy.sh
 │└──┬──┘└──┬──┘└──┬──┘
 │   │      │      └── Other: r-- (4)
 │   │      └───────── Group: r-x (5)
 │   └──────────────── Owner: rwx (7)

chmod 755 script.sh        # rwxr-xr-x
chmod +x script.sh         # Add execute for all
chown ubuntu:team file.txt # Change owner:group

Essential Commands

# Navigation
pwd; ls -la; cd /var/log; cd ~; cd -

# Files
cp source dest; mv file newname; rm -rf dir/
mkdir -p a/b/c; touch file.txt
cat file.txt; less file.txt; tail -f /var/log/syslog

# Search
find / -name "*.log" -type f
grep -r "ERROR" /var/log/
grep -i "warning" app.log
which python3; whereis nginx

# Archives
tar -czvf archive.tar.gz /mydir
tar -xzvf archive.tar.gz
zip -r backup.zip /mydir; unzip backup.zip

Processes & System Info

ps aux                  # All running processes
ps aux | grep nginx     # Filter for nginx
top                     # Real-time process viewer
kill PID                # Graceful stop (SIGTERM)
kill -9 PID             # Force kill (SIGKILL)
free -h                 # Memory usage
df -h                   # Disk usage
uname -a                # Kernel info
uptime                  # System uptime and load

Linux Networking Commands

ip addr show                  # Show IP addresses
ping -c 4 8.8.8.8             # ICMP ping
traceroute google.com         # Trace route to host
nslookup example.com          # DNS lookup
dig example.com               # Detailed DNS query
netstat -tulnp                # All listening ports
ss -tulnp                     # Modern netstat
curl -I https://example.com   # HTTP headers
curl -v https://api.example.com  # Verbose HTTP

Shell Scripting

#!/bin/bash
# EC2 UserData example
REGION="us-east-1"
INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)

if [ -f "/etc/nginx/nginx.conf" ]; then
  echo "Nginx config exists"
fi

for FILE in /var/log/*.log; do
  echo "Processing: $FILE"
done

# Error handling
set -e          # Exit on first error
set -u          # Treat unset vars as error

# UserData script pattern
yum update -y
yum install -y nginx
systemctl start nginx
systemctl enable nginx

SSH & EC2 Access

# Connect to EC2
chmod 400 my-key.pem
ssh -i my-key.pem ec2-user@54.123.45.67   # Amazon Linux
ssh -i my-key.pem ubuntu@54.123.45.67     # Ubuntu

# SSH config (~/.ssh/config)
Host my-ec2
    HostName 54.123.45.67
    User ec2-user
    IdentityFile ~/.ssh/my-key.pem
# Then just: ssh my-ec2

# SCP — copy files
scp -i key.pem file.txt ec2-user@IP:/home/ec2-user/

# Port forwarding (tunnel to private RDS)
ssh -i key.pem -L 5432:rds-endpoint:5432 ec2-user@IP

Systemd & Services

systemctl start nginx     # Start service
systemctl stop nginx      # Stop service
systemctl restart nginx   # Restart
systemctl enable nginx    # Auto-start on boot
systemctl status nginx    # Current status
journalctl -u nginx -f    # Follow service logs
journalctl -xe            # Recent errors