Linux Crash Course#

pwd                       # Print working directory
cd /var/log               # Change to absolute directory
cd ../                    # Move up one directory level
cd ~                      # Jump to user home directory ($HOME)

ls -la                    # List all files (including hidden .files) with permissions & sizes
mkdir -p project/src/api  # Create directory tree recursively
cp -r src/ dist/          # Copy directory recursively
mv old_name.txt new.txt   # Rename or move file
rm -rf temp/              # Forcefully remove directory without prompting (use with care!)
touch app.py              # Create empty file or update timestamp

cat file.txt              # Print entire file content to stdout
less large_log.log        # Interactive pager (search with '/', navigate with 'q', 'g', 'G')
head -n 20 data.csv       # View first 20 lines
tail -f /var/log/syslog   # Follow live appends to file in real-time
grep -rn "ERROR" ./src    # Recursively search for 'ERROR' with line numbers
wc -l access.log          # Count total lines in a file
sort -u users.txt         # Sort lines alphabetically and deduplicate

# Output redirection
echo "Starting build" > build.log     # Overwrite file with stdout
echo "Step completed" >> build.log    # Append stdout to file
python app.py 2> error.log            # Redirect stderr only
python app.py > output.log 2>&1       # Combine stdout and stderr into one file
python app.py > /dev/null 2>&1        # Discard all output completely

# Pipelines: Connect stdout of left command to stdin of right command
cat access.log | grep " 404 " | awk '{print $7}' | sort | uniq -c | sort -nr | head -n 10

chmod 755 script.sh       # rwxr-xr-x (Owner: read/write/exec, Group/Others: read/exec)
chmod 600 id_rsa          # rw------- (Owner: read/write only, Private SSH key requirement)
chmod +x deploy.sh        # Add execute permission for everyone
chown ec2-user:www-data site/ # Change owner to ec2-user and group to www-data
chown -R ec2-user:ec2-user .  # Recursively change ownership for current directory

ps aux                    # Snapshot of all running processes on the system
top                       # Real-time task manager (press 'q' to exit, 'M' to sort by RAM)
pgrep -l python           # Find PIDs matching name 'python'
kill -15 <PID>            # SIGTERM: Polite shutdown request (allows cleanup)
kill -9 <PID>             # SIGKILL: Immediate hardware termination by kernel (cannot be caught)

# Backgrounding and Job Control
python server.py &         # Launch process directly in background
jobs                      # List background jobs in current shell
fg %1                     # Bring job 1 to foreground
# Press Ctrl+Z to pause foreground job, then run:
bg %1                     # Resume paused job in background

whoami                    # Print current active username
id                        # Show UID, GID, and supplementary group memberships
sudo command              # Execute a single command with root privileges
sudo -i                   # Open interactive root shell
useradd -m -s /bin/bash devops # Create new user with home directory and default bash shell
passwd devops             # Set or update password
usermod -aG sudo devops   # Add user to sudoers group (Debian/Ubuntu)
usermod -aG wheel devops  # Add user to wheel group (RHEL/Fedora/Amazon Linux)

ip addr show              # Display IP addresses on all network interfaces
ss -tulpn                 # Show listening TCP/UDP ports and the process using each
curl -I https://example.com # Fetch HTTP response headers
ping -c 4 8.8.8.8         # Test network round-trip latency to Google DNS

# SSH and Remote File Copy
ssh -i key.pem ec2-user@44.193.134.238
scp -i key.pem file.tar.gz ec2-user@remote:/tmp/
rsync -avz -e "ssh -i key.pem" dist/ ec2-user@remote:/var/www/site/

#!/usr/bin/env bash
set -euo pipefail # Strict mode: exit immediately on error, unset vars, or pipe failure

APP_NAME="techtoday-worker"
TARGET_DIR="/var/www/${APP_NAME}"

echo "Deploying ${APP_NAME} to ${TARGET_DIR}..."

if [ ! -d "${TARGET_DIR}" ]; then
    echo "Directory does not exist. Creating..."
    sudo mkdir -p "${TARGET_DIR}"
fi

for file in *.html *.css; do
    if [ -f "$file" ]; then
        echo "Validating $file..."
    fi
done

echo "Deployment successful! Exit code: $?"