Shell Scripting on Linux

nano ~/scripts/hello.sh

#!/bin/bash
echo "Hello from my first script!"
echo "Server: $(hostname)"
echo "Date: $(date)"
echo "Uptime: $(uptime -p)"

chmod +x ~/scripts/hello.sh
~/scripts/hello.sh

#!/bin/bash

# Assign variables
SERVER_NAME="webserver-01"
PORT=8080

# Use with $
echo "Server: $SERVER_NAME on port $PORT"

# Command substitution
DISK_USAGE=$(df -h / | awk 'NR==2 {print $5}')
echo "Root disk usage: $DISK_USAGE"

# Default value if variable is empty
LOG_DIR="${LOG_DIR:-/var/log}"
echo "Log directory: $LOG_DIR"

#!/bin/bash

SERVICE="nginx"

# Check if a service is running
if systemctl is-active --quiet "$SERVICE"; then
    echo "$SERVICE is running"
else
    echo "$SERVICE is NOT running"
    sudo systemctl start "$SERVICE"
fi

# Check if a file exists
CONFIG="/etc/nginx/nginx.conf"
if [ -f "$CONFIG" ]; then
    echo "Config found: $CONFIG"
fi

# Check if a directory exists
if [ ! -d "/opt/docker" ]; then
    mkdir -p /opt/docker
    echo "Created /opt/docker"
fi

# Compare numbers
FREE_DISK=$(df / | awk 'NR==2 {print $4}')
if [ "$FREE_DISK" -lt 1000000 ]; then
    echo "WARNING: Less than 1GB free on root partition"
fi

#!/bin/bash

# Loop over a list
for SERVICE in nginx docker tailscaled; do
    if systemctl is-active --quiet "$SERVICE"; then
        echo "✅ $SERVICE"
    else
        echo "❌ $SERVICE"
    fi
done

# Loop over files
for FILE in /opt/docker/*/compose.yml; do
    DIR=$(dirname "$FILE")
    echo "Found stack: $DIR"
done

# While loop — retry until success
RETRIES=0
while ! ping -c 1 google.com > /dev/null 2>&1; do
    RETRIES=$((RETRIES + 1))
    echo "Network not ready, retrying... ($RETRIES)"
    sleep 5
    if [ "$RETRIES" -ge 10 ]; then
        echo "Network check failed after 10 retries"
        exit 1
    fi
done
echo "Network is up"

# Loop with a range
for i in {1..5}; do
    echo "Attempt $i"
done

#!/bin/bash

# Simple function
log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1"
}

# Function with return via echo
get_disk_usage() {
    df -h "$1" | awk 'NR==2 {print $5}' | tr -d '%'
}

# Function with local variables
check_service() {
    local SERVICE="$1"
    if systemctl is-active --quiet "$SERVICE"; then
        log "$SERVICE is running"
        return 0
    else
        log "$SERVICE is NOT running"
        return 1
    fi
}

# Usage
log "Script started"
check_service nginx
USAGE=$(get_disk_usage /)
log "Disk usage: ${USAGE}%"

#!/bin/bash

# Check required arguments
if [ $# -lt 1 ]; then
    echo "Usage: $0 <service> [action]"
    echo "Example: $0 nginx restart"
    exit 1
fi

SERVICE="$1"
ACTION="${2:-status}"

case "$ACTION" in
    start)    sudo systemctl start "$SERVICE" ;;
    stop)     sudo systemctl stop "$SERVICE" ;;
    restart)  sudo systemctl restart "$SERVICE" ;;
    status)   systemctl status "$SERVICE" ;;
    *)        echo "Unknown action: $ACTION"; exit 1 ;;
esac

#!/bin/bash

# Exit on any error
set -e

# Exit on undefined variables
set -u

# Catch errors in pipelines
set -o pipefail

# Log function
log() { echo "[$(date '+%H:%M:%S')] $1"; }
error() { echo "[ERROR] $1" >&2; exit 1; }

# Cleanup on exit
cleanup() {
    log "Cleaning up..."
    rm -f /tmp/deploy-lock
}
trap cleanup EXIT

# Check we're running as root
if [ "$EUID" -ne 0 ]; then
    error "This script must be run as root"
fi

log "Starting deployment..."

#!/bin/bash
# health-check.sh — quick server health summary

log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1"; }

echo "========================================"
echo "  Health Check — $(hostname)"
echo "  $(date)"
echo "========================================"

# Disk usage
DISK=$(df -h / | awk 'NR==2 {print $5}' | tr -d '%')
if [ "$DISK" -gt 85 ]; then
    echo "⚠️  Disk: ${DISK}% (WARNING)"
else
    echo "✅ Disk: ${DISK}%"
fi

# Memory
MEM_FREE=$(free -m | awk 'NR==2 {print $4}')
echo "💾 Free memory: ${MEM_FREE}MB"

# Load average
LOAD=$(uptime | awk -F'load average:' '{print $2}' | xargs)
echo "⚡ Load average: $LOAD"

# Services
echo ""
echo "Services:"
for SERVICE in nginx docker tailscaled ssh; do
    if systemctl is-active --quiet "$SERVICE" 2>/dev/null; then
        echo "  ✅ $SERVICE"
    else
        echo "  ❌ $SERVICE"
    fi
done

# Docker containers
if command -v docker &>/dev/null; then
    echo ""
    echo "Docker containers:"
    docker ps --format "  {{.Names}} — {{.Status}}" 2>/dev/null
fi

#!/bin/bash
# backup.sh — tar backup with timestamp and optional remote copy

SOURCE="${1:?Usage: $0 <source-dir> [dest-dir]}"
DEST_DIR="${2:-/mnt/backups}"
TIMESTAMP=$(date +%Y%m%d-%H%M%S)
FILENAME="$(basename "$SOURCE")-${TIMESTAMP}.tar.gz"
DEST="${DEST_DIR}/${FILENAME}"

mkdir -p "$DEST_DIR"

echo "Backing up $SOURCE → $DEST"
tar -czf "$DEST" "$SOURCE"

SIZE=$(du -sh "$DEST" | cut -f1)
echo "Done. Size: $SIZE"

# Keep only last 7 backups
ls -t "${DEST_DIR}/$(basename "$SOURCE")-"*.tar.gz 2>/dev/null | tail -n +8 | xargs rm -f
echo "Old backups cleaned up"

#!/bin/bash
# update-stacks.sh — pull and restart all Docker Compose stacks

DOCKER_BASE="/opt/docker"
LOG_FILE="/var/log/stack-updates.log"

log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a "$LOG_FILE"; }

log "Starting stack updates..."

for COMPOSE_FILE in "${DOCKER_BASE}"/*/compose.yml; do
    STACK_DIR=$(dirname "$COMPOSE_FILE")
    STACK_NAME=$(basename "$STACK_DIR")

    log "Updating: $STACK_NAME"
    cd "$STACK_DIR"

    if docker compose pull --quiet 2>&1 | tee -a "$LOG_FILE"; then
        docker compose up -d --quiet-pull 2>&1 | tee -a "$LOG_FILE"
        log "$STACK_NAME updated successfully"
    else
        log "ERROR: Failed to pull $STACK_NAME"
    fi
done

log "All stacks updated"

#!/bin/bash
# log-monitor.sh — watch a log file and alert on keywords

LOG_FILE="${1:-/var/log/syslog}"
KEYWORDS="error|critical|failed|panic"
ALERT_FILE="/tmp/alerts.log"

echo "Monitoring $LOG_FILE for: $KEYWORDS"
echo "Alerts logged to: $ALERT_FILE"

tail -f "$LOG_FILE" | grep --line-buffered -iE "$KEYWORDS" | while read -r LINE; do
    TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S')
    echo "[$TIMESTAMP] ALERT: $LINE" | tee -a "$ALERT_FILE"
done

crontab -e

# Create a scripts directory
mkdir -p ~/scripts

# Add to PATH in ~/.bashrc
echo 'export PATH="$HOME/scripts:$PATH"' >> ~/.bashrc
source ~/.bashrc

# Now run scripts from anywhere
health-check.sh
backup.sh /opt/docker