Bash Scripting for DevOps: From Basics to Advanced

#!/bin/bash

# ─── Variables ───
NAME="DevOps"
VERSION=1.5
IS_PROD=true

echo "App: $NAME v$VERSION"
echo "Production: ${IS_PROD}"   # Braces are good practice

# ─── Special Variables ───
echo "Script name: $0"          # Name of the script
echo "First arg: $1"            # First argument passed
echo "All args: $@"             # All arguments
echo "Arg count: $#"            # Number of arguments
echo "Last exit code: $?"       # Exit code of last command
echo "Process ID: $$"           # PID of current shell

# ─── Command Substitution ───
CURRENT_DATE=$(date +%Y-%m-%d)
DISK_USAGE=$(df -h / | awk 'NR==2 {print $5}')
echo "Date: $CURRENT_DATE  Disk: $DISK_USAGE"

# ─── Readonly Variables ───
readonly DB_HOST="prod-db.internal"
# DB_HOST="other"  # This would fail with an error

# ─── Arrays ───
SERVERS=("web01" "web02" "web03")
echo "${SERVERS[0]}"            # web01
echo "${SERVERS[@]}"            # All elements
echo "${#SERVERS[@]}"           # Array length
SERVERS+=("web04")              # Append element

# ─── Associative Arrays (Bash 4+) ───
declare -A CONFIG
CONFIG[host]="localhost"
CONFIG[port]=3000
CONFIG[env]="prod"
echo "Host: ${CONFIG[host]}, Port: ${CONFIG[port]}"

#!/bin/bash

# ─── if / elif / else ───
CPU_USAGE=85

if [ $CPU_USAGE -gt 90 ]; then
    echo "CRITICAL: CPU above 90%"
elif [ $CPU_USAGE -gt 75 ]; then
    echo "WARNING: CPU above 75%"
else
    echo "OK: CPU normal"
fi

# ─── String comparisons ───
ENV="production"
if [[ "$ENV" == "production" ]]; then
    echo "Running in PROD — be careful!"
fi

if [[ "$ENV" != "dev" && "$ENV" != "staging" ]]; then
    echo "Not a dev or staging environment"
fi

# ─── File tests ───
FILE="/etc/nginx/nginx.conf"
if [[ -f "$FILE" ]]; then
    echo "Config file exists"
elif [[ -d "/etc/nginx" ]]; then
    echo "Nginx directory exists but no config"
else
    echo "Nginx not installed"
fi

# Common file test operators:
# -f  : file exists
# -d  : directory exists
# -r  : file is readable
# -w  : file is writable
# -x  : file is executable
# -s  : file has size > 0
# -z  : string is empty
# -n  : string is not empty

# ─── case statement ───
ACTION="deploy"
case "$ACTION" in
    "deploy")   echo "Deploying application..." ;;
    "rollback") echo "Rolling back to previous version..." ;;
    "status")   echo "Checking service status..." ;;
    *)          echo "Unknown action: $ACTION"; exit 1 ;;
esac

#!/bin/bash

# ─── for loop ───
for SERVER in web01 web02 web03; do
    echo "Checking $SERVER..."
    ssh user@$SERVER "systemctl status nginx"
done

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

# ─── for loop over array ───
SERVICES=("nginx" "mysql" "redis" "node")
for SERVICE in "${SERVICES[@]}"; do
    if systemctl is-active --quiet "$SERVICE"; then
        echo "✅ $SERVICE is running"
    else
        echo "❌ $SERVICE is DOWN"
    fi
done

# ─── while loop ───
RETRIES=0
MAX_RETRIES=5
until curl -sf http://localhost:3000/health; do
    RETRIES=$((RETRIES + 1))
    if [ $RETRIES -ge $MAX_RETRIES ]; then
        echo "Service failed to start after $MAX_RETRIES attempts"
        exit 1
    fi
    echo "Waiting for service... (attempt $RETRIES)"
    sleep 5
done
echo "Service is up!"

# ─── Loop over file lines ───
while IFS= read -r line; do
    echo "Processing: $line"
done < servers.txt

# ─── Loop over command output ───
for PID in $(pgrep -f "node server.js"); do
    echo "Found Node process: $PID"
done

#!/bin/bash

# ─── Basic function ───
log() {
    local LEVEL="$1"
    local MESSAGE="$2"
    local TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S')
    echo "[$TIMESTAMP] [$LEVEL] $MESSAGE"
}

log "INFO"  "Starting deployment"
log "ERROR" "Database connection failed"

# ─── Function with return value ───
is_service_running() {
    local SERVICE="$1"
    if systemctl is-active --quiet "$SERVICE"; then
        return 0   # true / success
    else
        return 1   # false / failure
    fi
}

if is_service_running "nginx"; then
    log "INFO" "Nginx is running"
else
    log "ERROR" "Nginx is not running"
fi

# ─── Function returning output ───
get_container_ip() {
    local CONTAINER="$1"
    docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' "$CONTAINER"
}

APP_IP=$(get_container_ip "my-app")
echo "App container IP: $APP_IP"

#!/usr/bin/env bash
# ─── SAFETY FLAGS (Always use these!) ───
set -euo pipefail
# -e : Exit immediately on error
# -u : Treat unset variables as errors
# -o pipefail : Catch errors in pipes

# ─── Cleanup trap ───
TEMP_DIR=$(mktemp -d)
cleanup() {
    echo "Cleaning up temp files..."
    rm -rf "$TEMP_DIR"
}
trap cleanup EXIT           # Runs on script exit
trap 'log ERROR "Script failed at line $LINENO"' ERR

# ─── Real deployment script ───
deploy() {
    local APP_NAME="$1"
    local VERSION="$2"
    local ENV="$3"

    log INFO "Starting deployment of $APP_NAME:$VERSION to $ENV"

    # Validate inputs
    [[ -z "$APP_NAME" ]] && { log ERROR "APP_NAME required"; exit 1; }
    [[ -z "$VERSION" ]] && { log ERROR "VERSION required"; exit 1; }

    # Pull Docker image
    log INFO "Pulling image: $APP_NAME:$VERSION"
    docker pull "registry.example.com/$APP_NAME:$VERSION" || {
        log ERROR "Failed to pull image"
        exit 1
    }

    # Stop old container gracefully
    if docker ps -q -f name="$APP_NAME" | grep -q .; then
        log INFO "Stopping existing container"
        docker stop "$APP_NAME" --time=30
        docker rm "$APP_NAME"
    fi

    # Start new container
    log INFO "Starting new container"
    docker run -d \
        --name "$APP_NAME" \
        --restart unless-stopped \
        -e ENV="$ENV" \
        "registry.example.com/$APP_NAME:$VERSION"

    # Health check
    log INFO "Running health check..."
    sleep 10
    local MAX_CHECKS=6
    local CHECK=0
    while ! curl -sf "http://localhost:3000/health"; do
        CHECK=$((CHECK + 1))
        [[ $CHECK -ge $MAX_CHECKS ]] && { log ERROR "Health check failed after $MAX_CHECKS attempts"; exit 1; }
        log INFO "Waiting for health check ($CHECK/$MAX_CHECKS)..."
        sleep 5
    done

    log INFO "✅ Deployment of $APP_NAME:$VERSION to $ENV SUCCESSFUL"
}

deploy "$1" "$2" "${3:-staging}"