Shell Scripting Complete Guide

BASH#!/bin/bash # my_first_script.sh — The shebang line tells Linux which shell to use echo "Hello! I am a shell script" echo "Today is: $(date)" echo "You are logged in as: $(whoami)" echo "Current directory: $(pwd)" # Make it executable and run: # chmod +x my_first_script.sh # ./my_first_script.sh

BASH#!/bin/bash # Variables — NO spaces around the = sign! APP_NAME="order-service" VERSION="2.1.0" PORT=8080 DEPLOY_DIR="/opt/apps" echo "Deploying $APP_NAME version $VERSION on port $PORT" echo "Install directory: $DEPLOY_DIR" # Command output as variable CURRENT_DATE=$(date +%Y-%m-%d) HOSTNAME=$(hostname) FREE_MEM=$(free -m | awk "/Mem:/ {print \$4}") echo "Date: $CURRENT_DATE" echo "Host: $HOSTNAME" echo "Free memory: ${FREE_MEM}MB" # Read-only variable (cannot be changed) readonly DB_HOST="prod-db.company.com" # Environment variable (visible to child processes) export API_KEY="abc123"

BASH#!/bin/bash # Echo — print output echo "Simple message" echo -n "No newline at end" # stays on same line echo -e "Line 1\nLine 2" # \n = newline, \t = tab # Printf — formatted output (more control) printf "Name: %-15s Age: %d\n" "Suresh" 28 printf "Price: $%.2f\n" 49.9 # Read — get user input echo -n "Enter server name: " read SERVER_NAME echo "You entered: $SERVER_NAME" # Read with prompt (cleaner) read -p "Enter environment (dev/prod): " ENV read -sp "Enter password: " PASSWORD # -s = silent (hidden) echo "" # newline after hidden input # Read with timeout read -t 10 -p "Confirm deploy? (y/n): " CONFIRM # Waits max 10 seconds for input

BASH#!/bin/bash # Basic if-else SERVER_STATUS=$(systemctl is-active nginx) if [ "$SERVER_STATUS" = "active" ]; then echo "Nginx is running — all good" elif [ "$SERVER_STATUS" = "inactive" ]; then echo "Nginx is down — restarting..." sudo systemctl start nginx else echo "Nginx status unknown: $SERVER_STATUS" fi # File checks if [ -f "/etc/nginx/nginx.conf" ]; then echo "Config file exists" fi if [ -d "/var/log/nginx" ]; then echo "Log directory exists" fi if [ ! -f "/tmp/deploy.lock" ]; then echo "No deploy in progress — safe to deploy" fi # Number comparisons DISK_USAGE=$(df / | tail -1 | awk "{print \$5}" | tr -d "%") if [ "$DISK_USAGE" -gt 80 ]; then echo "WARNING: Disk usage is ${DISK_USAGE}%!" elif [ "$DISK_USAGE" -gt 60 ]; then echo "Disk usage is moderate: ${DISK_USAGE}%" else echo "Disk usage OK: ${DISK_USAGE}%" fi

BASH#!/bin/bash # For loop — iterate over a list for SERVER in web1 web2 web3 db1; do echo "Checking $SERVER..." ping -c 1 -W 2 "$SERVER" > /dev/null 2>&1 if [ $? -eq 0 ]; then echo " $SERVER is UP" else echo " $SERVER is DOWN!" fi done # For loop — iterate over files for FILE in /var/log/*.log; do SIZE=$(du -sh "$FILE" | cut -f1) echo "$FILE: $SIZE" done # For loop — number range for i in {1..5}; do echo "Attempt $i of 5" done # While loop — repeat until condition is false RETRIES=0 MAX_RETRIES=5 while [ $RETRIES -lt $MAX_RETRIES ]; do curl -sf http://localhost:8080/health > /dev/null if [ $? -eq 0 ]; then echo "App is healthy!" break fi RETRIES=$((RETRIES + 1)) echo "Retry $RETRIES/$MAX_RETRIES — waiting 5 seconds..." sleep 5 done # While read — process file line by line while IFS= read -r LINE; do echo "Processing: $LINE" done < servers.txt

BASH#!/bin/bash # Define a function log() { echo "[$(date +"%Y-%m-%d %H:%M:%S")] $1" } # Use it log "Starting deployment" log "Building application" log "Deployment complete" # Output: [2024-06-01 10:30:15] Starting deployment # Function with return value is_service_running() { local SERVICE=$1 systemctl is-active --quiet "$SERVICE" return $? # returns 0 (true) or 1 (false) } if is_service_running "nginx"; then log "Nginx is running" else log "Nginx is down — starting it" sudo systemctl start nginx fi # Function with multiple parameters deploy() { local APP=$1 local ENV=$2 local VERSION=$3 log "Deploying $APP v$VERSION to $ENV" # deployment logic here } deploy "order-service" "staging" "2.1.0" deploy "user-service" "production" "1.5.2"

BASH#!/bin/bash STR="Hello World DevOps" # Length echo ${#STR} # 18 # Substring echo ${STR:0:5} # Hello (from position 0, take 5 chars) echo ${STR:6} # World DevOps (from position 6 to end) # Replace FILE="app-v1.2.3.tar.gz" echo ${FILE%.tar.gz} # app-v1.2.3 (remove suffix) echo ${FILE##*.} # gz (get extension) echo ${FILE/v1/v2} # app-v2.2.3.tar.gz (replace first match) # Default values (super useful!) ENV=${1:-"development"} # Use $1 if provided, otherwise "development" echo "Environment: $ENV" DB_PORT=${DB_PORT:-5432} # Use env var if set, otherwise 5432 echo "DB Port: $DB_PORT"

BASH#!/bin/bash # Create array SERVERS=("web1" "web2" "web3" "db1" "db2") PORTS=(80 443 8080 3306) # Access elements echo ${SERVERS[0]} # web1 (first element) echo ${SERVERS[2]} # web3 (third element) echo ${SERVERS[@]} # all elements echo ${#SERVERS[@]} # 5 (count) # Loop through array for SERVER in "${SERVERS[@]}"; do echo "Pinging $SERVER..." done # Add element SERVERS+=("web4") # Real-world: deploy to multiple environments ENVIRONMENTS=("dev" "staging" "production") for ENV in "${ENVIRONMENTS[@]}"; do echo "Deploying to $ENV..." kubectl apply -f "k8s/$ENV/" 2>/dev/null done

BASH#!/bin/bash # Check if file/directory exists [ -f "config.yml" ] && echo "Config exists" || echo "Config missing!" [ -d "/opt/app" ] || mkdir -p /opt/app # Read file line by line while IFS= read -r LINE; do # Skip empty lines and comments [[ -z "$LINE" || "$LINE" =~ ^# ]] && continue echo "Server: $LINE" done < servers.txt # Write to file echo "Deployment log" > deploy.log # overwrite echo "$(date): Started" >> deploy.log # append # Create config file from script cat > /tmp/nginx.conf << EOF server { listen 80; server_name myapp.com; location / { proxy_pass http://localhost:3000; } } EOF # Find and process files find /var/log -name "*.log" -mtime +30 -exec gzip {} \; echo "Old logs compressed"

BASH#!/bin/bash set -euo pipefail # set -e → Stop if ANY command fails # set -u → Stop if undefined variable is used # set -o pipefail → Catch errors in piped commands # Trap — run cleanup on exit (even on error) cleanup() { echo "Cleaning up temp files..." rm -f /tmp/deploy_*.tmp rm -f /tmp/deploy.lock } trap cleanup EXIT # Runs cleanup when script exits (success or failure) trap cleanup ERR # Also runs on error # Lock file — prevent two deploys at same time LOCKFILE="/tmp/deploy.lock" if [ -f "$LOCKFILE" ]; then echo "ERROR: Deploy already in progress!" exit 1 fi touch "$LOCKFILE" # Exit codes check_service() { if ! systemctl is-active --quiet "$1"; then echo "ERROR: $1 is not running" return 1 # return non-zero = failure fi return 0 # return 0 = success } check_service "nginx" || exit 1

BASH#!/bin/bash # awk — extract columns (like a spreadsheet) # Print 1st column (IP addresses) from access log awk "{print \$1}" access.log | sort | uniq -c | sort -rn | head -10 # Print specific fields echo "John:28:Engineer" | awk -F: "{print \$1, \$3}" # Output: John Engineer # sed — find and replace in files sed -i "s/old-version/new-version/g" config.yml # replace in file sed -i "s/DEBUG=true/DEBUG=false/" .env # toggle debug off sed -n "10,20p" logfile.txt # print lines 10-20 # cut — extract by delimiter echo "user:password:uid:gid" | cut -d: -f1,3 # Output: user:uid # tr — translate/delete characters echo "HELLO WORLD" | tr "A-Z" "a-z" # hello world echo " extra spaces " | tr -s " " # extra spaces # sort + uniq — count unique values cat access.log | awk "{print \$1}" | sort | uniq -c | sort -rn # Shows: count IP_address (most frequent first) # Combine them (the UNIX philosophy) # Find top 5 error types in last 1000 lines of log tail -1000 app.log | grep "ERROR" | awk -F":" "{print \$4}" | sort | uniq -c | sort -rn | head -5

DEPLOYMENT#!/bin/bash set -euo pipefail APP="order-service" DEPLOY_DIR="/opt/$APP" BACKUP_DIR="/opt/backups" DATE=$(date +%Y%m%d_%H%M%S) log() { echo "[$(date +\"%H:%M:%S\")] $1"; } log "Starting deployment of $APP" # Step 1: Backup log "Creating backup..." tar -czf "$BACKUP_DIR/${APP}_${DATE}.tar.gz" "$DEPLOY_DIR" 2>/dev/null || true # Step 2: Pull latest code log "Pulling latest code..." cd "$DEPLOY_DIR" git pull origin main # Step 3: Build log "Building..." npm ci && npm run build # Step 4: Restart log "Restarting service..." sudo systemctl restart "$APP" # Step 5: Health check sleep 5 if curl -sf http://localhost:3000/health > /dev/null; then log "Deployment successful!" else log "HEALTH CHECK FAILED! Rolling back..." tar -xzf "$BACKUP_DIR/${APP}_${DATE}.tar.gz" -C / sudo systemctl restart "$APP" log "Rolled back to previous version" exit 1 fi

BACKUP#!/bin/bash set -euo pipefail # Run daily via cron: 0 2 * * * /opt/scripts/backup.sh BACKUP_DIR="/opt/backups" DATE=$(date +%Y%m%d) RETENTION_DAYS=14 # Backup database mysqldump -u root mydb | gzip > "$BACKUP_DIR/db_${DATE}.sql.gz" # Backup application tar -czf "$BACKUP_DIR/app_${DATE}.tar.gz" /opt/myapp/ # Upload to S3 aws s3 cp "$BACKUP_DIR/db_${DATE}.sql.gz" s3://my-backups/db/ aws s3 cp "$BACKUP_DIR/app_${DATE}.tar.gz" s3://my-backups/app/ # Delete old local backups find "$BACKUP_DIR" -name "*.gz" -mtime +$RETENTION_DAYS -delete echo "[$(date)] Backup complete"

BASH#!/bin/bash # disk_alert.sh — Alert if disk usage exceeds threshold THRESHOLD=80 USAGE=$(df / | tail -1 | awk "{print \$5}" | tr -d "%") if [ "$USAGE" -gt "$THRESHOLD" ]; then echo "ALERT: Disk usage is ${USAGE}% (threshold: ${THRESHOLD}%)" # Send alert to Slack/email here fi

BASH#!/bin/bash # health_monitor.sh — Check multiple services SERVICES=("nginx" "docker" "mysql") for SVC in "${SERVICES[@]}"; do if systemctl is-active --quiet "$SVC"; then echo "[ OK ] $SVC" else echo "[FAIL] $SVC — attempting restart..." sudo systemctl start "$SVC" fi done

BASH#!/bin/bash # cleanup_logs.sh — Delete old logs, compress recent ones LOG_DIR="/var/log/myapp" # Compress logs older than 1 day find "$LOG_DIR" -name "*.log" -mtime +1 ! -name "*.gz" -exec gzip {} \; # Delete compressed logs older than 30 days find "$LOG_DIR" -name "*.gz" -mtime +30 -delete echo "Cleaned up: $(du -sh $LOG_DIR | cut -f1) remaining"

BASH#!/bin/bash # git_deploy.sh — Pull, build, deploy across multiple repos REPOS=("frontend" "backend" "worker") BASE_DIR="/opt/apps" for REPO in "${REPOS[@]}"; do echo "Updating $REPO..." cd "$BASE_DIR/$REPO" git pull origin main if [ -f "package.json" ]; then npm ci && npm run build elif [ -f "pom.xml" ]; then mvn clean package -DskipTests fi sudo systemctl restart "$REPO" echo "$REPO updated and restarted" done