Shell Scripting — Loops: until & select

# ── Basic until ───────────────────────────────────────────
count=1
until (( count > 5 )); do
  echo "  count = ${count}"
  (( count++ ))
done

# ── Wait until service is up ──────────────────────────────
echo "Waiting for MySQL to accept connections..."
until mysql -h "${DB_HOST}" -u root -e "SELECT 1" >/dev/null 2>&1; do
  echo "  Not ready yet — retrying in 3s..."
  sleep 3
done
echo "  ✔ MySQL is ready"

# ── Wait with timeout ─────────────────────────────────────
timeout=60
elapsed=0

until curl -sf "http://localhost:8080/health" >/dev/null; do
  if (( elapsed >= timeout )); then
    echo "ERROR: Service did not start within ${timeout}s" >&2
    exit 1
  fi
  echo "  Waiting... ${elapsed}s / ${timeout}s"
  sleep 2
  (( elapsed += 2 ))
done
echo "  ✔ Service ready after ${elapsed}s"

# These two are exactly equivalent — pick the more readable one

# Option A: while (not ready)
while ! ping -c1 -W1 db-server >/dev/null 2>&1; do
  echo "  Waiting for db-server..."
  sleep 2
done

# Option B: until (ready)
until ping -c1 -W1 db-server >/dev/null 2>&1; do
  echo "  Waiting for db-server..."
  sleep 2
done
# Option B is cleaner — no negation ! needed

# ── Practical rule ────────────────────────────────────────
# Use until when your success condition is a simple command
# Use while when your condition uses comparisons like (( n < max ))

# until for deploy readiness check
until [[ $(kubectl get deployment myapp \
  -o jsonpath='{.status.readyReplicas}' 2>/dev/null) -ge 3 ]]; do
  echo "  Waiting for 3 replicas to be ready..."
  sleep 5
done
echo "  ✔ Deployment ready"

#!/usr/bin/env bash
# Simple select menu

PS3="Choose an option: "   # PS3 is the select prompt

select choice in "Start service" "Stop service" "Check status" "Quit"; do
  case "${choice}" in
    "Start service")
      echo "  Starting myapp..."
      systemctl start myapp
      ;;
    "Stop service")
      echo "  Stopping myapp..."
      systemctl stop myapp
      ;;
    "Check status")
      systemctl status myapp --no-pager
      ;;
    "Quit")
      echo "  Bye!"
      break
      ;;
    *)
      echo "  Invalid option: ${REPLY}"
      ;;
  esac
done

#!/usr/bin/env bash
# Dynamic database selection menu

PS3="Select database to backup: "

# Build list dynamically from MySQL
databases=()
while IFS= read -r db; do
  [[ "${db}" =~ ^(information_schema|performance_schema|mysql|sys)$ ]] \
    && continue
  databases+=("${db}")
done < <(mysql -Nse "SHOW DATABASES" 2>/dev/null)

databases+=("ALL" "Cancel")

select db in "${databases[@]}"; do
  case "${db}" in
    "Cancel")
      echo "  Cancelled."
      break
      ;;
    "ALL")
      echo "  Backing up all databases..."
      mysqldump --all-databases > "/backups/all_$(date +%Y%m%d).sql"
      echo "  ✔ Done"
      break
      ;;
    "")
      echo "  Invalid choice: ${REPLY}"
      ;;
    *)
      echo "  Backing up: ${db}..."
      mysqldump "${db}" > "/backups/${db}_$(date +%Y%m%d).sql"
      echo "  ✔ Backup complete"
      break
      ;;
  esac
done

#!/usr/bin/env bash
# admin_menu.sh — Production server admin interface

set -euo pipefail

show_header() {
  clear
  echo "╔══════════════════════════════════╗"
  echo "║   VRIDDH SERVER ADMIN v1.0       ║"
  echo "║   Host: $(hostname -s)           ║"
  echo "║   User: ${USER}                  ║"
  echo "╚══════════════════════════════════╝"
  echo
}

disk_usage() {
  echo "── Disk Usage ──────────────────────"
  df -h --output=source,size,used,avail,pcent,target \
     | grep -v tmpfs | head -10
}

top_procs() {
  echo "── Top Processes ───────────────────"
  ps aux --sort=-%cpu | head -8
}

mem_usage() {
  echo "── Memory Usage ────────────────────"
  free -h
}

PS3=$'\nAdmin action: '

while true; do
  show_header
  select opt in \
    "Disk usage" \
    "Top processes" \
    "Memory usage" \
    "Restart nginx" \
    "Tail app log" \
    "Exit"; do
    case "${opt}" in
      "Disk usage")     disk_usage;   break ;;
      "Top processes")  top_procs;    break ;;
      "Memory usage")   mem_usage;    break ;;
      "Restart nginx")
        echo "Restarting nginx..."
        systemctl restart nginx && echo "✔ Done"
        break ;;
      "Tail app log")
        tail -f /var/log/myapp/app.log
        break ;;
      "Exit")
        echo "Goodbye."
        exit 0 ;;
      *) echo "Invalid: ${REPLY}" ;;
    esac
  done
  echo $'\nPress Enter to return to menu...'
  read -r
done