Add initial Linux operations Bash toolkit with network diagnostics

infra-run/scripts/bash/service_check.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+set -o errexit
+set -o nounset
+set -o pipefail
+
+services=("$@")
+
+service_exists() {
+  local service="$1"
+  systemctl list-unit-files "${service}.service" --no-legend 2>/dev/null | awk '{print $1}' | grep -qx "${service}.service"
+}
+
+pick_default_scheduler() {
+  if service_exists cron; then
+    printf 'cron'
+  elif service_exists crond; then
+    printf 'crond'
+  else
+    printf 'cron'
+  fi
+}
+
+pick_default_ssh() {
+  if service_exists sshd; then
+    printf 'sshd'
+  elif service_exists ssh; then
+    printf 'ssh'
+  else
+    printf 'sshd'
+  fi
+}
+
+if ! command -v systemctl >/dev/null 2>&1; then
+  printf 'CRITICAL: systemctl command not available; cannot check services\n' >&2
+  exit 1
+fi
+
+if (( ${#services[@]} == 0 )); then
+  services=("$(pick_default_ssh)" "$(pick_default_scheduler)")
+fi
+
+status=0
+
+for service in "${services[@]}"; do
+  if ! service_exists "$service"; then
+    printf 'CRITICAL: %s service not found\n' "$service"
+    status=1
+    continue
+  fi
+
+  if systemctl is-active --quiet "$service"; then
+    printf 'OK: %s is active\n' "$service"
+  else
+    state="$(systemctl is-active "$service" 2>/dev/null || true)"
+    printf 'CRITICAL: %s is %s\n' "$service" "${state:-unknown}"
+    status=1
+  fi
+done
+
+exit "$status"