#!/usr/bin/env bash
#
# Generate a concise system health report for Linux servers.
#
# This script is useful for incident triage, routine maintenance, and
# collecting baseline information before deployments or infrastructure changes.
#
# Usage:
#   bash system_health_report.sh
#   bash system_health_report.sh --output /tmp/health-report.txt
#   bash system_health_report.sh --service nginx --service docker
#
# Options:
#   --output FILE       Write the report to FILE instead of stdout.
#   --service NAME     Include systemd status for a service. Can be repeated.
#   --help             Show this help message.

set -Eeuo pipefail

output_file=""
services=()

usage() {
  sed -n '2,18p' "$0" | sed 's/^# \{0,1\}//'
}

require_command() {
  local command_name="$1"
  if ! command -v "$command_name" >/dev/null 2>&1; then
    printf 'ERROR: required command not found: %s\n' "$command_name" >&2
    exit 1
  fi
}

section() {
  printf '\n## %s\n' "$1"
}

collect_report() {
  section "Host"
  printf 'Hostname: %s\n' "$(hostname -f 2>/dev/null || hostname)"
  printf 'Kernel: %s\n' "$(uname -srmo)"
  printf 'Uptime: %s\n' "$(uptime -p 2>/dev/null || uptime)"
  printf 'Date: %s\n' "$(date -Is)"

  section "CPU"
  if command -v lscpu >/dev/null 2>&1; then
    lscpu | awk -F: '/Model name|CPU\(s\)|Thread|Core|Socket/ {gsub(/^[ \t]+/, "", $2); print $1 ": " $2}'
  else
    grep -m1 'model name' /proc/cpuinfo || true
  fi
  printf 'Load average: %s\n' "$(awk '{print $1, $2, $3}' /proc/loadavg)"

  section "Memory"
  free -h

  section "Disk"
  df -hT -x tmpfs -x devtmpfs

  section "Network"
  ip -brief address 2>/dev/null || hostname -I || true

  section "Top Processes By Memory"
  ps -eo pid,ppid,comm,%mem,%cpu --sort=-%mem | head -n 11

  if [[ "${#services[@]}" -gt 0 ]]; then
    section "Service Status"
    for service in "${services[@]}"; do
      printf '\nService: %s\n' "$service"
      systemctl is-active "$service" 2>/dev/null || true
      systemctl --no-pager --lines=5 status "$service" 2>/dev/null || true
    done
  fi
}

while [[ "$#" -gt 0 ]]; do
  case "$1" in
    --output)
      output_file="${2:-}"
      [[ -n "$output_file" ]] || { printf 'ERROR: --output requires a file path.\n' >&2; exit 1; }
      shift 2
      ;;
    --service)
      [[ -n "${2:-}" ]] || { printf 'ERROR: --service requires a service name.\n' >&2; exit 1; }
      services+=("$2")
      shift 2
      ;;
    --help|-h)
      usage
      exit 0
      ;;
    *)
      printf 'ERROR: unknown option: %s\n' "$1" >&2
      usage >&2
      exit 1
      ;;
  esac
done

require_command free
require_command df
require_command ps

if [[ -n "$output_file" ]]; then
  collect_report > "$output_file"
  printf 'Health report written to %s\n' "$output_file"
else
  collect_report
fi