#!/bin/bash
################################################################################
# Script Name: systemd-service-exporter.sh
# Version: 1.0
# Description: Prometheus textfile collector exporter for systemd service status
#              Monitors service state, uptime, restart count, and enabled status
#
# Author: Phil Connor
# Contact: contact@mylinux.work
# Website: https://mylinux.work
# License: MIT
# Date: 2026-03-03
#
# Prerequisites:
#   - systemctl command available (systemd)
#   - node_exporter with textfile collector enabled
#   - /var/lib/node_exporter directory exists
#
# Usage:
#   # Configure services via environment variable
#   SERVICE_LIST="nginx,sshd,cron" ./systemd-service-exporter.sh
#
#   # Configure services via config file
#   echo -e "nginx\nsshd\ncron" > /etc/systemd-service-exporter.conf
#   ./systemd-service-exporter.sh
#
#   # Debug mode
#   DEBUG=1 SERVICE_LIST="nginx" ./systemd-service-exporter.sh
#
#   # Dry run (output to stdout)
#   ./systemd-service-exporter.sh --dry-run
#
# Metrics Exported:
#   - linux_systemd_service_state{service,state} - Service state (1=current, 0=other)
#   - linux_systemd_service_uptime_seconds{service} - Seconds since service became active
#   - linux_systemd_service_restarts_total{service} - Number of times the service restarted
#   - linux_systemd_service_enabled{service} - Whether the service is enabled (1/0)
#
# Configuration:
#   Environment: SERVICE_LIST (comma-separated)
#   Config file: /etc/systemd-service-exporter.conf (one per line)
#   Textfile directory: /var/lib/node_exporter
#
################################################################################

set -o pipefail

# ============================================================================
# CONFIGURATION
# ============================================================================

readonly VERSION="1.0"
readonly SCRIPT_NAME="${0##*/}"
readonly TEXTFILE_DIR="${TEXTFILE_DIR:-/var/lib/node_exporter}"
readonly OUTPUT_FILE="${TEXTFILE_DIR}/systemd_services.prom"
readonly CONFIG_FILE="${CONFIG_FILE:-/etc/systemd-service-exporter.conf}"
readonly TMP_FILE="${OUTPUT_FILE}.$$"

# Runtime flags
DRY_RUN=false
DEBUG=${DEBUG:-}

# ============================================================================
# HELPER FUNCTIONS
# ============================================================================

debug_echo() {
    if [[ -n "$DEBUG" ]]; then
        echo "[DEBUG] $*" >&2
    fi
}

log_error() {
    echo "[ERROR] $*" >&2
}

cleanup() {
    rm -f "$TMP_FILE"
}

trap cleanup EXIT

show_help() {
    cat <<EOF
Usage: $SCRIPT_NAME [OPTIONS]

Prometheus textfile collector exporter for systemd service status.
Monitors service state, uptime, restart count, and enabled status.

OPTIONS:
    --dry-run       Output metrics to stdout instead of writing to file
    --debug         Enable debug output
    --help          Show this help message
    --version       Show version

CONFIGURATION:
    Services can be configured in two ways:

    1. Environment variable (comma-separated):
       SERVICE_LIST="nginx,sshd,cron" $SCRIPT_NAME

    2. Config file (one service per line):
       /etc/systemd-service-exporter.conf

    The environment variable takes precedence over the config file.

ENVIRONMENT VARIABLES:
    SERVICE_LIST      Comma-separated list of services to monitor
    CONFIG_FILE       Path to config file (default: /etc/systemd-service-exporter.conf)
    TEXTFILE_DIR      Textfile collector directory (default: /var/lib/node_exporter)
    DEBUG             Enable debug output when set to any value

EXAMPLES:
    SERVICE_LIST="nginx,sshd,cron" $SCRIPT_NAME
    SERVICE_LIST="docker" $SCRIPT_NAME --dry-run
    DEBUG=1 $SCRIPT_NAME

EOF
    exit 0
}

show_version() {
    echo "$SCRIPT_NAME version $VERSION"
    exit 0
}

# ============================================================================
# SERVICE DISCOVERY
# ============================================================================

load_services() {
    local services=()

    if [[ -n "${SERVICE_LIST:-}" ]]; then
        debug_echo "Loading services from SERVICE_LIST environment variable"
        IFS=',' read -ra services <<< "$SERVICE_LIST"
    elif [[ -f "$CONFIG_FILE" ]]; then
        debug_echo "Loading services from config file: $CONFIG_FILE"
        while IFS= read -r line; do
            line="${line%%#*}"
            line="${line// /}"
            if [[ -n "$line" ]]; then
                services+=("$line")
            fi
        done < "$CONFIG_FILE"
    else
        log_error "No services configured. Set SERVICE_LIST or create $CONFIG_FILE"
        exit 1
    fi

    if [[ ${#services[@]} -eq 0 ]]; then
        log_error "No services found in configuration"
        exit 1
    fi

    debug_echo "Monitoring ${#services[@]} services: ${services[*]}"
    printf '%s\n' "${services[@]}"
}

# ============================================================================
# METRICS COLLECTION
# ============================================================================

get_service_state() {
    local service="$1"
    local state
    state=$(systemctl is-active "$service" 2>/dev/null) || true
    echo "${state:-unknown}"
}

get_service_uptime() {
    local service="$1"
    local timestamp
    timestamp=$(systemctl show "$service" --property=ActiveEnterTimestamp --value 2>/dev/null) || true

    if [[ -z "$timestamp" || "$timestamp" == "" ]]; then
        echo "0"
        return
    fi

    local active_epoch
    active_epoch=$(date -d "$timestamp" +%s 2>/dev/null) || true

    if [[ -z "$active_epoch" ]]; then
        echo "0"
        return
    fi

    local now
    now=$(date +%s)
    local uptime=$((now - active_epoch))

    if [[ $uptime -lt 0 ]]; then
        echo "0"
    else
        echo "$uptime"
    fi
}

get_restart_count() {
    local service="$1"
    local count
    count=$(systemctl show "$service" --property=NRestarts --value 2>/dev/null) || true
    echo "${count:-0}"
}

get_enabled_status() {
    local service="$1"
    local status
    status=$(systemctl is-enabled "$service" 2>/dev/null) || true

    if [[ "$status" == "enabled" ]]; then
        echo "1"
    else
        echo "0"
    fi
}

state_to_value() {
    local current_state="$1"
    local check_state="$2"

    if [[ "$current_state" == "$check_state" ]]; then
        echo "1"
    else
        echo "0"
    fi
}

collect_metrics() {
    local services=()
    while IFS= read -r svc; do
        services+=("$svc")
    done < <(load_services)

    local output=""

    # Header comments
    output+="# HELP linux_systemd_service_state Current state of the systemd service\n"
    output+="# TYPE linux_systemd_service_state gauge\n"

    for service in "${services[@]}"; do
        local state
        state=$(get_service_state "$service")
        debug_echo "Service $service: state=$state"

        for s in active inactive failed; do
            local val
            val=$(state_to_value "$state" "$s")
            output+="linux_systemd_service_state{service=\"${service}\",state=\"${s}\"} ${val}\n"
        done
    done

    output+="# HELP linux_systemd_service_uptime_seconds Time in seconds since the service became active\n"
    output+="# TYPE linux_systemd_service_uptime_seconds gauge\n"

    for service in "${services[@]}"; do
        local uptime
        uptime=$(get_service_uptime "$service")
        debug_echo "Service $service: uptime=${uptime}s"
        output+="linux_systemd_service_uptime_seconds{service=\"${service}\"} ${uptime}\n"
    done

    output+="# HELP linux_systemd_service_restarts_total Total number of service restarts\n"
    output+="# TYPE linux_systemd_service_restarts_total counter\n"

    for service in "${services[@]}"; do
        local restarts
        restarts=$(get_restart_count "$service")
        debug_echo "Service $service: restarts=$restarts"
        output+="linux_systemd_service_restarts_total{service=\"${service}\"} ${restarts}\n"
    done

    output+="# HELP linux_systemd_service_enabled Whether the service is enabled to start at boot\n"
    output+="# TYPE linux_systemd_service_enabled gauge\n"

    for service in "${services[@]}"; do
        local enabled
        enabled=$(get_enabled_status "$service")
        debug_echo "Service $service: enabled=$enabled"
        output+="linux_systemd_service_enabled{service=\"${service}\"} ${enabled}\n"
    done

    printf '%b' "$output"
}

# ============================================================================
# OUTPUT
# ============================================================================

write_metrics() {
    local metrics
    metrics=$(collect_metrics)

    if [[ "$DRY_RUN" == "true" ]]; then
        echo "$metrics"
        return
    fi

    if [[ ! -d "$TEXTFILE_DIR" ]]; then
        log_error "Textfile collector directory does not exist: $TEXTFILE_DIR"
        exit 1
    fi

    echo "$metrics" > "$TMP_FILE"
    mv "$TMP_FILE" "$OUTPUT_FILE"
    debug_echo "Metrics written to $OUTPUT_FILE"
}

# ============================================================================
# MAIN
# ============================================================================

main() {
    while [[ $# -gt 0 ]]; do
        case "$1" in
            --dry-run)
                DRY_RUN=true
                shift
                ;;
            --debug)
                DEBUG=1
                shift
                ;;
            --help|-h)
                show_help
                ;;
            --version|-v)
                show_version
                ;;
            *)
                log_error "Unknown option: $1"
                echo "Use --help for usage information" >&2
                exit 1
                ;;
        esac
    done

    if ! command -v systemctl &>/dev/null; then
        log_error "systemctl not found — this script requires systemd"
        exit 1
    fi

    write_metrics
}

main "$@"