Project Zomboid dedicated-server watchdog: ensures the server is always running inside a GNU screen session, periodically checks for Steam Workshop mod updates via the server console, and applies them by restarting only when no players are online, otherwise it warns connected players in-game and defers the restart until the server is empty.

project-zomboid-watchdog.sh

#!/usr/bin/env bash
set -euo pipefail

# Update the following parameters to reflect your server's setup.
START_CMD="/home/pzserver/start-server.sh"
STEAMCMD="/home/steamcmd/steamcmd.sh"
LOG_DIR="/root/Zomboid/Logs"
LOG_GLOB="*DebugLog-server.txt"

# Those parameters should be okay as they are.
SCREEN_SERVER="project-zomboid-server"
PZ_PGREP_PATTERN="ProjectZomboid"
WATCHDOG_TICK="15s"
CHECK_EVERY="3m"
CHECK_TIMEOUT="60s"
LOCKFILE="/tmp/project-zomboid-watchdog.lock"
SCREEN_WATCHDOG="project-zomboid-watchdog"
SELF="$(readlink -f "$0")"

is_pm2() { [[ -n "${PM2_HOME:-}" || -n "${pm_id:-}" ]]; }
screen_exists() {
    screen -list 2>/dev/null | grep -qE "[[:space:]][0-9]+\.${1}([[:space:]]|$)"
}

case "${1:-}" in
    start)
        if is_pm2; then
            echo "Running under pm2 — use 'pm2 start' instead."
            exit 1
        fi
        if screen_exists "$SCREEN_WATCHDOG" 2>/dev/null; then
            echo "Watchdog is already running (screen: $SCREEN_WATCHDOG)."
            exit 0
        fi
        screen -dmS "$SCREEN_WATCHDOG" bash -lc "exec \"$SELF\" run"
        echo "Watchdog started in screen '$SCREEN_WATCHDOG'."
        exit 0
        ;;
    stop)
        if screen_exists "$SCREEN_WATCHDOG" 2>/dev/null; then
            screen -S "$SCREEN_WATCHDOG" -X quit
            echo "Watchdog stopped."
        else
            echo "Watchdog is not running."
        fi
        exit 0
        ;;
    status)
        if screen_exists "$SCREEN_WATCHDOG" 2>/dev/null; then
            echo "Watchdog is running (screen: $SCREEN_WATCHDOG)."
        else
            echo "Watchdog is not running."
        fi
        exit 0
        ;;
    run|"")
        ;;
    *)
        echo "Usage: $0 {start|stop|status}"
        exit 1
        ;;
esac

ts() { date +"%F %T"; }
log() { echo "[$(ts)] $*"; }

start_server_screen() {
    log "Starting server in screen '$SCREEN_SERVER' -> $START_CMD"
    screen -dmS "$SCREEN_SERVER" bash -lc "exec \"$START_CMD\""
}

wait_for_stop() {
    while pgrep -f "$PZ_PGREP_PATTERN" >/dev/null 2>&1; do sleep 1; done
}

latest_log() {
    ls -1t "$LOG_DIR"/$LOG_GLOB 2>/dev/null | head -n1 || true
}

screen_send() {
    screen -S "$SCREEN_SERVER" -X stuff $"$1\n"
}

get_player_count() {
    local logf
    logf="$(latest_log)"
    [[ -z "${logf:-}" ]] && echo "-1" && return

    screen_send "players"

    local count=""
    count="$(
        timeout 10s stdbuf -oL -eL tail -n0 -F "$logf" \
            | awk -F'[()]' '/Players connected/ {print $2; exit}' \
            || true
    )"

    echo "${count:--1}"
}

ensure_server_running() {
    local has_screen=0 has_proc=0
    screen_exists "$SCREEN_SERVER" && has_screen=1 || true
    pgrep -f "$PZ_PGREP_PATTERN" >/dev/null 2>&1 && has_proc=1 || true

    if [[ $has_screen -eq 0 && $has_proc -eq 0 ]]; then
        start_server_screen
        sleep 2
        return
    fi

    if [[ $has_screen -eq 1 && $has_proc -eq 0 ]]; then
        log "Screen exists but server process is dead -> restarting"
        screen -S "$SCREEN_SERVER" -X quit || true
        start_server_screen
        sleep 2
        return
    fi

    if [[ $has_screen -eq 0 && $has_proc -eq 1 ]]; then
        log "Server process running but screen missing -> restarting into screen"
        pkill -f "$PZ_PGREP_PATTERN" || true
        wait_for_stop
        start_server_screen
        sleep 2
        return
    fi
}

check_mods_need_update() {
    ensure_server_running

    local logf
    logf="$(latest_log)"
    if [[ -z "${logf:-}" ]]; then
        log "No log found at $LOG_DIR/$LOG_GLOB (will retry)"
        return 2
    fi

    log "Checking mods (log: $logf)"
    screen_send "checkModsNeedUpdate"

    local result=""
    result="$(
        timeout "$CHECK_TIMEOUT" stdbuf -oL -eL tail -n0 -F "$logf" \
            | awk '/need update/ {print "NEEDUPDATE"; exit} /updated/ {print "UPTODATE"; exit}' \
            || true
    )"

    [[ "$result" == "NEEDUPDATE" ]] && return 0
    [[ "$result" == "UPTODATE"   ]] && return 1
    log "No clear result within $CHECK_TIMEOUT"
    return 2
}

restart_and_update() {
    log "Mods need update -> restarting (no players connected)"

    log "Stopping server..."
    screen_send "quit"
    wait_for_stop
    screen -S "$SCREEN_SERVER" -X quit || true
    log "Server stopped."

    start_server_screen
    log "Server restarted."
}

exec 9>"$LOCKFILE"
flock -n 9 || { log "Another Watchdog instance is already running."; exit 0; }

log "Watchdog started. watchdog tick=$WATCHDOG_TICK, mod check=$CHECK_EVERY"

parse_interval_to_seconds() {
    local s="$1"
    case "$s" in
        *s) echo "${s%s}";;
        *m) echo $(( ${s%m} * 60 ));;
        *h) echo $(( ${s%h} * 3600 ));;
        *)  echo "$s";;
    esac
}

check_interval_sec="$(parse_interval_to_seconds "$CHECK_EVERY")"
tick_sec="$(parse_interval_to_seconds "$WATCHDOG_TICK")"
next_check_epoch=$(( $(date +%s) + check_interval_sec ))
update_pending=0
update_notified=0

while true; do
    ensure_server_running

    now=$(date +%s)

    if [[ $update_pending -eq 1 ]]; then
        player_count="$(get_player_count)"
        if [[ "$player_count" -eq 0 ]]; then
            restart_and_update
            update_pending=0
            update_notified=0
            next_check_epoch=$(( $(date +%s) + check_interval_sec ))
        else
            log "Waiting for $player_count player(s) to disconnect..."
        fi
    elif (( now >= next_check_epoch )); then
        if check_mods_need_update; then
            player_count="$(get_player_count)"
            if [[ "$player_count" -eq 0 ]]; then
                restart_and_update
            else
                log "Mods need update but $player_count player(s) connected — deferring restart"
                screen_send 'servermsg "Mod updates pending, the server will restart once all players disconnect."'
                update_pending=1
                update_notified=1
            fi
        fi
        next_check_epoch=$(( now + check_interval_sec ))
    fi

    sleep "$tick_sec"
done

The best way to use this is with pm2, otherwise just start it and it will self-contain in a screen. The watchdog will start your server for you and monitor it as described.