suspend.nix

{ config, lib, pkgs, ...}:

with lib;

let
  cfg = config.services.batteryNotifier;
in {
  options = {
    services.batteryNotifier = {
      enable = mkOption {
        default = false;
        description = ''
          Whether to enable battery notifier.
        '';
      };
      device = mkOption {
        default = "BAT0";
        description = ''
          Device to monitor.
        '';
      };
      notifyCapacity = mkOption {
        default = 10;
        description = ''
          Battery level at which a notification shall be sent.
        '';
      };
      suspendCapacity = mkOption {
        default = 5;
        description = ''
          Battery level at which a suspend unless connected shall be sent.
        '';
      };
    };
  };

  config = mkIf cfg.enable {
    systemd.user.timers."lowbatt" = {
      description = "check battery level";
      timerConfig.OnBootSec = "1m";
      timerConfig.OnUnitInactiveSec = "1m";
      timerConfig.Unit = "lowbatt.service";
      wantedBy = ["timers.target"];
    };
    systemd.user.services."lowbatt" = {
      description = "battery level notifier";
      serviceConfig.PassEnvironment = "DISPLAY";
      script = ''
        export battery_capacity=$(${pkgs.coreutils}/bin/cat /sys/class/power_supply/${cfg.device}/capacity)
        export battery_status=$(${pkgs.coreutils}/bin/cat /sys/class/power_supply/${cfg.device}/status)

        if [[ $battery_capacity -le ${builtins.toString cfg.notifyCapacity} && $battery_status = "Discharging" ]]; then
            ${pkgs.libnotify}/bin/notify-send --urgency=critical --hint=int:transient:1 --icon=battery_empty "Battery Low" "You should probably plug-in."
        fi

        if [[ $battery_capacity -le ${builtins.toString cfg.suspendCapacity} && $battery_status = "Discharging" ]]; then
            ${pkgs.libnotify}/bin/notify-send --urgency=critical --hint=int:transient:1 --icon=battery_empty "Battery Critically Low" "Computer will suspend in 60 seconds."
            sleep 60s

            battery_status=$(${pkgs.coreutils}/bin/cat /sys/class/power_supply/${cfg.device}/status)
            if [[ $battery_status = "Discharging" ]]; then
                systemctl suspend
            fi
        fi
      '';
    };
  };
}

Managed to narrow it down to this line: ${pkgs.libnotify}/bin/notify-send --urgency=critical --hint=int:transient:1 --icon=battery_empty "Battery Low" "You should probably plug-in."

I added echoes to each line of the script that write to a log. The script is successfully executing every 1-2 minutes, and after the battery level drops below my notify capacity, the echo inside the $battery_capacity -le ${builtins.toString cfg.notifyCapacity} && $battery_status = "Discharging" condition is successfully logged. But I'm not seeing the notification.

Running notify-send directly from my own shell works fine. It makes me wonder if this is user account related? i.e. Because it's a system configuration, the notify-send call in the script isn't pinging the notification server associated with my user's login session?

Do you see it's been executed if you run $ systemctl status --user lowbatt.service as your user?

Systemd should make sure it executes under your user, and the module sets DISPLAY variable to make sure it can find the display. Maybe you have a custom setup? I'd guess Sway is to blame here, probably needs something else than DISPLAY variable.

the module sets DISPLAY variable to make sure it can find the display

Nice, this was part of the issue: I realized that systemctl --user show-environment | grep DISPLAY actually outputs nothing as Sway doesn't pass those vars to systemd (see here).

I initially tried to fix this by passing DISPLAY, WAYLAND_DISPLAY and SWAYSOCK to systemd with no luck. Then, like this person, I just passed my whole env and the notifications now work 🍾.

tl;dr: If you have the issue I'm describing, you can resolve it by adding this to the end of your sway config: exec systemctl --user import-environment.

Thank you very much for your help! 🙏

Thank you very much for your help