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
      '';
    };
  };
}

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