configuration.nix

{ config, lib, pkgs, ... }:
let
  rotate = pkgs.writeShellScriptBin "rotate" ''
     ${pkgs.python3}/bin/python3 ${rotate-py} 
  '';

  rotate-py = pkgs.writeText "rotate-py" ''
import subprocess as sp
import os

ORIENTATIONS = {
        "bottom-up": "inverted",
        "normal": "normal",
        "left-up": "left",
        "right-up": "right",
        }

def rotar(output):
  cosa = output.rstrip().decode("utf-8")
  if "orientation changed:" in cosa:
    _, iio_orientation = cosa.split(": ")
    xrandr_orientation = ORIENTATIONS[iio_orientation]
    xrandr_cmd = ["/run/current-system/sw/bin/xrandr", "--output", "eDP-1", "--rotate", xrandr_orientation]
    print(" ".join(xrandr_cmd))
    os.system(" ".join(xrandr_cmd))

def run_long_running_cmd(args, responders=None):
  process = sp.Popen(args, stdout=sp.PIPE, stderr=sp.STDOUT)
  with process.stdout:
    while True:
      output = process.stdout.readline()
      exit_status = process.poll()
      if responders is not None:
        for responder in responders:
          response = responder(output)
          if response is not None:
            process.stdin.write(response)
      if output:
        print(output)
      elif exit_status is not None:
        break

    logger.info('Finished running subprocess.')
    return exit_status

if __name__=="__main__":
    run_long_running_cmd("${pkgs.iio-sensor-proxy}/bin/monitor-sensor", responders=[ rotar,])
'';
in {
  
  systemd.services.voltear = {
   enable = true;
   description = "Darle vuelta a la pantalla";
   serviceConfig = {
     LimitNOFILE=65536;
     PermissionsStartOnly=true;
     Type="simple";
     ExecStart = "${pkgs.python3}/bin/python3 ${rotate-py}";
     ExecStop = "/run/current-system/sw/bin/true";
     User = "ani";
   };
   wantedBy = [ "default.target" ];
  };


}

status.log

[root@fonseca:/etc/nixos]# systemctl status voltear.service 
● voltear.service - Darle vuelta a la pantalla
   Loaded: loaded (/nix/store/83mh5mv6vd9m79jgyhl1zind2v1qwhbs-unit-voltear.service/voltear.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2019-04-05 11:45:54 -05; 2min 8s ago
 Main PID: 28983 (python3)
    Tasks: 4 (limit: 4915)
   Memory: 6.9M
   CGroup: /system.slice/voltear.service
           ├─28983 /nix/store/0n8slcq8p5x31kc9hncabsqq9y3fpkzp-python3-3.7.3/bin/python3 /nix/store/5lgi26vrh98r6vdsl91rj6xzc994a0wa-rotate-py
           └─28984 /nix/store/xxgwk7qjy0x81b2sclics10ykhdxbaf5-iio-sensor-proxy-2.5/bin/monitor-sensor

abr 05 11:45:54 fonseca systemd[1]: Started Darle vuelta a la pantalla.
abr 05 11:45:58 fonseca python3[28983]: Can't open display
abr 05 11:46:00 fonseca python3[28983]: Can't open display