First commit

.gitignore

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+/mqtt-backlight-control
+/mqtt-backlight-control.exe
+/.vscode

README.md

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+# MQTT backlight control
+
+Simple mqtt command executor to turn screen on or off via MQTT.
+
+Config example:
+
+```yaml
+mqtt:
+  address: "tcp://example.com:1883"
+  username: "mqtt"
+  password: "${MQTT_PASSWORD}"
+  command_topic: "light/screen/command"
+  state_topic: "light/screen/state"
+commands:
+  on:
+    program: "xrandr"
+    args:
+      - "--display"
+      - ":0"
+      - "--output"
+      - "HDMI-0"
+      - "--auto"
+  off:
+    program: "xrandr"
+    args:
+      - "--display"
+      - ":0"
+      - "--output"
+      - "HDMI-0"
+      - "--off"
+```
+
+Password is provided via **MQTT_PASSWORD** environment variable.
+
+Usage:
+
+```
+mqtt-backlight-control config.yaml
+```
+
+Systemd unit example:
+
+```ini
+[Unit]
+Description=Control screen backlight via mqtt
+After=network-online.target
+StartLimitIntervalSec=500
+StartLimitBurst=5
+
+[Service]
+Type=simple
+User=mqtt_backlight_control
+Environment=MQTT_PASSWORD=xxxxxxxxxxxxx
+WorkingDirectory=/opt/backlight-control/
+ExecStart=/opt/backlight-control/mqtt-backlight-control config.yaml
+Restart=on-failure
+RestartSec=5s
+
+[Install]
+WantedBy=multi-user.target
+```

go.mod

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+module mqtt-backlight-control
+
+go 1.19
+
+require (
+	github.com/eclipse/paho.mqtt.golang v1.4.3 // indirect
+	github.com/gorilla/websocket v1.5.0 // indirect
+	golang.org/x/net v0.8.0 // indirect
+	golang.org/x/sync v0.1.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

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+github.com/eclipse/paho.mqtt.golang v1.4.3 h1:2kwcUGn8seMUfWndX0hGbvH8r7crgcJguQNCyp70xik=
+github.com/eclipse/paho.mqtt.golang v1.4.3/go.mod h1:CSYvoAlsMkhYOXh/oKyxa8EcBci6dVkLCbo5tTC1RIE=
+github.com/gorilla/websocket v1.5.0 h1:PPwGk2jz7EePpoHN/+ClbZu8SPxiqlu12wZP/3sWmnc=
+github.com/gorilla/websocket v1.5.0/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
+golang.org/x/net v0.8.0 h1:Zrh2ngAOFYneWTAIAPethzeaQLuHwhuBkuV6ZiRnUaQ=
+golang.org/x/net v0.8.0/go.mod h1:QVkue5JL9kW//ek3r6jTKnTFis1tRmNAW2P1shuFdJc=
+golang.org/x/sync v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
+golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

main.go

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+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"os/signal"
+	"syscall"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+	"gopkg.in/yaml.v3"
+)
+
+var config = Config{}
+
+type MqttConfig struct {
+	Address      string
+	Username     string
+	Password     string
+	CommandTopic string `yaml:"command_topic"`
+	StateTopic   string `yaml:"state_topic"`
+}
+
+type CommandDef struct {
+	Program string
+	Args    []string `yaml:",flow"`
+}
+
+type CommandConfig struct {
+	On  CommandDef
+	Off CommandDef
+}
+
+type Config struct {
+	Mqtt     MqttConfig
+	Commands CommandConfig
+}
+
+var onCommandMessage mqtt.MessageHandler = func(client mqtt.Client, msg mqtt.Message) {
+	value := string(msg.Payload())
+	fmt.Println(value)
+
+	var cmd *exec.Cmd
+
+	if value == "ON" {
+		cmd = exec.Command(config.Commands.On.Program, config.Commands.On.Args...)
+	} else {
+		cmd = exec.Command(config.Commands.Off.Program, config.Commands.Off.Args...)
+	}
+
+	_, err := cmd.Output()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Execution error: %v\n", err.Error())
+		return
+	}
+
+	// fmt.Printf("Command output: %v\n", string(stdout))
+
+	token := client.Publish(config.Mqtt.StateTopic, 0, false, value)
+	token.Wait()
+}
+
+var onConnected mqtt.OnConnectHandler = func(client mqtt.Client) {
+	fmt.Println("Connected")
+
+	token := client.Subscribe(config.Mqtt.CommandTopic, 1, onCommandMessage)
+	token.Wait()
+}
+
+var onDisconnected mqtt.ConnectionLostHandler = func(client mqtt.Client, err error) {
+	fmt.Fprintf(os.Stderr, "Connection lost: %v\n", err)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s config.yaml\n", os.Args[0])
+		os.Exit(2)
+	}
+
+	yamlData, err := os.ReadFile(os.Args[1])
+
+	if err != nil {
+		panic(err.Error())
+	}
+
+	yamlStr := os.ExpandEnv(string(yamlData))
+	err = yaml.Unmarshal([]byte(yamlStr), &config)
+
+	if err != nil {
+		panic(err.Error())
+	}
+
+	opts := mqtt.NewClientOptions()
+	opts.AddBroker(config.Mqtt.Address)
+	opts.SetClientID("screen_backlight_listener")
+	opts.SetUsername(config.Mqtt.Username)
+	opts.SetPassword(config.Mqtt.Password)
+	opts.SetAutoReconnect(true)
+
+	opts.OnConnect = onConnected
+	opts.OnConnectionLost = onDisconnected
+	client := mqtt.NewClient(opts)
+
+	if token := client.Connect(); token.Wait() && token.Error() != nil {
+		panic(token.Error())
+	}
+
+	// wait forever for interrupt
+	exit := make(chan os.Signal, 1)
+	defer close(exit)
+	signal.Notify(exit, syscall.SIGINT, syscall.SIGTERM)
+	<-exit
+
+	client.Disconnect(250)
+}