HC-06 Bluetooth chat on an Arduino Uno

// HC-06 Bluetooth chat: the four-pin module on a SoftwareSerial port.
//
//   HC-06 TXD  -> pin 2   (the sketch's RX)
//   HC-06 RXD  <- pin 3   (the sketch's TX)
//   HC-06 VCC  -> 5V, GND -> GND
//
// Simulated Bluetooth: a phone in Mokxi, not a real radio. Press Run, open
// the Bluetooth tab, pair with the HC-06 (PIN 1234) and connect, the way a
// phone's terminal app does. Then send 1 or 0 and the LED on pin 13 follows.
//
// An HC-06 comes out of the bag with nothing connected, and until something
// connects, whatever the sketch sends it is an AT command: the hello below
// is only sent once a phone is there to read it.

#include <SoftwareSerial.h>

SoftwareSerial BTSerial(2, 3); // RX, TX

char line[32];
byte length = 0;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  BTSerial.begin(9600);
  Serial.println("HC-06 chat ready at 9600 baud. Connect from the Bluetooth tab.");
}

// A whole line from the phone. The answer waits until the line has ended:
// SoftwareSerial cannot listen while it talks.
void command() {
  line[length] = '\0';
  if (strcmp(line, "1") == 0) {
    digitalWrite(LED_BUILTIN, HIGH);
    BTSerial.println("LED on");
  } else if (strcmp(line, "0") == 0) {
    digitalWrite(LED_BUILTIN, LOW);
    BTSerial.println("LED off");
  } else {
    BTSerial.println("Send 1 or 0 for the LED.");
  }
  length = 0;
}

void loop() {
  if (BTSerial.available()) {
    char c = BTSerial.read();
    Serial.write(c);
    if (c == '\n') {
      command();
    } else if (c != '\r' && length < sizeof(line) - 1) {
      line[length++] = c;
    }
  }
}