With JavaScript (nodejs)

  var lightsOn = true;

  setInterval(blink, 2000);

  function blink() {
    var fs = require("fs");
	
    fs.writeFile("/sys/class/leds/beaglebone:green:usr0/brightness", lightsOn, function (err) {
      if (err) throw err;
    });
	
    lightsOn = lightsOn ? false : true;

  }