Java-Nashorn based file watcher, execute tasks on file creation/rename

watch.js

#!/usr/bin/jjs -scripting
if ($ARG.length < 2) {
  print("Usage: jjs -scripting watch.js -- <fileToWatch> <command> [args]");
  exit();
}

var FileSystems = Java.type("java.nio.file.FileSystems");
var StandardWatchEventKinds = Java.type("java.nio.file.StandardWatchEventKinds");
var Runtime = Java.type("java.lang.Runtime");

var fileToWatch = FileSystems.default.getPath($ARG[0]);
var file = fileToWatch.fileName;
var directory = fileToWatch.parent;

if (directory === null) {
  print("Usage: jjs -scripting watch.js -- <fileToWatch> <command> [args]");
  exit();
}

var progArgs = $ARG.slice(1);

var watcher = FileSystems.default.newWatchService();

directory.register(watcher, StandardWatchEventKinds.ENTRY_CREATE);

for (;;) {
  var key = watcher.take();

  for each (event in key.pollEvents()) {
    if (event.kind() === StandardWatchEventKinds.ENTRY_CREATE) {
      if (event.context().equals(file)) {
        Runtime.runtime.exec(progArgs);
      }
    }
  }

  var valid = key.reset();
  if (!valid) {
    exit();
  }
}