Gruntfile.js

module.exports = function(grunt) {
  grunt.loadNpmTasks('grunt-contrib-watch');
  grunt.registerTask('check', function() {
    grunt.log.writeln('Checking!');
  });
  grunt.registerTask('buildFoo', function() {
    grunt.log.writeln('Building Foo!');
  });
  grunt.registerTask('buildBar', function() {
    grunt.log.writeln('Building Bar!');
  });
  grunt.initConfig({
    watch: {
      check: {
        files: ['*'],
        tasks: ['check']
      },
      foo: {
        files: ['foo'],
        tasks: ['buildFoo']
      },
      bar: {
        files: ['bar'],
        tasks: ['buildBar']
      }
    }
  });
};

package.json

{
  "name": "Grunttest",
  "version": "0.0.1",
  "dependencies": {
    "grunt": "0.4.1",
    "grunt-contrib-watch": "0.4.0"
  }
}