gruntfile.js

module.exports = function(grunt) {
  grunt.initConfig({
    less: {
      development: {
        options: {
          compress: true,
          yuicompress: true,
          optimization: 2
        },
        files: {
          // target.css file: source.less file
          "src/index.css": "src/style/index.less"
        }
      }
    },
    watch: {
      styles: {
        // Which files to watch (all .less files recursively in the less directory)
        files: ['src/style/**/*.less'],
        tasks: ['less']
      }
    }
  });

  grunt.loadNpmTasks('grunt-contrib-less');
  grunt.loadNpmTasks('grunt-contrib-watch');

  grunt.registerTask('default', ['watch']);
};