gulpfile.js

var gulp = require('gulp');
var concat = require('gulp-concat');
var stylus = require('gulp-stylus');

gulp.task('styles', function() {
  gulp.src('./bower_components/bootstrap/dist/css/bootstrap.css')
    .pipe(concat('vendor.css'))
    .pipe(gulp.dest('./public'))

  gulp.src('./assets/**/*.styl')
    .pipe(stylus())
    .pipe(concat('app.css'))
    .pipe(gulp.dest('./public'))
})

gulp.task('scripts', function() {
  gulp.src('./assets/**/*.js')
    .pipe(concat('app.js'))
    .pipe(gulp.dest('build'))
})

gulp.task('default', function(){
  gulp.run('scripts', 'styles');

  gulp.watch('./assets/js/**', function(event) {
    gulp.run('scripts');
  })

  gulp.watch('./assets/css/**', function(event) {
    gulp.run('styles');
  })
});