gulpfile.js

var gulp = require('gulp');
var $    = require('gulp-load-plugins')();

var sassPaths = [];

gulp.task('sass', function() {
  return gulp.src('scss/app.scss')
    .pipe($.sourcemaps.init())
    .pipe($.sass({
      includePaths: sassPaths,
      outputStyle: 'expanded' // compressed or use expanded for dev
    })
      .on('error', $.sass.logError))
    .pipe($.autoprefixer({
      browsers: ['last 2 versions', 'ie >= 9']
    }))
    .pipe($.sourcemaps.write('.'))
    .pipe(gulp.dest('css'));
});

gulp.task('minify-css', function() {
  return gulp.src('css/app.css')
    .pipe($.cleanCss())
    .pipe($.rename({suffix: '.min'}))
    .pipe(gulp.dest('css'));
});

gulp.task('default', ['sass'], function() {
  gulp.watch(['scss/**/*.scss'], ['sass']);
});

gulp.task('build', ['sass', 'minify-css'], function() {
  console.log('Building minified css files');
});

package.json

{
  "name": "",
  "version": "1.0.0",
  "description": "",
  "main": "gulpfile.js",
  "scripts": {
    "start": "gulp",
    "build": "gulp build"
  },
  "repository": {
    "type": "git",
    "url": ""
  },
  "author": "",
  "license": "",
  "bugs": {
    "url": ""
  },
  "homepage": "",
  "devDependencies": {
    "gulp": "^3.9.1",
    "gulp-autoprefixer": "^4.1.0",
    "gulp-load-plugins": "^1.5.0",
    "gulp-sass": "^3.1.0",
    "normalize.css": "^7.0.0",
    "gulp-clean-css": "^3.9.2",
    "gulp-rename": "^1.2.2",
    "gulp-sourcemaps": "^2.6.3"
  }
}