gulpfile.js

var gulp = require('gulp'),
    watch = require('gulp-watch'),
    sass = require('gulp-sass'),
    neat = require('node-neat').includePaths,
    cp = require('child_process'),
    rename = require('gulp-rename'),
    cssmin = require('gulp-cssmin'),
    concat = require('gulp-concat'),
    browserSync = require('browser-sync'),
    autoprefixer = require('gulp-autoprefixer'),
    uglify = require('gulp-uglify'),
    del = require('del'),
    notify = require('gulp-notify'),
    plumber = require('gulp-plumber'),
    critical = require('critical'),
    imagemin = require('gulp-imagemin'),
    pngquant = require('imagemin-pngquant'),
    reload = browserSync.reload();

var paths = {
  scss: '_src/scss/**/*',
  html: '**/*.html',
  js: '_src/js/**/*',
  images: '_src/img/**/*',
  posts:  '_posts/**/*.html',
  dest: 'dist/'
};

gulp.task('sass', function () {
    gulp.src('_src/scss/*.scss')
      .pipe(plumber())
      .pipe(sass({
        includePaths: ['sass'].concat(neat) // required for node-neat to work
      }))
      .on('error', function (err) { // custom error logging since Gulp crashes without it
        console.log(err);
	    })
      .pipe(autoprefixer('last 2 version', 'ie 9')) // prefix up to ie9
      .pipe(cssmin()) // minify the css while we're at it
      .pipe(gulp.dest('./dist/css')) // specify the destination for the minified css created by sass
      .pipe(gulp.dest('./_site/dist/css')) // also output to the _site directory for live injecting
      .pipe(browserSync.reload({stream:true}))
      .pipe(notify({ message: 'Compiled Sass!' }));
});

gulp.task('js', function() {
  gulp.src('_src/js/**/*.js')
    .pipe(concat('scripts.js'))
    .pipe(rename({suffix: '.min'}))
    .pipe(uglify())
    .pipe(gulp.dest('./dist/js'))
    .pipe(notify({ message: 'Minified JS.'}));
});

gulp.task('clean', function() {
  return del([
    'dist/css','dist/js', 'dist/img'
  ]);
});

gulp.task('critical-css', function() {
  critical.generate({
    base: '_site',
    src: 'index.html',
    dest: './dist/css/critical.min.css',
    width: 1200,
    height: 900,
    minify: true
  });
});

gulp.task('images', function() {
  return gulp.src(paths.images)
    .pipe(imagemin({
      progressive: true,
      svgoPlugins: [
        { removeViewBox: false },
        { cleanupIDs: false }
      ],
      use: [pngquant()]
    }))
    .pipe(gulp.dest('_site/dist/img'))
    .pipe(gulp.dest('dist/img'));
});

gulp.task('jekyll-build', function(done) {
  browserSync.notify('<span style="color: grey">Running:</span> $ jekyll build');
  return cp.spawn('jekyll', ['build'], { stdio: 'inherit' })
  .on('close', done);
});

gulp.task('jekyll-rebuild', ['jekyll-build'], function() {
  browserSync.reload();
});

gulp.task('browsersync', ['jekyll-build'], function() {
  browserSync({
    server: {
          baseDir: './_site/'
      },
      host: 'localhost'
  });
});

gulp.task('watch', function() {
  gulp.watch(paths.scss, ['sass']);
  gulp.watch(paths.js, ['js']);
  gulp.watch(paths.html, ['jekyll-rebuild']);
  gulp.watch(paths.posts, ['jekyll-rebuild']);
});

// The default task (called when you run `gulp` from cli)
gulp.task('default', ['clean'], function() {
  gulp.start('sass', 'js', 'images', 'browsersync', 'watch');
});