gulpfile.js

var gulp        = require('gulp');
var browserSync = require('browser-sync');
var sass        = require('gulp-sass');
var prefix      = require('gulp-autoprefixer');
var cp          = require('child_process');
var sourcemaps  = require('gulp-sourcemaps');
var include     = require('gulp-include')

// @see https://gist.github.com/LoyEgor/e9dba0725b3ddbb8d1a68c91ca5452b5
var imagemin         = require('gulp-imagemin');
var imageminPngquant = require('imagemin-pngquant');
var imageminZopfli   = require('imagemin-zopfli');
var imageminMozjpeg  = require('imagemin-mozjpeg'); //need to run 'brew install libpng'
var imageminGiflossy = require('imagemin-giflossy');

var jekyll   = process.platform === 'win32' ? 'jekyll.bat' : 'jekyll';
var messages = {
    jekyllBuild: '<span style="color: grey">Running:</span> $ jekyll build'
};

/**
 * Build the Jekyll Site
 */
gulp.task('jekyll-build', function (done) {
    browserSync.notify(messages.jekyllBuild);
    return cp.spawn( jekyll , ['build', '--baseurl', ''], {stdio: 'inherit'})
        .on('close', done);
});

/**
 * Rebuild Jekyll & do page reload
 */
gulp.task('jekyll-rebuild', ['jekyll-build'], function () {
    browserSync.reload();
});

/**
 * Wait for jekyll-build, then launch the Server
 */
gulp.task('browser-sync', ['sass', 'jekyll-build'], function() {
    browserSync({
        server: {
            baseDir: '_site',
            injectChanges: true
        }
    });
});

/**
 * Minify image assets
 */
gulp.task('imagemin', function(){
  return gulp.src(['images/**/*.{gif,png,jpg,JPG}'])
        .pipe(imagemin([
            //png
            imageminPngquant({
                speed: 1,
                quality: 98 //lossy settings
            }),
            imageminZopfli({
                more: true
            }),
            //gif
            // imagemin.gifsicle({
            //     interlaced: true,
            //     optimizationLevel: 3
            // }),
            //gif very light lossy, use only one of gifsicle or Giflossy
            imageminGiflossy({
                optimizationLevel: 3,
                optimize: 3, //keep-empty: Preserve empty transparent frames
                lossy: 2
            }),
            //svg
            imagemin.svgo({
                plugins: [{
                    removeViewBox: false
                }]
            }),
            //jpg lossless
            imagemin.jpegtran({
                progressive: true
            }),
            //jpg very light lossy, use vs jpegtran
            imageminMozjpeg({
                quality: 90
            })
        ]))
        .pipe(gulp.dest('images'));
});

/**
 * Compile files from _scss into both _site/css (for live injecting) and site (for future jekyll builds)
 */
gulp.task('sass', function () {
    return gulp.src('_sass/main.scss')
        .pipe(sass({
            // includePaths: ['scss'],
            onError: browserSync.notify
        }))
        .pipe(gulp.dest('_site/css'))
        .pipe(gulp.dest('css'))
        .pipe(prefix(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true }))
        .pipe(browserSync.reload({stream:true}));
        // .pipe(gulp.dest('css'));
});

/**
 * Watch scss files for changes & recompile
 * Watch html/md files, run jekyll & reload BrowserSync
 */
gulp.task('watch', function () {
    gulp.watch('_sass/*.scss', ['sass']);
    gulp.watch(['*.html', '**/*.md', 'tests/*.js', '_includes/**/*.html', '_layouts/*.html', 'js/*.js', '_posts/*', '_institutes/*'], ['jekyll-rebuild']);
    // gulp.watch('assets/**/*.{gif,png,jpg}', ['imagemin']);
});

/**
 * Default task, running just `gulp` will compile the sass,
 * compile the jekyll site, launch BrowserSync & watch files.
 */
gulp.task('default', ['browser-sync', 'watch', 'sass']);