An example file for creating multi-site build scripts using Gulp

gulpfile.js

// Require Packages
var gulp = require('gulp');

// Paths variable
var paths = {
  themes: 'wp-content/themes/',
  shared: 'shared/',
  src: 'src/',
  dest: 'dist/'
};

// Themes variable
var themes = [{
  id : 'astra'
}, {
  id : 'bistro'
}, {
  id : 'cell'
}, {
  id : 'nova'
}, {
  id : 'quest'
}];

// Tasks object
var tasks = {
  'css' : [],
  'js' : [],
  'img' : []
};

// Loop through theme objects
themes.forEach(function (theme, index) {

  // Construct our unique source and destination paths
  var src  = paths.themes + theme.id + '/' + paths.src;
  var dest = paths.themes + theme.id + '/' + paths.dest;

  // Save our tasks for later
  tasks.css.push(theme.id + ':css');
  tasks.js.push(theme.id + ':js');
  tasks.img.push(theme.id + ':img');
  
  // CSS task
  gulp.task(theme.id + ':css', function() {
    // Our task code goes here...
  });

  // JavaScript task
  gulp.task(theme.id + ':js', function() {
    // Our task code goes here...
  });

  // Image processing task
  gulp.task(theme.id + ':img', function() {
    // Our task code goes here...
  });

  // Watch task
  gulp.task(theme.id + ':watch', function() {
    gulp.watch(src + 'scss/**/*', [theme.id + ':css']);
    gulp.watch(src + 'js/**/*', [theme.id + ':js']);
    gulp.watch(src + 'img/**/*', [theme.id + ':img']);
  });

  // Creates the :go tasks to run everything
  gulp.task(theme.id + ':go', [
    theme.id + ':css',
    theme.id + ':js',
    theme.id + ':img'
  ]);

  // Creates default task
  gulp.task(theme.id, [
    theme.id + ':go',
    theme.id + ':watch'
  ]);

});

// Global CSS task
gulp.task('css', tasks.css);

// Global JavaScript task
gulp.task('js', tasks.js);

// Global image processing task
gulp.task('img', tasks.img);

// Global watch task
gulp.task('watch', function() {

  // Loop through theme objects
  themes.forEach(function (theme, index) {

    // Construct our unique source path
    var src  = paths.themes + theme.id + '/' + paths.src;

    // Theme specific watch tasks
    gulp.watch(src + 'css/**/*', [theme.id + ':css']);
    gulp.watch(src + 'js/**/*', [theme.id + ':js']);
    gulp.watch(src + 'img/**/*', [theme.id + ':img']);

  });

  // Shared watch tasks
  gulp.watch(paths.shared + 'scss/**/*', ['css']);
  gulp.watch(paths.shared + 'js/**/*', ['js']);
  gulp.watch(paths.shared + 'img/**/*', ['img']);

});