index.js

var gulp = require('gulp');
var fs = require('fs');
var argv = require('yargs').argv;
var replace = require('gulp-replace');

function BootswatchExtension(config) {
    gulp.task('set-theme', function () {
        var theme = argv.name;
        if (!theme) {
          console.log('ERROR: set-theme needs name argument like --name=paper');
        } else {
          var p = 'node_modules/bootswatch/';
          try {
            if (fs.statSync(p + theme).isDirectory()) {
              if (fs.statSync(p + theme + '/_bootswatch.scss')){
                return gulp.src(config.assetsPath.styles + 'bootstrap.scss')
                    .pipe(replace(/bootswatch\/[^\/]+\//g, 'bootswatch/'+ theme + '/'))
                    .pipe(gulp.dest(config.assetsPath.styles));
              }
            }
          } catch (err) {
            console.log('ERROR: theme "' + theme + '" is not found');
          }
        }
        process.exit(1);
    });
}

module.exports = BootswatchExtension;