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Note: This question is only relevant for Grunt 0.3.x and has been left for reference. For help with the latest Grunt 1.x release please see my comment below this question.

I'm currently trying to use Grunt.js to setup an automatic build process for first concatenating and then minifying CSS and JavaScript files.

I have been able to successfully concatenate and minify my JavaScript files, although each time I run grunt it seems to just append to the file instead of overwriting them.

As for the minifying or even concatenating CSS, I have been unable to do this as of yet!

In terms of grunt CSS modules I have tried using consolidate-cssgrunt-css & cssmin but to no avail. Could not get my head around how to use them!

My directory structure is as follows (being a typical node.js application):

  • app.js
  • grunt.js
  • /public/index.html
  • /public/css/[various css files]
  • /public/js/[various javascript files]

Here is what my grunt.js file currently looks like in the root folder of my application:

module.exports = function(grunt) {

  // Project configuration.
    pkg: '<json:package.json>',
    concat: {
      dist: {
        src: 'public/js/*.js',
        dest: 'public/js/concat.js'
    min: {
      dist: {
        src: 'public/js/concat.js',
        dest: 'public/js/concat.min.js'
    jshint: {
      options: {
        curly: true,
        eqeqeq: true,
        immed: true,
        latedef: true,
        newcap: true,
        noarg: true,
        sub: true,
        undef: true,
        boss: true,
        eqnull: true,
        node: true
      globals: {
        exports: true,
        module: false
    uglify: {}

  // Default task.
  grunt.registerTask('default', 'concat min');


So just to summarise I need help with two questions:

  1. How to concatenate and minify all my css files under the folder /public/css/ into one file, say main.min.css
  2. Why does grunt.js keep on appending to the concatenated and minified javascript files concat.js and concat.min.js under /public/js/ instead of overwriting them each time the command grunt is run?

Updated 5th of July 2016 - Upgrading from Grunt 0.3.x to Grunt 0.4.x or 1.x

Grunt.js has moved to a new structure in Grunt 0.4.x (the file is now called Gruntfile.js). Please see my open source project Grunt.js Skeleton for help with setting up a build process for Grunt 1.x.

Moving from Grunt 0.4.x to Grunt 1.x should not introduce many major changes.

concat.js is being included in the concat task's source files public/js/*.js. You could have a task that removes concat.js (if the file exists) before concatenating again, pass an array to explicitly define which files you want to concatenate and their order, or change the structure of your project.

If doing the latter, you could put all your sources under ./src and your built files under ./dest

├── css
│   ├── 1.css
│   ├── 2.css
│   └── 3.css
└── js
    ├── 1.js
    ├── 2.js
    └── 3.js

Then set up your concat task

concat: {
  js: {
    src: 'src/js/*.js',
    dest: 'dest/js/concat.js'
  css: {
    src: 'src/css/*.css',
    dest: 'dest/css/concat.css'

Your min task

min: {
  js: {
    src: 'dest/js/concat.js',
    dest: 'dest/js/concat.min.js'

The build-in min task uses UglifyJS, so you need a replacement. I found grunt-css to be pretty good. After installing it, load it into your grunt file


And then set it up

cssmin: {
    src: 'dest/css/concat.css',
    dest: 'dest/css/concat.min.css'

Notice that the usage is similar to the built-in min.

Change your default task to

grunt.registerTask('default', 'concat min cssmin');

Now, running grunt will produce the results you want.

├── css
│   ├── concat.css
│   └── concat.min.css
└── js
    ├── concat.js
    └── concat.min.js
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