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I have the following Gruntfile.js:

module.exports = function(grunt) 
 var config = 
 pkg: grunt.file.readJSON('package.json'),

 /* Some other tasks... */

 uglify: 
 options: 
 banner: '/*! <%= pkg.name %> <%= grunt.template.today("yyyy-mm-dd") %> */n'
 ,
 def: 
 files: 
 'out/src.js': 'out/src.min.js'
 
 
 
 ;

 grunt.initConfig(config);
 grunt.loadNpmTasks('grunt-contrib-uglify');

 grunt.registerTask('default', [/* <other-tasks>, */ 'uglify:def']);
;

Folder structure is as follows:

project
 |
 +-out (folder)
 +-Gruntfile.js

Important: I run grunt from the project folder.

When running grunt, there is a task before uglify:def which is responsible for generating src.js into project/out.

When I run grunt I can see src.js being generated into project/out, but when Grunt runs uglisy:def I get the following error:

Running "uglify:def" (uglify) task.

Destination out/src.js not written because src files were empty.

No files created.

What am i doing wrong?

Log

When running with --verbose I get:

Running "uglify:def" (uglify) task
Verifying property uglify.def exists in config...OK
Files: [no src] -> out/src.js
Options: banner="/*! My Pack 2015-07-19 */rn", footer="", compress="warnings":false, mangle=, beautify=false, report="
min", expression=false, maxLineLen=32000, ASCIIOnly=false, screwIE8=false, quoteStyle=0
>> Destination out/src.js not written because src files were empty.
>> No files created.

I've a configuration like the following, and it works fine for me.

 // uglify javascript
 uglify: 
 dev: 
 options: 
 mangle: true
 ,
 files: 
 'js/dest.min.js': 'js/source.js'
 
 
 ,

Probably you confused the destination with the source. Try to switch them.

It were happening due to, you are not registering above given tasks.

OK, lets start with concatenation in grunt:

concat: 
 css: 
 src: ['./assets/css/*.css', './assets/css/**/*.css'],
 dest: './dist/css/style.css'
 ,
 js: 
 src: ['./assets/js/*.js', './assets/js/**/*.js'],
 dest: './dist/js/script.js'
 
,

so, this concat is supposed to collect all css files from above given url / directories and concatenate to given destination in one place and so with js.

this will be simple concatenated style.css and script.js at dest destination directory.

but it won't work, till you not register this concat task inside below line:

grunt.registerTask('default', ['concat', 'cssmin', 'uglify']);

So, till concat will not concatenate those files in dest directory, how the uglify will collect and work!

Conclusion: task won't get execute till you not mention them inside grunt.registerTask function.