Have nonull: true cause the task to fail.

Gruntfile.js

@@ -48,11 +48,6 @@ module.exports = function(grunt) {
           'tmp/custom_options': ['test/fixtures/file1', 'test/fixtures/file2']
         }
       },
-      handling_invalid_files: {
-        src: ['test/fixtures/file1', 'invalid_file/should_warn/but_not_fail', 'test/fixtures/file2'],
-        dest: 'tmp/handling_invalid_files',
-        nonull: true
-      },
       process_function: {
         options: {
           process: function(src, filepath) {
@@ -160,6 +155,31 @@ module.exports = function(grunt) {
 
   });
 
+  // Encapsulates tasks which invoke `grunt.fail.warn` and should abort.
+  grunt.registerTask('concat-warn', function() {
+    grunt.config('concat', {
+      handling_invalid_files: {
+        src: ['test/fixtures/file1', 'invalid_file/should_warn/and_abort', 'test/fixtures/file2'],
+        dest: 'tmp/handling_invalid_files',
+        nonull: true
+      }
+    });
+    grunt.task.run(['concat']);
+  });
+
+  // Encapsulates tasks which invoke `grunt.fail.warn` and should abort, but the
+  // test is checking behavior in the case of a user-specified `--force` flag.
+  grunt.registerTask('concat-force', function() {
+    grunt.config('concat', {
+      handling_invalid_files_force: {
+        src: ['test/fixtures/file1', 'invalid_file/should_warn/but_not_fail', 'test/fixtures/file2'],
+        dest: 'tmp/handling_invalid_files_force',
+        nonull: true
+      }
+    });
+    grunt.task.run(['concat']);
+  });
+
   // Actually load this plugin's task(s).
   grunt.loadTasks('tasks');
 

README.md

@@ -235,7 +235,7 @@ grunt.initConfig({
 ```
 
 #### Invalid or Missing Files Warning
-If you would like the `concat` task to warn if a given file is missing or invalid be sure to set `nonull` to `true`:
+If you would like the `concat` task to warn and abort if a given file is missing or invalid be sure to set `nonull` to `true`:
 
 ```js
 grunt.initConfig({
@@ -249,6 +249,8 @@ grunt.initConfig({
 });
 ```
 
+Additionally invoke grunt with `--force` to skip over missing files.
+
 See [configuring files for a task](http://gruntjs.com/configuring-tasks#files) for how to configure file globbing in Grunt.
 
 
@@ -298,4 +300,4 @@ grunt.initConfig({
 
 Task submitted by ["Cowboy" Ben Alman](http://benalman.com/)
 
-*This file was generated on Wed Apr 20 2016 08:41:44.*
+*This file was generated on Sun Jun 05 2016 08:36:20.*

docs/concat-examples.md

@@ -136,7 +136,7 @@ grunt.initConfig({
 ```
 
 ## Invalid or Missing Files Warning
-If you would like the `concat` task to warn if a given file is missing or invalid be sure to set `nonull` to `true`:
+If you would like the `concat` task to warn and abort if a given file is missing or invalid be sure to set `nonull` to `true`:
 
 ```js
 grunt.initConfig({
@@ -150,6 +150,8 @@ grunt.initConfig({
 });
 ```
 
+Additionally invoke grunt with `--force` to skip over missing files.
+
 See [configuring files for a task](http://gruntjs.com/configuring-tasks#files) for how to configure file globbing in Grunt.
 
 

tasks/concat.js

@@ -71,9 +71,10 @@ module.exports = function(grunt) {
 
       // Concat banner + specified files + footer.
       var src = banner + f.src.filter(function(filepath) {
-        // Warn on and remove invalid source files (if nonull was set).
+        // Warn on invalid source files (if nonull was set). They will be
+        // removed if --force is specified.
         if (!grunt.file.exists(filepath)) {
-          grunt.log.warn('Source file "' + filepath + '" not found.');
+          grunt.fail.warn('Source file "' + filepath + '" not found.');
           return false;
         }
         return true;

test/concat_test.js

@@ -2,6 +2,11 @@
 
 var grunt = require('grunt');
 var comment = require('../tasks/lib/comment').init(grunt);
+var exec = require('child_process').exec;
+var path = require('path');
+var fs = require('fs');
+
+var execOptions = {cwd: path.join(__dirname, '..')};
 
 function getNormalizedFile(filepath) {
   return grunt.util.normalizelf(grunt.file.read(filepath));
@@ -27,13 +32,30 @@ exports.concat = {
     test.done();
   },
   handling_invalid_files: function(test) {
-    test.expect(1);
+    test.expect(3);
 
-    var actual = getNormalizedFile('tmp/handling_invalid_files');
-    var expected = getNormalizedFile('test/expected/handling_invalid_files');
-    test.equal(actual, expected, 'will have warned, but should not fail.');
+    exec('grunt concat-warn', execOptions, function(error, stdout) {
+      test.ok(stdout.indexOf('Warning:') > -1, 'should print a warning.');
+      test.ok(stdout.indexOf('Aborted due to warnings.') > -1, 'should abort.');
 
-    test.done();
+      fs.exists('tmp/handling_invalid_files', function(exists) {
+        test.ok(!exists, 'should not have created a file.');
+        test.done();
+      });
+    });
+  },
+  handling_invalid_files_force: function(test) {
+    test.expect(2);
+
+    exec('grunt concat-force --force', execOptions, function(error, stdout) {
+      test.ok(stdout.indexOf('Warning:') > -1, 'should print a warning.');
+
+      var actual = getNormalizedFile('tmp/handling_invalid_files_force');
+      var expected = getNormalizedFile('test/expected/handling_invalid_files_force');
+      test.equal(actual, expected, 'should not fail.');
+
+      test.done();
+    });
   },
   strip_banner: function(test) {
     test.expect(10);