Grunt Watch repeatedly showing "Warning: must provide pattern"

Question

I'm having trouble configuring Grunt to watch my project files, rebuild and update a page hosted in a connect server. If I run any of the build tasks and then 'watch' as part of a combined task, then 'watch' seems to get stuck in a loop, endlessly printing the message.

Running "watch" task
Waiting...
Warning: must provide pattern

If instead I just run $ grunt watch, it will happily watch my source files and compile/build as appropriate.

I think the relevant task configurations are these:

watch: {
  html: {
    files: [ '<%= site.partials %>', '<%= site.layouts %>', '<%= site.pages %>' ],
    tasks: [ 'html' ]
  },
  sass: {
    files: [ '<%= site.src %>sass/*.scss' ],
    tasks: [ 'styles' ]
  }
},

// development server
connect: {
  options: {
    port: 8080,
    livereload: 35729,
    hostname: 'localhost',
  },
  dev: {
    options: {
      directory: 'build',
    }
  }
},

and the task definitions:

grunt.registerTask( 'build', [ 'styles', 'html', ] );
grunt.registerTask( 'default', [  'build','connect:dev', 'watch' ] );

The 'styles' and 'html' tasks run grunt-sass and assemble. As stated above, running any of these tasks, or even 'watch' on its own yields the expected results. This suggests my config object has site.partials, site.dest etc defined correctly. The problem only happens when I run any task and then 'watch', as in the default task.

Answer 1

I just encountered a similar problem when I had been editing my Gruntfile and left a field (that should have had a file pattern) blank.

Check your Gruntfile for an empty file field.

In my specific example:

wiredep: {
    options: {
        overrides: {
          "jquery-ui": {
              "main": [
                 "jquery-ui.js",
                 "themes/base/jquery-ui.css",
                 ""
              ]
          }
        }
    }
}

Note the empty string above. That generated an error very similar to yours. It seems that Grunt doesn't tell you where the error is, unfortunately. You'll just need to scan through your Gruntfile manually to find the error.