hexo gruntfile generate example

Gruntfile.js

module.exports = function(grunt) {
  grunt.loadNpmTasks('grunt-shell');
  grunt.loadNpmTasks('grunt-contrib-watch');

  grunt.initConfig({
    watch: {
      sourceFiles: {
        files: ['themes/**','scaffolds/**','scripts/**','source/**','app.js'],
        tasks: 'shell:hexoGen'
      },
    },
    shell: {
      hexoGen: {
          command: 'hexo generate'
      }
    },
  });
  
  // Default task.
  grunt.registerTask('default', 'watch');

};

If you want it to clean first you can add another shell task

clean: {
    command: 'rm -rf public'
}

and change your watch tasks to

tasks: ['shell:clean','shell:generate']