A simple sample gruntfile. If you are trying this, make sure to run `npm install`.

app.js

var args = process.argv.slice(2);

switch (args[0]) {
    // cmd: node app a
    case 'a':
        console.log('Hello!');
        break;

    // -cmd: node app b
    case 'b':
        console.log('World!');
        break;

    // -cmd: node app
    default:
        console.log('No parames matched :(');
}

Gruntfile.js

function commentCommand(cmd) {
    cmd = cmd || 'cmd';
    var result = require('execSync')
                    .exec('grep -nr "// '+cmd+':" ./')
                    .stdout
                    .split('\n')[0];
    if (result !== '') {
        var reg = new RegExp('// '+cmd+':(.+)', 'i');
        result = result.match(reg)[1];
    }
    return result;
}

module.exports = function (grunt) {

    // config
    grunt.initConfig({

        exec: {
            run: {
                cmd: commentCommand
            }
        },

        watch: {
            scripts: {
                files: 'app.js',
                tasks: ['jshint', 'exec']
            }
        },

        jshint: {
            files: ['**/*.js', '!**/node_modules/**']
        }
    });

    // load
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-contrib-jshint');
    grunt.loadNpmTasks('grunt-exec');

    // register
    grunt.registerTask('default', ['watch']);
};

package.json

{
  "name": "grunt-demo",
  "version": "0.1.0",
  "devDependencies": {
    "execSync": "~0.0.4",
    "grunt": "~0.4.1",
    "grunt-contrib-jshint": "~0.4.3",
    "grunt-contrib-watch": "~0.4.0",
    "grunt-exec": "~0.4.0"
  }
}