Example... with Jekull

Gruntfile.js

// This is the main application configuration file. It is a Grunt
// configuration file, which you can learn more about here:
// https://github.com/cowboy/grunt/blob/master/docs/configuring.md
/*jslint nomen: true*/
'use strict';
module.exports = function (grunt) {
    'use strict';
    grunt.initConfig({
        // The clean task ensures the entire XXX folder is removed
        clean: {
            development: ['_site'],
            staging: ['_staging'],
            production: ['_production']
        },
 
        // Compress generated css files
        cssmin: {
                'css/screen.css': ['css/screen.css']
        },
 
        // Run Jekyll
        jekyll: {
            server: {
                auto: true,
                server: true,
                server_port: 5000,
                exclude: ['node_modules', 'less'],
                dest: './_site'
            },
            staging: {
                url: 'http://some.url/',
                exclude: ['node_modules', 'less'],
                dest: './_staging'
            },
            production: {
                exclude: ['node_modules', 'less'],
                dest: './_production'
            }
        },
 
        // Automatically run a task when a file changes
        watch: {
            styles: {
                files: [
                    'css/less/*'
                ],
                tasks: 'less'
            }
        },
 
        // Compile specified less files
        less: {
            compile: {
                options: {
                    // These paths are searched when trying to resolve @import in less file
                    paths: [
                        'css/less'
                    ]
                },
                files: {
                    'css/screen.css': 'css/less/screen.less'
                }
            }
        },
 
        // Add shell tasks
        shell: {
            copyCss: {
                command: 'cp css/screen.css _site/css/screen.css'
            }
        }
    });
 
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-contrib-clean');
    grunt.loadNpmTasks('grunt-contrib-cssmin');
    grunt.loadNpmTasks('grunt-contrib-less');
    grunt.loadNpmTasks('grunt-jekyll');
    grunt.loadNpmTasks('grunt-shell');
 
    // The default task will show the usage
    grunt.registerTask('default', 'Prints usage', function () {
        grunt.log.writeln('');
        grunt.log.writeln('Product site development');
        grunt.log.writeln('------------------------');
        grunt.log.writeln('');
        grunt.log.writeln('* run "grunt --help" to get an overview of all commands.');
        grunt.log.writeln('* run "grunt dev"" to start developing.');
    });
 
    // The dev task will be used during development
    grunt.registerTask('dev', ['clean:development', 'less:compile', 'jekyll:server', 'watch:jekyll', 'watch:styles']);
 
    // The stag task will be used before "deploying" to staging (subfolder of a domain)
    grunt.registerTask('stag', ['clean:staging', 'less:compile', 'jekyll:staging']);
 
    // The prod task will be used before "deploying" to product (root of a domain)
    grunt.registerTask('prod', ['clean:production', 'less:compile', 'jekyll:production']);
};