gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var sourcemaps = require('gulp-sourcemaps');

var input = './sass/**/*.scss';
var output = './sass/css';

var sassOptions = {
    errLogToConsole: true,
    outputStyle: 'compressed'
};

gulp.task('sass', function () {
    return gulp
        .src(input)
        .pipe(sourcemaps.init())
        .pipe(sass(sassOptions).on('error', sass.logError))
        .pipe(sourcemaps.write('./'))
        .pipe(gulp.dest(output));
});

gulp.task('watch', function() {
    return gulp
    // Watch the input folder for change,
    // and run `sass` task when something happens
        .watch(input, ['sass'])
        // When there is a change,
        // log a message in the console
        .on('change', function(event) {
            console.log('File ' + event.path + ' was ' + event.type + ', running tasks...');
        });
});

gulp.task('default', ['sass', 'watch' /*, possible other tasks... */]);