This is a sample gulp file created for learning

gulpfile.js

var changed = require('gulp-changed');
var imagemin = require('gulp-imagemin');
var minifyHTML = require('gulp-minify-html');
var gulp = require('gulp');
var jshint = require('gulp-jshint');
var concat = require('gulp-concat');
var stripDebug = require('gulp-strip-debug');
var uglify = require('gulp-uglify');
var autoprefix = require('gulp-autoprefixer');
var minifyCSS = require('gulp-minify-css');
 
// JS hint task
gulp.task('jshint', function() {
  gulp.src('./src/scripts/*.js')
    .pipe(jshint())
    .pipe(jshint.reporter('default'));
});

// minify new images
gulp.task('imagemin', function() {
  var imgSrc = './src/images/**/*',
      imgDst = './build/images';
 
  gulp.src(imgSrc)
    .pipe(changed(imgDst))
    .pipe(imagemin())
    .pipe(gulp.dest(imgDst));

    
});

// minify new or changed HTML pages


gulp.task('htmlpage', function() {
  var htmlSrc = './src/*.html',
      htmlDst = './build';
 
  gulp.src(htmlSrc)
    .pipe(changed(htmlDst))
    .pipe(minifyHTML())
    .pipe(gulp.dest(htmlDst));
});

// JS concat, strip debugging and minify
gulp.task('scripts', function() {
  gulp.src(['./src/scripts/lib.js','./src/scripts/*.js'])
    .pipe(concat('script.js'))
    .pipe(stripDebug())
    .pipe(uglify())
    .pipe(gulp.dest('./build/scripts/'));
});

// CSS concat, auto-prefix and minify
gulp.task('styles', function() {
  gulp.src(['./src/styles/*.css'])
    .pipe(concat('styles.css'))
    .pipe(autoprefix('last 2 versions'))
    .pipe(minifyCSS())
    .pipe(gulp.dest('./build/styles/'));
});

// default gulp task
gulp.task('default', ['imagemin', 'htmlpage', 'scripts', 'styles'], function() {
});

 // watch for HTML changes
  gulp.watch('./src/*.html', function() {
    gulp.run('htmlpage');
  });
 
  // watch for JS changes
  gulp.watch('./src/scripts/*.js', function() {
    gulp.run('jshint', 'scripts');
  });
 
  // watch for CSS changes
  gulp.watch('./src/styles/*.css', function() {
    gulp.run('styles');
  });
});