我正在使用gulp和jekyll构建一个站点，我正在尝试设置gulpfile，以便它自动浏览所有的html文件，并添加srcset信息以进行响应图像交付。
基本上，我想把这个：

<img src="/img/agency/agency-lobby.jpg" />

导入此文件（对于HTML文件中的每个图像）：

<img src="/img/agency/agency-lobby.jpg" srcset="/img/agency/agency-lobby400.jpg 400w, /img/agency/agency-lobby600.jpg 600w, /img/agency/agency-lobby900.jpg 900w, /img/agency/agency-lobby900.jpg 1200w, /img/agency/agency-lobby1800.jpg 1800w"/>

我的gulpfile显然被配置为执行许多自动化任务，其中之一是查看unsized-img文件夹，并为我设置的每个断点创建图像大小，然后将其输出到img文件夹 （我这样做是为了避免Jekyll只使用原始图像覆盖我的*_sites/img文件夹）*。然后，我有一个优化任务，将优化所有这些图像的最终输出在_sites/img**文件夹。
现在我想让gulp检查并找到所有的img标签，并自动化它，这样它就可以为每个图像添加srcset属性，这样我就不必每次都用5个srcset属性手动写入每个图像。

var gulp        = require('gulp');
var browserSync = require('browser-sync');
var sass        = require('gulp-sass');
var prefix      = require('gulp-autoprefixer');
var cp          = require('child_process');
var cleanCSS    = require('gulp-clean-css');
var uglify      = require('gulp-uglify');
var pump        = require('pump');
var imagemin    = require('gulp-imagemin');
var imageresize = require('gulp-image-resize');
var rename      = require("gulp-rename");

var jekyll   = process.platform === 'win32' ? 'jekyll.bat' : 'jekyll';
var messages = {
    jekyllBuild: '<span style="color: grey">Running:</span> $ jekyll build'
};

/**
 * Build the Jekyll Site
 */
gulp.task('jekyll-build', function (done) {
    browserSync.notify(messages.jekyllBuild);
    return cp.spawn( jekyll , ['build'], {stdio: 'inherit'})
        .on('close', done);
});

/**
 * Rebuild Jekyll & do page reload
 */
gulp.task('jekyll-rebuild', ['jekyll-build'], function () {
    browserSync.reload();
});

/**
 * Wait for jekyll-build, then launch the Server
 */
gulp.task('browser-sync', ['sass', 'jekyll-build'], function() {
    browserSync({
        server: {
            baseDir: '_site'
        }
    });
});

/**
 * Compile files from _scss into both _site/css (for live injecting) and site (for future jekyll builds)
 */
gulp.task('sass', function () {
    return gulp.src('_scss/main.scss')
        .pipe(sass({
            includePaths: ['scss'],
            onError: browserSync.notify
        }))
        .pipe(prefix(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true }))
        .pipe(cleanCSS({compatibility: 'ie8'}))
        .pipe(gulp.dest('_site/css'))
        .pipe(browserSync.reload({stream:true}))
        .pipe(gulp.dest('css'));
});

/**
 * Minify javascript files (must be run separatly after gulp)
 */
gulp.task('compress', function(cb){
  pump([
        gulp.src('js/*.js'),
        uglify(),
        gulp.dest('_site/js')
    ],
    cb
  );
});

/**
 * Compress image files
 */

   var resizeImageTasks = [];
   [400,600,900,1200,1800].forEach(function(size) {
     var resizeImageTask = 'resize_' + size;
     gulp.task(resizeImageTask, function() {
         gulp.src('unsized-img/**/*.{jpg,png,gif}')
             .pipe(imageresize({
               width: size,
               upscale: false,
             }))
             .pipe(rename(function (path) { path.basename += size; }))
             .pipe(gulp.dest('img'));
           gulp.src('img/**/*.{svg,ico}')
            .pipe(gulp.dest('img'));
     });
     resizeImageTasks.push(resizeImageTask);
   });

gulp.task('resize', resizeImageTasks);

gulp.task('optimize', function() {
    gulp.src('img/**/*.{jpg,png,gif}')
        .pipe(imageresize({
          quality: 0.8,
          samplingFactor: [2, 2],
          noProfile: true,
          interlace: true,
        }))
        .pipe(imagemin())
        .pipe(gulp.dest('_site/img'));
      gulp.src('img/**/*.{svg,ico}')
       .pipe(gulp.dest('_site/img'));
});
/**
 * Watch scss files for changes & recompile
 * Watch html/md files, run jekyll & reload BrowserSync
 */
gulp.task('watch', function () {
    gulp.watch('_scss/*.scss', ['sass']);
    gulp.watch(['*.html', '_layouts/*.html', '_posts/*', 'work/*.html', 'services/*.html', 'culture/*.html', 'blog/*.html', 'contact/*.html', 'agency/*.html', 'blog/**/*.md','work/**/*.md'], ['jekyll-rebuild']);
});

/**
 * Default task, running just `gulp` will compile the sass,
 * compile the jekyll site, launch BrowserSync & watch files.
 */
gulp.task('default', ['browser-sync', 'watch']);