You can filter out unchanged files between runs of a task using

the `gulp.src` function's `since` option and `gulp.lastRun`:

gulp.task('images', function() {

return gulp.src(paths.images, {since: gulp.lastRun('images')})

.pipe(imagemin({optimizationLevel: 5}))

.pipe(gulp.dest('build/img'));

});

gulp.task('watch', function() {

gulp.watch(paths.images, 'images');

});

Task run times are saved in memory and are lost when gulp exits. It will only

save time during the `watch` task when running the `images` task

for a second time.

If you want to compare modification time between files instead, we recommend these plugins:

- [gulp-changed];

- or [gulp-newer] - supports many:1 source:dest.

[gulp-newer] example:

gulp.task('images', function() {

var dest = 'build/img';

return gulp.src(paths.images)

.pipe(newer(dest)) // pass through newer images only

.pipe(imagemin({optimizationLevel: 5}))

.pipe(gulp.dest(dest));

});

If you can't simply filter out unchanged files, but need them in a later phase

of the stream, we recommend these plugins:

- [gulp-cached] - in-memory file cache, not for operation on sets of files

- [gulp-remember] - pairs nicely with gulp-cached

[gulp-remember] example:

gulp.task('scripts', function () {

return gulp.src(scriptsGlob)

.pipe(cache('scripts')) // only pass through changed files

.pipe(header('(function () {')) // do special things to the changed files...

.pipe(footer('})();')) // for example,

// add a simple module wrap to each file

.pipe(remember('scripts')) // add back all files to the stream

.pipe(concat('app.js')) // do things that require all files

.pipe(gulp.dest('public/'))

});

[gulp-cached]: https://github.com/contra/gulp-cached

[gulp-remember]: https://github.com/ahaurw01/gulp-remember

[gulp-changed]: https://github.com/sindresorhus/gulp-changed

[gulp-newer]: https://github.com/tschaub/gulp-newer

Returns the timestamp of the last time the task ran successfully. The time

will be the time the task started. Returns `undefined` if the task has

not run yet.

Type: `String`

The name of the registered task or of a function.

Type: `Number`.

Default: `1000` on node v0.10, `0` on node v0.12 (and iojs v1.5).

Set the time resolution of the returned timestamps. Assuming

the task named "someTask" ran at `1426000004321`:

- `gulp.lastRun('someTask', 1000)` would return `1426000004000`.

- `gulp.lastRun('someTask', 100)` would return `1426000004300`.

`timeResolution` allows you to compare a run time to a file [mtime stat][fs stats]

attribute. This attribute time resolution may vary depending of the node version

and the file system used:

- on node v0.10, a file [mtime stat][fs stats] time resolution of any files will be 1s at best;

- on node v0.12 and iojs v1.5, 1ms at best;

- for files on FAT32, the mtime time resolution is 2s;

- on HFS+ and Ext3, 1s;

- on NTFS, 1s on node v0.10, 100ms on node 0.12;

- on Ext4, 1s on node v0.10, 1ms on node 0.12.

[fs stats]: https://nodejs.org/api/fs.html#fs_class_fs_stats