grunt-contrib-watch

What is grunt-contrib-watch?

grunt-contrib-watch is a Grunt plugin that watches files and directories for changes. When changes are detected, it can run predefined tasks, making it useful for automating workflows such as compiling code, running tests, or refreshing a browser.

What are grunt-contrib-watch's main functionalities?

Watch Files for Changes

This feature allows you to watch JavaScript files for changes and run the 'jshint' task whenever a change is detected. The 'spawn' option is set to false to improve performance.

{
  "watch": {
    "scripts": {
      "files": ["**/*.js"],
      "tasks": ["jshint"],
      "options": {
        "spawn": false
      }
    }
  }
}

Live Reload

This feature enables live reloading of the browser when files change. The 'livereload' option is set to true, which will trigger a browser refresh whenever a watched file changes.

{
  "watch": {
    "options": {
      "livereload": true
    },
    "scripts": {
      "files": ["**/*.js"],
      "tasks": ["jshint"]
    }
  }
}

Custom Event Handling

This feature allows you to specify custom events to watch for, such as 'added' or 'deleted'. In this example, the 'jshint' task will run only when JavaScript files are added or deleted.

{
  "watch": {
    "scripts": {
      "files": ["**/*.js"],
      "tasks": ["jshint"],
      "options": {
        "event": ["added", "deleted"]
      }
    }
  }
}

Other packages similar to grunt-contrib-watch

gulp-watch

gulp-watch is a Gulp plugin that provides file watching capabilities similar to grunt-contrib-watch. It allows you to watch files and directories for changes and run Gulp tasks in response. Compared to grunt-contrib-watch, gulp-watch is designed to work within the Gulp ecosystem, which is known for its simplicity and stream-based approach.

chokidar

chokidar is a highly efficient and flexible file watcher library for Node.js. It provides a rich API for watching files and directories, and it can be used independently or integrated into other build tools. Compared to grunt-contrib-watch, chokidar offers more advanced features and better performance, making it suitable for large projects with complex file-watching needs.

nodemon

nodemon is a utility that monitors for changes in your source and automatically restarts your Node.js application. While it is primarily used for restarting server applications, it can also be configured to watch for changes in other types of files. Compared to grunt-contrib-watch, nodemon is more focused on server-side development and provides automatic restarts rather than running arbitrary tasks.

README

grunt-contrib-watch

Run predefined tasks whenever watched file patterns are added, changed or deleted.

Getting Started

This plugin requires Grunt ~0.4.0

If you haven't used Grunt before, be sure to check out the Getting Started guide, as it explains how to create a Gruntfile as well as install and use Grunt plugins. Once you're familiar with that process, you may install this plugin with this command:

npm install grunt-contrib-watch --save-dev

Once the plugin has been installed, it may be enabled inside your Gruntfile with this line of JavaScript:

grunt.loadNpmTasks('grunt-contrib-watch');

Watch task

Run this task with the grunt watch command.

Settings

There are a number of options available. Please review the minimatch options here. As well as some additional options as follows:

files

Type: String|Array

This defines what file patterns this task will watch. Can be a string or an array of files and/or minimatch patterns.

tasks

Type: String|Array

This defines which tasks to run when a watched file event occurs.

options.spawn

Type: Boolean Default: true

Whether to spawn task runs in a child process. Setting this option to false speeds up the reaction time of the watch (usually 500ms faster for most) and allows subsequent task runs to share the same context. Not spawning task runs can make the watch more prone to failing so please use as needed.

Example:

watch: {
  scripts: {
    files: ['**/*.js'],
    tasks: ['jshint'],
    options: {
      spawn: false,
    },
  },
},

For backwards compatibility the option nospawn is still available and will do the opposite of spawn.

options.interrupt

Type: Boolean Default: false

As files are modified this watch task will spawn tasks in child processes. The default behavior will only spawn a new child process per target when the previous process has finished. Set the interrupt option to true to terminate the previous process and spawn a new one upon later changes.

Example:

watch: {
  scripts: {
    files: '**/*.js',
    tasks: ['jshint'],
    options: {
      interrupt: true,
    },
  },
},

options.debounceDelay

Type: Integer Default: 500

How long to wait before emitting events in succession for the same filepath and status. For example if your Gruntfile.js file was changed, a changed event will only fire again after the given milliseconds.

Example:

watch: {
  scripts: {
    files: '**/*.js',
    tasks: ['jshint'],
    options: {
      debounceDelay: 250,
    },
  },
},

options.interval

Type: Integer Default: 100

The interval is passed to fs.watchFile. Since interval is only used by fs.watchFile and this watcher also uses fs.watch; it is recommended to ignore this option. Default is 100ms.

options.event

Type: String|Array Default: 'all'

Specify the type watch event that trigger the specified task. This option can be one or many of: 'all', 'changed', 'added' and 'deleted'.

Example:

watch: {
  scripts: {
    files: '**/*.js',
    tasks: ['generateFileManifest'],
    options: {
      event: ['added', 'deleted'],
    },
  },
},

options.forever

Type: Boolean Default: true

This is only a task level option and cannot be configured per target. By default the watch task will duck punch grunt.fatal and grunt.warn to try and prevent them from exiting the watch process. If you don't want grunt.fatal and grunt.warn to be overridden set the forever option to false.

options.dateFormat

Type: Function

This is only a task level option and cannot be configured per target. By default when the watch has finished running tasks it will display the message Completed in 1.301s at Thu Jul 18 2013 14:58:21 GMT-0700 (PDT) - Waiting.... You can override this message by supplying your own function:

watch: {
  options: {
    dateFormat: function(time) {
      grunt.log.writeln('The watch finished in ' + time + 'ms at' + (new Date()).toString()));
      grunt.log.writeln('Waiting for more changes...');
    },
  },
  scripts: {
    files: '**/*.js',
    tasks: 'jshint',
  },
},

options.atBegin

Type: Boolean Default: false

This option will trigger the run of each specified task at startup of the watcher.

options.livereload

Type: Boolean|Number|Object Default: false

Set to true or set livereload: 1337 to a port number to enable live reloading. Default and recommended port is 35729.

If enabled a live reload server will be started with the watch task per target. Then after the indicated tasks have ran, the live reload server will be triggered with the modified files.

Example:

watch: {
  css: {
    files: '**/*.sass',
    tasks: ['sass'],
    options: {
      livereload: true,
    },
  },
},

Examples

// Simple config to run jshint any time a file is added, changed or deleted
grunt.initConfig({
  watch: {
    files: ['**/*'],
    tasks: ['jshint'],
  },
});

// Advanced config. Run specific tasks when specific files are added, changed or deleted.
grunt.initConfig({
  watch: {
    gruntfile: {
      files: 'Gruntfile.js',
      tasks: ['jshint:gruntfile'],
    },
    src: {
      files: ['lib/*.js', 'css/**/*.scss', '!lib/dontwatch.js'],
      tasks: ['default'],
    },
    test: {
      files: '<%= jshint.test.src %>',
      tasks: ['jshint:test', 'qunit'],
    },
  },
});

Using the watch event

This task will emit a watch event when watched files are modified. This is useful if you would like a simple notification when files are edited or if you're using this task in tandem with another task. Here is a simple example using the watch event:

grunt.initConfig({
  watch: {
    scripts: {
      files: ['lib/*.js'],
    },
  },
});
grunt.event.on('watch', function(action, filepath, target) {
  grunt.log.writeln(target + ': ' + filepath + ' has ' + action);
});

The watch event is not intended for replacing the standard Grunt API for configuring and running tasks. If you're trying to run tasks from within the watch event you're more than likely doing it wrong. Please read configuring tasks.

Compiling Files As Needed

A very common request is to only compile files as needed. Here is an example that will only lint changed files with the jshint task:

grunt.initConfig({
  watch: {
    scripts: {
      files: ['lib/*.js'],
      tasks: ['jshint'],
      options: {
        nospawn: true,
      },
    },
  },
  jshint: {
    all: ['lib/*.js'],
  },
});

// on watch events configure jshint:all to only run on changed file
grunt.event.on('watch', function(action, filepath) {
  grunt.config(['jshint', 'all'], filepath);
});

If you need to dynamically modify your config, the nospawn option must be enabled to keep the watch running under the same context.

If you save multiple files simultaneously you may opt for a more robust method:

var changedFiles = Object.create(null);
var onChange = grunt.util._.debounce(function() {
  grunt.config(['jshint', 'all'], Object.keys(changedFiles));
  changedFiles = Object.create(null);
}, 200);
grunt.event.on('watch', function(action, filepath) {
  changedFiles[filepath] = action;
  onChange();
});

Live Reloading

Live reloading is built into the watch task. Set the option livereload to true to enable on the default port 35729 or set to a custom port: livereload: 1337.

The simplest way to add live reloading to all your watch targets is by setting livereload to true at the task level. This will run a single live reload server and trigger the live reload for all your watch targets:

grunt.initConfig({
  watch: {
    options: {
      livereload: true,
    },
    css: {
      files: ['public/scss/*.scss'],
      tasks: ['compass'],
    },
  },
});

You can also configure live reload for individual watch targets or run multiple live reload servers. Just be sure if you're starting multiple servers they operate on different ports:

grunt.initConfig({
  watch: {
    css: {
      files: ['public/scss/*.scss'],
      tasks: ['compass'],
      options: {
        // Start a live reload server on the default port 35729
        livereload: true,
      },
    },
    another: {
      files: ['lib/*.js'],
      tasks: ['anothertask'],
      options: {
        // Start another live reload server on port 1337
        livereload: 1337,
      },
    },
    dont: {
      files: ['other/stuff/*'],
      tasks: ['dostuff'],
    },
  },
});

Enabling Live Reload in Your HTML

Once you've started a live reload server you'll be able to access the live reload script. To enable live reload on your page, add a script tag before your closing </body> tag pointing to the livereload.js script:

<script src="http://localhost:35729/livereload.js"></script>

Feel free to add this script to your template situation and toggle with some sort of dev flag.

Using Live Reload with the Browser Extension

Instead of adding a script tag to your page, you can live reload your page by installing a browser extension. Please visit how do I install and use the browser extensions for help installing an extension for your browser.

Once installed please use the default live reload port 35729 and the browser extension will automatically reload your page without needing the <script> tag.

Using Connect Middleware

Since live reloading is used when developing, you may want to disable building for production (and are not using the browser extension). One method is to use Connect middleware to inject the script tag into your page. Try the connect-livereload middleware for injecting the live reload script into your page.

Rolling Your Own Live Reload

Live reloading is made easy by the library tiny-lr. It is encouraged to read the documentation for tiny-lr. If you would like to trigger the live reload server yourself, simply POST files to the URL: http://localhost:35729/changed. Or if you rather roll your own live reload implementation use the following example:

// Create a live reload server instance
var lrserver = require('tiny-lr')();

// Listen on port 35729
lrserver.listen(35729, function(err) { console.log('LR Server Started'); });

// Then later trigger files or POST to localhost:35729/changed
lrserver.changed({body:{files:['public/css/changed.css']}});

Live Reload with Preprocessors

Any time a watched file is edited with the livereload option enabled, the file will be sent to the live reload server. Some edited files you may desire to have sent to the live reload server, such as when preprocessing (sass, less, coffeescript, etc). As any file not recognized will reload the entire page as opposed to just the css or javascript.

The solution is to point a livereload watch target to your destination files:

grunt.initConfig({
  sass: {
    dev: {
      src: ['src/sass/*.sass'],
      dest: 'dest/css/index.css',
    },
  },
  watch: {
    sass: {
      // We watch and compile sass files as normal but don't live reload here
      files: ['src/sass/*.sass'],
      tasks: ['sass'],
    },
    livereload: {
      // Here we watch the files the sass task will compile to
      // These files are sent to the live reload server after sass compiles to them
      options: { livereload: true },
      files: ['dest/**/*'],
    },
  },
});

FAQs

How do I fix the error EMFILE: Too many opened files.?

This is because of your system's max opened file limit. For OSX the default is very low (256). Temporarily increase your limit with ulimit -n 10480, the number being the new max limit.

In some versions of OSX the above solution doesn't work. In that case try launchctl limit maxfiles 10480 10480 and restart your terminal. See here.

Can I use this with Grunt v0.3?

Yes. Although grunt-contrib-watch is a replacement watch task for Grunt v0.4, version grunt-contrib-watch@0.1.x is compatible with Grunt v0.3. grunt-contrib-watch >= 0.2.x is *only compatible and recommended to use with Grunt v0.4.

Why is the watch devouring all my memory/cpu?

Likely because of an enthusiastic pattern trying to watch thousands of files. Such as '**/*.js' but forgetting to exclude the node_modules folder with '!**/node_modules/**'. Try grouping your files within a subfolder or be more explicit with your file matching pattern.

Another reason if you're watching a large number of files could be the low default interval. Try increasing with options: { interval: 5007 }. Please see issues #35 and #145 for more information.

Why spawn as child processes as a default?

The goal of this watch task is as files are changed, run tasks as if they were triggered by the user themself. Each time a user runs grunt a process is spawned and tasks are ran in succession. In an effort to keep the experience consistent and continually produce expected results, this watch task spawns tasks as child processes by default.

Sandboxing task runs also allows this watch task to run more stable over long periods of time. As well as more efficiently with more complex tasks and file structures.

Spawning does cause a performance hit (usually 500ms for most environments). It also cripples tasks that rely on the watch task to share the context with each subsequent run (i.e., reload tasks). If you would like a faster watch task or need to share the context please set the nospawn option to true. Just be aware that with this option enabled, the watch task is more prone to failure.

Release History

• 2013-07-18   v0.5.0   Added target name to watch event. Added atBegin option to run tasks when watcher starts. Changed nospawn option to spawn (nospawn still available for backwards compatibility). Moved libs/vars into top scope to prevent re-init. Bumped Gaze version to ~0.4. Re-grab task/target options upon each task run. Add dateFormat option to override the date/time output upon completion.
• 2013-05-27   v0.4.4   Remove gracefully closing SIGINT. Not needed and causes problems for Windows. Ensure tasks are an array to not conflict with cliArgs.
• 2013-05-11   v0.4.3   Only group changed files per target to send correct files to live reload.
• 2013-05-09   v0.4.2   Fix for closing watchers.
• 2013-05-09   v0.4.1   Removed "beep" notification. Tasks now optional with livereload option. Reverted "run again" with interrupt off to fix infinite recursion issue. Watchers now close more properly on task run.
• 2013-05-03   v0.4.0   Option livereload to start live reload servers. Will reload a Gruntfile before running tasks if Gruntfile is modified. Option event to only trigger watch on certain events. Refactor watch task into separate task runs per target. Option forever to override grunt.fatal/warn to help keeping the watch alive with nospawn enabled. Emit a beep upon complete. Logs all watched files with verbose flag set. If interrupt is off, will run the tasks once more if watch triggered during a previous task run. tasks property is optional for use with watch event. Watchers properly closed when exiting.
• 2013-02-28   v0.3.1   Fix for top level options.
• 2013-02-27   v0.3.0   nospawn option added to run tasks without spawning as child processes. Watch emits 'watch' events upon files being triggered with grunt.event. Completion time in seconds and date/time shown after tasks ran. Negate file patterns fixed. Tasks debounced individually to handle simultaneous triggering for multiple targets. Errors handled better and viewable with --stack cli option. Code complexity reduced making the watch task code easier to read.
• 2013-02-15   v0.2.0   First official release for Grunt 0.4.0.
• 2013-01-18   v0.2.0rc7   Updating grunt/gruntplugin dependencies to rc6. Changing in-development grunt/gruntplugin dependency versions from tilde version ranges to specific versions.
• 2013-01-09   v0.2.0rc5   Updating to work with grunt v0.4.0rc5.
• 2012-12-15   v0.2.0a   Conversion to grunt v0.4 conventions. Remove node v0.6 and grunt v0.3 support. Allow watch task to be renamed. Use grunt.util.spawn "grunt" option. Updated to gaze@0.3.0, forceWatchMethod option removed.
• 2012-11-01   v0.1.4   Prevent watch from spawning duplicate watch tasks
• 2012-10-28   v0.1.3   Better method to spawn the grunt bin Bump gaze to v0.2.0. Better handles some events and new option forceWatchMethod Only support Node.js >= v0.8
• 2012-10-17   v0.1.2   Only spawn a process per task one at a time Add interrupt option to cancel previous spawned process Grunt v0.3 compatibility changes
• 2012-10-16   v0.1.1   Fallback to global grunt bin if local doesnt exist. Fatal if bin cannot be found Update to gaze 0.1.6
• 2012-10-08   v0.1.0   Release watch task Remove spawn from helper Run on Grunt v0.4

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Task submitted by Kyle Robinson Young

This file was generated on Thu Jul 18 2013 20:14:03.