Command Line Usage

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Compile .less files to .css using the command line

Heads up! If the command line isn't your thing, learn more about GUIs for Less.


Install with npm

npm install less -g

The -g option installs the lessc command available globally. For a specific version (or tag) you can add @VERSION after our package name, e.g. npm install less@2.7.1 -g.

Installing for Node Development

Alternatively, if you don't want to use the compiler globally, you may be after

npm i less --save-dev

This will install the latest official version of lessc in your project folder, also adding it to the devDependencies in your project's package.json.

Beta releases of lessc

Periodically, as new functionality is being developed, lessc builds will be published to npm, tagged as beta. These builds will not be published as a @latest official release, and will typically have beta in the version (use lessc -v to get current version).

Since patch releases are non-breaking we will publish patch releases immediately and alpha/beta/candidate versions will be published as minor or major version upgrades (we endeavour since 1.4.0 to follow semantic versioning).

Server-Side and Command Line Usage

The binary included in this repository, bin/lessc works with Node.js on *nix, OS X and Windows.

Usage: lessc [option option=parameter ...] <source> [destination]

Command Line Usage

lessc [option option=parameter ...] <source> [destination]

If source is set to `-' (dash or hyphen-minus), input is read from stdin.


Compile bootstrap.less to bootstrap.css

lessc bootstrap.less bootstrap.css

Options specific to lessc

For all other options, see Less Options.


lessc -s lessc --silent

Stops any warnings from being shown.


lessc -v
lessc --version


lessc --help
lessc -h

Prints a help message with available options and exits.


lessc -M
lessc --depends

Outputs a makefile import dependency list to stdout.

No Color

lessc --no-color

Clean CSS

In v2 of less, Clean CSS is no longer included as a direct dependency. To use clean css with lessc, use the clean css plugin.

Browser Usage

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Using Less.js in the browser is the easiest way to get started and convenient for developing with Less, but in production, when performance and reliability is important, we recommend pre-compiling using Node.js or one of the many third party tools available.

To start off, link your .less stylesheets with the rel attribute set to "stylesheet/less":

<link rel="stylesheet/less" type="text/css" href="styles.less" />

Next, download less.js and include it in a <script></script> tag in the <head> element of your page:

<script src="less.js" type="text/javascript"></script>

Setting Options

You can set options either programmatically, by setting them on a less object before the script tag - this then affects all initial link tags and programmatic usage of less.

  less = {
    env: "development",
    async: false,
    fileAsync: false,
    poll: 1000,
    functions: {},
    dumpLineNumbers: "comments",
    relativeUrls: false,
    rootpath: ":/"
<script src="less.js"></script>

The other way is to specify the options on the script tag, e.g.

  less = {
    env: "development"
<script src="less.js" data-env="development"></script>

Or for brevity they can be set as attributes on the script and link tags:

<script src="less.js" data-poll="1000" data-relative-urls="false"></script>
<link data-dump-line-numbers="all" data-global-vars='{ "myvar": "#ddffee", "mystr": "\"quoted\"" }' rel="stylesheet/less" type="text/css" href="less/styles.less">

Browser Support

Less.js supports all modern browsers (recent versions of Chrome, Firefox, Safari, IE11+, and Edge). While it is possible to use Less on the client side in production, please be aware that there are performance implications for doing so (although the latest releases of Less are quite a bit faster). Also, sometimes cosmetic issues can occur if a JavaScript error occurs. This is a trade off of flexibility vs. speed. For the fastest performance possible for a static web site, we recommend compiling Less on the server side.

There are reasons to use client-side less in production, such as if you want to allow users to tweak variables which will affect the theme and you want to show it to them in real-time - in this instance a user is not worried about waiting for a style to update before seeing it.


  • Make sure you include your stylesheets before the script.
  • When you link more than one .less stylesheet each of them is compiled independently. So any variables, mixins or namespaces you define in a stylesheet are not accessible in any other.
  • Due to the same origin policy of browsers, loading external resources requires enabling CORS

Watch Mode

To enable Watch mode, option env must be set to development. Then AFTER the less.js file is included, call, like this:

<script>less = { env: 'development'};</script>
<script src="less.js"></script>

Alternatively, you can enable Watch mode temporarily by appending #!watch to the URL.

Modify Variables

Enables run-time modification of Less variables. When called with new values, the Less file is recompiled without reloading. Simple basic usage:

  '@buttonFace': '#5B83AD',
  '@buttonText': '#D9EEF2'


It is possible to output rules in your CSS which allow tools to locate the source of the rule.

Either specify the option dumpLineNumbers as above or add !dumpLineNumbers:mediaquery to the url.

You can use the mediaquery option with FireLESS (it is identical to the SCSS media query debugging format). Also see FireLess and Less v2. The comment option can be used to display file information and line numbers in the inline compiled CSS code.


Set options in a global less object before loading the less.js script:

<!-- set options before less.js script -->
  less = {
    env: "development",
    logLevel: 2,
    async: false,
    fileAsync: false,
    poll: 1000,
    functions: {},
    dumpLineNumbers: "comments",
    relativeUrls: false,
    globalVars: {
      var1: '"quoted value"',
      var2: 'regular value'
    rootpath: ":/"
<script src="less.js"></script>

Options specific to Less.js in the browser

For all other options, see Less Options.


Type: Boolean

Default: false

Whether to request the import files with the async option or not. See fileAsync.


Type: String Default: depends on page URL

Environment to run may be either development or production.

In production, your css is cached in local storage and information messages are not output to the console.

If the document's URL starts with file:// or is on your local machine or has a non standard port, it will automatically be set to development.


less = { env: 'production' };


Type: String

Options: html|console|function

Default: html

Set the method of error reporting when compilation fails.


Type: Boolean

Default: false

Whether to request the import asynchronously when in a page with a file protocol.

functions (Deprecated - use @plugin)

Type: object

User functions, keyed by name.

less = {
    functions: {
        myfunc: function() {
            return less.Dimension(1);

and it can be used like a native Less function e.g.

.my-class {
  border-width: unit(myfunc(), px);


Type: Number

Default: 2

The amount of logging in the javascript console. Note: If you are in the production environment you will not get any logging.

2 - Information and errors
1 - Errors
0 - Nothing


Type: Integer

Default: 1000

The amount of time (in milliseconds) between polls while in watch mode.


Type: Boolean

Default: false

Optionally adjust URLs to be relative. When false, URLs are already relative to the entry less file.


Type: Boolean

Default: true (previously false in before v2)

Whether to use the per session file cache. This caches less files so that you can call modifyVars and it will not retrieve all the less files again. If you use the watcher or call refresh with reload set to true, then the cache will be cleared before running.

Less.js Options

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Cross-Platform Options

Include Paths

lessc --include-path=PATH1;PATH2 { paths: ['PATH1', 'PATH2'] }

If the file in an @import rule does not exist at that exact location, less will look for it at the location(s) passed to this option. You might use this for instance to specify a path to a library which you want to be referenced simply and relatively in the less files.


lessc -rp=resources/
lessc --rootpath=resources/
{ rootpath: 'resources/' }

Allows you to add a path to every generated import and url in your css. This does not affect less import statements that are processed, just ones that are left in the output css.

For instance, if all the images the css use are in a folder called resources, you can use this option to add this on to the URL's and then have the name of that folder configurable.

Relative URLs

lessc -ru
lessc --relative-urls
{ relativeUrls: true }

By default URLs are kept as-is, so if you import a file in a sub-directory that references an image, exactly the same URL will be output in the css. This option allows you to re-write URL's in imported files so that the URL is always relative to the base imported file. E.g.

# main.less
@import "files/backgrounds.less";
# files/backgrounds.less
.icon-1 {
  background-image: url('images/lamp-post.png');

this will output the following normally

.icon-1 {
  background-image: url('images/lamp-post.png');

but with relativeUrls: true, it will instead output

.icon-1 {
  background-image: url('files/images/lamp-post.png');

You may also want to consider using the data-uri function instead of this option, which will embed images into the css.

Strict Math

lessc -sm=on
lessc --strict-math=on
{ strictMath: true }

Defaults to off/false.

Without this option on, Less will try and process all math in your css e.g.

.class {
  height: calc(100% - 10px);

will be processed currently.

With strict math on, only math that is inside un-necessary parenthesis will be processed. For instance.

.class {
  width: calc(100% - ((10px  - 5px)));
  height: (100px / 4px);
  font-size: 1 / 4;
.class {
  width: calc(100% - 5px);
  height: 25px;
  font-size: 1 / 4;

We originally planned to default this to true in the future, but it has been a controversial option and we are considering whether we have solved the problem in the right way, or whether less should just have exceptions for instances where / is valid or calc is used.

Strict Units

lessc -su=on
lessc --strict-units=on
{ strictUnits: true }

Defaults to off/false.

Without this option, less attempts to guess at the output unit when it does maths. For instance

.class {
  property: 1px * 2px;

In this case, things are clearly not right - a length multiplied by a length gives an area, but css does not support specifying areas. So we assume that the user meant for one of the values to be a value, not a unit of length and we output 2px.

With strict units on, we assume this is a bug in the calculation and throw an error.

IE8 Compatibility (Deprecated)

lessc --ie-compat { ieCompat: true }

False by default starting in v3.0.0. Currently only used for the data-uri function to ensure that images aren't created that are too large for the browser to handle.

Enable Inline JavaScript (Deprecated)

lessc --js { inlineJavaScript: true }

False by default starting in v3.0.0. Enables evaluation of JavaScript inline in .less files. This created a security problem for some developers who didn't expect user input for style sheets to have executable code.

Replaced with the @plugin option.

Global Variables

lessc --global-var="color1=red" { globalVars: { color1: 'red' } }

This option defines a variable that can be referenced by the file. Effectively the declaration is put at the top of your base Less file, meaning it can be used but it also can be overridden if this variable is defined in the file.

Modify Variables

lessc --modify-var="color1=red" { modifyVars: { color1: 'red' } }

As opposed to the global variable option, this puts the declaration at the end of your base file, meaning it will override anything defined in your Less file.

URL Arguments

lessc --url-args="cache726357" { urlArgs: 'cache726357' }

This option allows you to specify a argument to go on to every URL. This may be used for cache-busting for instance.

Line Numbers (Deprecated)

lessc --line-numbers=comments
lessc --line-numbers=mediaquery
lessc --line-numbers=all
{ dumpLineNumbers: 'comments' }

Generates inline source-mapping. This was the only option before browsers started supporting sourcemaps.

Pre-Loaded Plugin

See: Pre-Loaded Plugins


lessc --lint -l { lint: true }

Runs the less parser and just reports errors without any output.


lessc --compress -x { compress: true }

Compress using less built-in compression. This does an okay job but does not utilise all the tricks of dedicated css compression. Please feel free to improve our compressed output with a pull request.

Allow Imports from Insecure HTTPS Hosts

lessc --insecure { insecure: true }

Source Map Options

Most of these options are not applicable to using Less.js in the browser, as you should generate a source map with your pre-compiled Less files.

Generate a Source Map

lessc --source-map { sourceMap: {} }

Tells less to generate a sourcemap.

Source Map Output Filename

lessc { sourceMap: { outputFilename: '' } }

Source Map Rootpath

lessc --source-map-rootpath=dev-files/ { sourceMap: { sourceMapRootpath: 'dev-files/' } }

Specifies a rootpath that should be prepended to each of the less file paths inside the sourcemap and also to the path to the map file specified in your output css.

Because the basepath defaults to the directory of the input less file, the rootpath defaults to the path from the sourcemap output file to the base directory of the input less file.

Use this option if for instance you have a css file generated in the root on your web server but have your source less/css/map files in a different folder. So for the option above you might have


Source Map Basepath

lessc --source-map-basepath=less-files/ { sourceMap: { sourceMapBasepath: 'less-files/' } }

This is the opposite of the rootpath option, it specifies a path which should be removed from the output paths. For instance if you are compiling a file in the less-files directory but the source files will be available on your web server in the root or current directory, you can specify this to remove the additional less-files part of the path.

It defaults to the path to the input less file.

Include Less Source in the Source Map

lessc --source-map-include-source { sourceMap: { outputSourceFiles: true } }

This option specifies that we should include all of the Less files in to the sourcemap. This means that you only need your map file to get to your original source.

This can be used in conjunction with the map inline option so that you do not need to have any additional external files at all.

Source Map Map Inline

lessc --source-map-inline { sourceMap: { sourceMapFileInline: true } }

This option specifies that the map file should be inline in the output CSS. This is not recommended for production, but for development it allows the compiler to produce a single output file which in browsers that support it, use the compiled css but show you the non-compiled less source.

Source Map URL

lessc --source-map-url=../my-map.json { sourceMap: { sourceMapURL: '../my-map.json' } }

Allows you to override the URL in the css that points at the map file. This is for cases when the rootpath and basepath options are not producing exactly what you need.

Pre-Loaded Plugins

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Loading plugins before parsing begins in Less.js

While the easiest way to use a plugin is using the @plugin at-rule, you can pre-load a global Less.js plugin via the command line or by specifying it in the Less options.


Pre-loading plugins is necessary if you want to add a Less.js Pre-processor. That is, a plugin that gets called and passed the raw Less source before parsing even starts. An example of this would be a Sass-To-Less Pre-processor plugin.

Note: pre-loading is not necessary for pre-evaluation plugins (after Less source is parsed, but before it is evaluated). See: API.


Using the Command Line

If you are using lessc, the first thing you need to do is install that plugin. In registries like NPM, we recommend a Less.js plugin is registered with the "less-plugin-" prefix (for easier searching), though that isn't required. So, for a custom plugin, you might install with:

npm install less-plugin-myplugin

To use the plugin, you can pass this on the command line by simply writing:

lessc --myplugin

Less.js will try to load either the "less-plugin-myplugin" and the "myplugin" modules as plugins whenever there's an unknown Less option (like "myplugin").

You can also explicitly specify the plugin with:

lessc --plugin=myplugin

To pass options to the plugin, you can write that in one of two ways.

lessc --myplugin="advanced"
lessc --plugin=myplugin=advanced

Loading a Plugin via Less.js

In Node, pass it to less in an array on the options object e.g.

TODO: Just passing the string name of the plugin to the render function doesn't actually work yet in 3.0 alpha.

less.render(myCSS, { plugins: ["less-plugin-myplugin"] })
    function(output) { },
    function(error) { }

Pre-Loading a Plugin In the Browser

To pre-load a plugin in the browser, include the plugin script before you load Less.js.

<script src="less-plugin-myplugin.js"></script>
<script src="less.min.js"></script>

Programmatic Usage

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The main entry point into less is the less.render function. This takes the following format

less.render(lessInput, options)
    .then(function(output) {
        // output.css = string of css
        // = string of sourcemap
        // output.imports = array of string filenames of the imports referenced
    function(error) {

// or...

less.render(css, options, function(error, output) {})

The options argument is optional. If you specify a callback then a promise will not be returned, where as if you do not specify a callback a promise will be given. Under the hood, the callback version is used so that less can be used synchronously.

If you wanted to render a file, you would first read it into a string (to pass to less.render) and then set the filename field on options to be the filename of the main file. less will handle all the processing of the imports.

The sourceMap option is an object which enables you to set sub sourcemap options. Available sub options are: sourceMapURL,sourceMapBasepath,sourceMapRootpath,outputSourceFiles and sourceMapFileInline. Notice that the sourceMap option is not available for the less.js in browser compiler now.

    .then(function(output) {
        // output.css = string of css
        // = undefined
less.render(lessInput, {sourceMap: {}})
    .then(function(output) {
        // output.css = string of css
        // = string of sourcemap
less.render(lessInput, {sourceMap: {sourceMapFileInline: true}})
    .then(function(output) {
        // output.css = string of css \n /*# sourceMappingURL=data:application/json;base64,eyJ2ZXJ..= */
        // = undefined

Getting Access to the Log

You can add a log listener with the following code

    debug: function(msg) {
    info: function(msg) {
    warn: function(msg) {
    error: function(msg) {

Note: all functions are optional. An error will not be logged, but instead is passed back to the callback or promise in less.render

Contributing to Less.js

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Thanks for thinking about contributing! Please read the contributing instructions carefully to avoid wasted work.

Install These Tools

make sure the paths are setup. If you start your favourite command line and type node -v you should see the node compiler. If you run phantomjs -v you should see the phantomjs version number.

  • clone your less.js repository locally
  • navigate to your local less.js repository and run npm install this installs less' npm dependencies.
  • If you haven't used grunt before, run npm install grunt-cli -g - this allows you to use the "grunt" command anywhere


If you go to the root of the less repository you should be able to do grunt test - this should run all the tests. For the browser specific only, run grunt browsertest If you want to try out the current version of less against a file, from here do node bin/lessc path/to/file.less

To debug the browser tests, run grunt browsertest-server then go to http://localhost:8088/tmp/browser/ to see the test runner pages.

Optional: To get the current version of the less compiler do npm -g install less - npm is the node package manager and "-g" installs it to be available globally.

You should now be able to do lessc file.less and if there is an appropriate file.less then it will be compiled and output to the stdout. You can then compare it to running locally (node bin/lessc file.less).

Other grunt commands

  • grunt benchmark - run our benchmark tests to get some numbers on performance
  • grunt stable to create a new release
  • grunt readme to generate a new in the root directory (after each release)

How to Run Less in Other Environments

If you look in the libs folder you will see less, less-node, less-browser. The less folder is pure javascript with no environment specifics. if you require less/libs/less, you get a function that takes an environment object and an array of file managers. The file managers are the same file managers that can also be written as a plugin.

var createLess = require("less/libs/less"),
    myLess = createLess(environment, [myFileManager]);

The environment api is specified in less/libs/less/environment/environment-api.js and the file manager api is specified in less/libs/less/environment/file-manager-api.js.

For file managers we highly recommend setting the prototype as a new AbstractFileManager - this allows you to override what is needed and allows us to implement new functions without breaking existing file managers. For an example of file managers, see the 2 node implementations, the browser implementation or the npm import plugin implementation.


If you look at, This is an overview diagram of how less works. Warning! It needs updating with v2 changes.