Node Enhanced Shell

An enhanced extensible interactive interpreter (REPL) for Node.js and languages that compile to Javascript, like CoffeeScript and Babel (ES6/7). Some features:

Dependency Status Build Status


You can install and start using nesh with npm (note: you may need to use sudo to install globally):

npm install -g nesh

# Run nesh

# Run nesh with CoffeeScript
nesh -c

# Run nesh with ES6 through Babel
nesh -b

If you wish to use nesh within your own project with require 'nesh' (i.e. to embed within your app) you can use the following non-global install instead. See Embedding the Interpreter below for more information.

npm install nesh

Basic Usage

The nesh command starts an interactive interpreter with a default set of plugins loaded. You can type commands and they will be executed, with the output or any errors displayed below the command.

Command Help

You can get a list of options and help via:

nesh --help

Setting a Language

Nesh supports multiple languages, and ships with both CoffeeScript and ES6/7 (through Babel) support out of the box. To select a language:

nesh --language coffee
nesh --language babel

You can get a list of supported built-in languages via:

nesh --list-languages

As a shortcut for CoffeeScript, you can use nesh -c. It's also pretty easy to set up an alias for this, e.g. alias cs='nesh -c' in bash. For ES6, you would use nesh -b instead.

Setting a Prompt & Welcome Message

A prompt can be set with the --prompt parameter, e.g. nesh --prompt "test> ". The welcome message can be set the same way with the --welcome parameter. You can disable the welcome message via e.g. nesh --no-welcome.

Preloading Code

You can preload a script with the --eval option, which will evaluate either a file or string in the context of the interpreter, so anything that you define will be available in the interpreter after startup. This is similar to using ipython -i

echo 'var hello = function (name) { return "Hello, " + name; }' >hello.js
nesh --eval hello.js

Now you can run hello('world'); in the interpreter. A string can also be used:

nesh --eval "var test = 1;"

Languages other than Javascript can also be used. When using a non-Javascript language, the code loaded will use that language's compile function before running if the loaded filename does not end in .js. This means it is possible to load code both in the loaded language and in plain javascript. For example:

# Load code from a language-specific file
nesh -c -e
nesh -b -e hello.es6

# Load code from a plain javascript file
nesh -c -e hello.js


Plugins can add functionality to Nesh. Plugins are published via NPM just like any other Node.js package. Plugins published via NPM should use nesh- as the naming prefix, which makes them easier to find.

You can install and enable new plugins easily:

# Install and enable a plugin called nesh-hello
nesh --enable nesh-hello

# Remove and disable a plugin called nesh-hello
nesh --disable nesh-hello

It's also possible to blacklist built-in plugins from loading on startup:

# Prevent welcome message from ever showing
nesh --disable welcome

You can see a list of loaded plugins via the --plugins option:

nesh --plugins

See the section below on Extending the Interpreter for information on how to write plugins.

Convenience Functions

When run from the nesh command several built-in convenience functions are available.

REPL Commands


Clears the screen

.require module

Shortcut for requiring a module and assigning it to a variable of the same name, e.g:

nesh> .require fs
nesh> fs.readFileSync(...);

.doc object or method

Get documentation about an object. Can be invoked with either .doc or ctrl-q. Pressing ctrl-q twice will dump out the source of the object or method.

nesh> .doc fs.readFileSync
[Function] (path, options)

// Or with ctrl-q
nesh> path.extname
[Function] (path)

// And ctrl-q again
function (path) {
  return splitPath(path)[3];



Exposes the Underscore.js library. Two underscores are used because a single underscore is reserved for the response of the last run command.


md5 (value)

Return an MD5 hash of a value as a hexadecimal string.

sha (value)

Return a SHA1 hash of a value as a hexadecimal string.


rand ([start], [end])

Generate a random number. If neither start nor end are given, it returns a number between 0 and 1. If only start is given, a number between 0 and start is returned. Otherwise, a number between start and end is returned.

randInt ([start], [end])

Generate a random integer. This is a shortcut for Math.round(rand(start, end)) and follows the same rules as rand for start and end.

randChoices (choices, [length])

Select an array of random choices of length length from an array choices.

randString (length, [charSet])

Return a random string with characters selected from charSet, which defaults to case-sensitive alphanumeric characters.

randHex (length)

Return a random lowercase hexadecimal string.

Number Encoding

bin (value)

Convert a number into a binary string. For example, bin(22) would return '10110'.

oct (value)

Convert a number into an octal string. For example, oct(22) would return '26'.

hex (value)

Convert a number into a hexadecimal string. For example, hex(22) would return '16'.

URL Encoding

urlenc (value)

Return a URL-encoded version of a string. For example, a+1/% would become a%2B1%2F%25.

urldec (value)

Return a URL-decoded version of a string. For example, a%2B1%2F%25 would become a+1/%.

Embedding the Interpreter

The Nesh interpreter can be embedded into your application, whether it is written in Javascript, Coffeescript, or another language that runs on Node. For example, to start an interactive CoffeeScript session on stdin/stdout from Javascript with a custom prompt and welcome message:

nesh = require('nesh');

opts = {
    welcome: 'Welcome!',
    prompt: 'test> '

// Load user configuration

// Load CoffeeScript

// Start the REPL
nesh.start(opts, function (err) {
    if (err) {

Embedding Reference


The Nesh version.


The configuration module. See below in the Configuration section for more information.


An object containing default values that are set when no such value is passed to nesh.start's opts parameter.


A function to compile a snippet of code into javascript.


An object with a start function to create a new REPL-like object. Defaults to the built-in Node.js repl module. This can be set when a language is loaded or by plugins to provide extra functionality.


A list of loaded plugins. This is usually populated by the nesh.loadPlugin function.

nesh.languages ()

Get a list of supported built-in languages that can be passed as strings to nesh.loadLanguage.

nesh.loadLanguage (language)

Load a language to be interpreted, e.g. coffee for CoffeeScript. Can also take in a function to be called to load the language. See below in the Extending the Interpreter section for details.

nesh.loadPlugin (plugin)

Loads a plugin by name or as an object - see below in the Extending the Interpreter section for details.

nesh.init (autoload, [callback])

Initialize the Nesh module. If autoload is true then a default set of plugins is loaded, as well as any plugins defined in the user configuration in ~/.nesh_config.json. This function doesn't need to be called explicitly as it will be called by nesh.start, but is provided to give you more control over the loading process.

nesh.start ([opts], [callback])

Create a new nesh REPL with the passed options opts. Allowed options include the defaults from the Node REPL module ( as well as the following:


Nesh provides a basic configuration system that by default stores data in ~/.nesh_config.json. This system is usable by plugins, languages, etc. The nesh command loads this configuration file on startup. When embedding the interpreter, you may wish to do this as well via the nesh.config.load() function.

Configuration Reference


The path to the default configuration file location.

nesh.config.reset ()

Reset the config to a blank state, i.e. {}.

nesh.config.load ([path])

Loads a configuration file. Once loaded, the config may be accessed via nesh.config.get(). Note: this may throw errors if parsing the file fails. ([path])

Saves a configuration file. Note: this may throw errors if the path cannot be written to, you are over your disk quota, etc.

nesh.config.get ()

Get the currently loaded configuration. This defaults to {}.


Nesh comes with a built-in logging framework to make it easy for plugins to log information. By default, each message will be sent to stdout, which is not ideal for many applications. Therefore, it is possible to modify the logger to provide integration with whatever logging framework your application is using. There are even convenience functions to do so for popular logging frameworks. For example, if your application is using Winston:

nesh = require 'nesh'


nesh.start (err) ->
    nesh.log.error err if err

If using a different logging framework or custom log output, you can manually override the logger functions. For example:

nesh = require 'nesh'

nesh.log.log = (level, message) ->
    console.log "#{level}: #{message}" if level >= nesh.log.level

nesh.log.color = false

Logging Reference


The debug logging level.


The informational logging level.


The warning logging level.


The error logging level.


The current logging level.

nesh.log.levelName ()

Get the name of the log level, e.g. 'warn' for nesh.log.WARN

nesh.log.log (level, message)

Log a message at a particular level if nesh.log.level allows it.

nesh.log.debug (message)

Log a debug message. (message)

Log an info message.

nesh.log.warn (message)

Log a warning message.

nesh.log.error (message)

Log an error message.

nesh.log.test ()

Reconfigure the logging to store sent messages in nesh.log.output and disable console colors. This makes testing log output much simpler.

nesh.log.winston ()

Reconfigure the logging to use Winston to output messages.

Extending the Interpreter

The Nesh interpreter can be easily extended with new languages and plugins.

Languages can be added using the nesh.loadLanguage function. New languages should override nesh.compile, nesh.repl, and probably nesh.defaults.historyFile. The nesh.repl object should provide a Node REPL-like interface with a start function and return a REPL-like object which may be modified by plugins. The context object has the following attributes:

Attribute Description Introduced
nesh A reference to the Nesh module 1.0.0

For example:

nesh = require 'nesh'
mylang = require 'mylang'

nesh.loadLanguage (context) ->
    nesh.compile = (data) ->
        # Compile to js here
        mylang.compile data, {bare: true}
    nesh.repl =
        start: (opts) ->
            # Do stuff here!
            opts.eval = mylang.eval
            repl = require('repl').start opts
            # Don't forget to return the REPL!
            return repl
    nesh.defaults.welcome = 'Welcome to my interpreter!'
    nesh.defaults.historyFile = path.join(nesh.config.home, '.mylang_history')

nesh.start (err) ->
    nesh.log.error err if err

Plugins should set a name and description. Plugins may also define setup, preStart, and postStart functions that are called when the plugin is loaded, before a REPL is created, and after a REPL has been created respectively. Plugins are loaded via the nesh.loadPlugin function. A very simple example plugin written in CoffeeScript might look like this:

nesh = require 'nesh'
util = require 'util'

myPlugin =
    name: "myplugin"
    description: "Some description here"
    setup: (context) ->
        {defaults} = context 'Setting up my plugin! Defaults:' util.inspect defaults

    preStart: (context) ->
        {options} = context 'About to start the interpreter with these options:' util.inspect options

    postStart: (context) ->
        {repl} = context 'Interpreter started! REPL:' util.inspect repl

nesh.loadPlugin myPlugin, (err) ->
    nesh.log.error err if err

    nesh.start (err) ->
        nesh.log.error err if err

Several plugins ship with Nesh, just take a look at the src/plugins directory. If these ever need to be removed then you can do so by accessing the nesh.plugins array. You can also prevent loading the default set of plugins by manually calling nesh.init with autoload set to false.

Asynchronous Plugins

Sometimes, a plugin may take actions that must run asynchronously. To support these cases, each of the plugin's functions can take a callback parameter next which must be called when finished. For example, if we were loading the welcome message's default value from a database with an asynchronous call:

myPlugin =
    setup: (context, next) ->
        {defaults} = context.nesh
        mongodb.findOne name: 'defaultWelcome', (err, item) ->
            return next(err) if err

            defaults.welcome = item.message

Default Plugins

Nesh ships with several default plugins:

Plugin Reference

Plugin.setup (context, [next])

Called when the plugin is first loaded. If next is defined, then the function is treated as asyncronous and next will be passed a function that must be called when finished. If an error occurs, then the error should be passed to next. The context passed in is an object containing the values defined below:

Attribute Description Introduced
nesh A reference to the Nesh module 1.0.0

This is a good place to add or modify default values via context.nesh.defaults.

Plugin.preStart (context, [next])

Called when nesh.start has been called but before the REPL is created and started. If next is defined, then the function is treated as asyncronous and next will be passed a function that must be called when finished. If an error occurs, then the error should be passed to next. The context passed in is an object containing the values defined below:

Attribute Description Introduced
nesh A reference to the Nesh module 1.0.0
options The options passed to nesh.start 1.0.0

This is a good place to print out information or modify the passed in options before they are sent to the REPL, e.g. context.options.welcome = 'foo'.

Plugin.postStart (context, [next])

Called when nesh.start has been called and the REPL is started. The repl passed in is the newly created and started REPL from the nesh.start call and includes the opts from above as repl.opts. If next is defined, then the function is treated as asyncronous and next will be passed a function that must be called when finished. If an error occurs, then the error should be passed to next.

Attribute Description Introduced
nesh A reference to the Nesh module 1.0.0
options The options passed to nesh.start 1.0.0
repl The created REPL instance 1.0.0

This is a good place to modify the REPL, e.g. adding new commands, modifying history, listening for specific key strokes, etc.


Nesh development is easy! Just grab the source with git and start hacking around. Contributions, especially interesting languages and plugins, are always welcome!


After making changes it is important to run a build step to generate the Javascript which gets loaded when you import the nesh module, which makes it work across all Node languages.

cake build

Running a local nesh

You can run the nesh command from your local checkout:


It is also possible to use npm to link your local checkout globally (note: this may require sudo):

npm link

Now you should be able to run nesh from anywhere and have it use your development version.

Unit Tests

The unit test suite can be run via the following:

cake test

Testing new plugins

You can test out new plugins that have their own NPM package by linking them into the plugins directory:

ln -s ~/Projects/nesh-myplugin ~/.nesh_modules/node_modules/

Then modify your ~/.nesh_config.json file to enable the plugin. A minimal configuration looks like the following:

    plugins: ["nesh-myplugin"]


Copyright (c) 2015 Daniel G. Taylor

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.