node.js helper lib for creating CLI tools with subcommands; think `git`, `svn`, `zfs`
JavaScript Makefile

node-cmdln is a node.js helper lib for creating CLI tools with subcommands (think git, svn, zfs, brew, etc.). It is a sister of my earlier Python lib for this.

Follow @trentmick for updates to node-cmdln.


You define a subclass of Cmdln and subcommands as do_NAME methods. Minimally you could have a "conan.js" as follows:

#!/usr/bin/env node
var util = require('util');
var cmdln = require('cmdln');

function Conan() {, {
        name: 'conan',
        desc: 'What is best in life?'
util.inherits(Conan, cmdln.Cmdln);

Conan.prototype.do_crush = function do_crush(subcmd, opts, args, cb) {
}; = 'Crush your enemies.';

cmdln.main(new Conan());  // mainline

With this, you get the following behaviour:

$ node examples/conan.js
What is best in life?

    conan [OPTIONS] COMMAND [ARGS...]
    conan help COMMAND

    -h, --help      Show this help message and exit.

    help (?)        Help on a specific sub-command.
    crush           Crush your enemies.

$ node examples/conan.js help crush
Crush your enemies.

$ node examples/conan.js crush

Option processing

Option processing (using dashdash) is integrated. do_crush above could be replaced with:

Conan.prototype.do_crush = function (subcmd, opts, args, cb) {
    if ( {
        this.do_help('help', {}, [subcmd], cb);
    if (!args.length) {
        console.log('No enemies? Yarg!');
    } else {
        args.forEach(function (enemy) {
            console.log('Smite %s with a %s!', enemy, opts.weapon);
Conan.prototype.do_crush.options = [
        names: ['help', 'h'],
        type: 'bool',
        help: 'Show this help.'
        names: ['weapon', 'w'],
        helpArg: 'WEAPON',
        type: 'string',
        default: 'sword',
        help: 'Weapon with which to smite.'
]; = (
    'Crush your enemies.\n'
    + '\n'
    + 'Usage:\n'
    + '     {{name}} {{cmd}} [OPTIONS] [ENEMIES...]\n'
    + '\n'
    + '{{options}}'

Then we get this behaviour:

$ node examples/conan.js crush Bob
Smite Bob with a sword!

$ node examples/conan.js crush Bob Linda --weapon mattock
Smite Bob with a mattock!
Smite Linda with a mattock!

$ node examples/conan.js crush -h
Crush your enemies.

     conan crush [OPTIONS] [ENEMIES...]

    -h, --help                  Show this help.
    -w WEAPON, --weapon=WEAPON  Weapon with which to smite.

See examples/conan.js for the complete example. Run node example/conan.js ... to try it out.

Bash completion

One can generate Bash completion code for a Cmdln subclass via


One possible usage is to add a completion subcmd to your CLI:

CLI.prototype.do_completion = function (subcmd, opts, args, cb) {
    console.log( this.bashCompletion() );

and get users to use that to setup Bash completion:

$ alias conan="node examples/conan.js"
$ conan completion > conan.completion
$ source conan.completion

$ conan <TAB>
crush      hear       help       pulverize  see        smash
$ conan -<TAB>
--help     --verbose  --version  -h         -v         -x
$ conan crush --weapon <TAB>            # custom 'weapon' completion type
bow-and-array  mattock        spear          sword
$ conan crush --weapon spear <TAB>      # custom 'enemy' completion type
King-Osric    Subotai       Thulsa-Doom   _mbsetupuser  trentm

See the do_completion subcommand on "examples/conan.js" for a complete example of this. See the equivalent in the larger triton tool for another example:

Another potential usage could be to pre-generate a completion file and distribute it with your tool.


In general, also please read the comments in the source and browse the examples.


To use this module you create a class that inherits from cmdln.Cmdln; add some methods to that class that define the tool's commands, options, etc.; then pass an instance to cmdln.main(). Roughly like this:

function CLI() {, {<config>});
util.inherits(CLI, cmdln.Cmdln);
var cli = new CLI();

We'll use the CLI and cli names as used above in the following reference:

  • new Cmdln(<config>) Create a Cmdln subclass instance. See the block comment in the code for full documentation on the config options.

  • CLI.prototype.do_<subcmd> = function (subcmd, opts, args, cb) is how a subcommand is defined. How the subcmd is handled can be customize with some properties (e.g. options, help) on the handler function.

  • CLI.prototype.do_<subcmd> = <SubCLI>; Instead of a function handler for a subcommand, a do_<subcmd> can be set to another Cmdln subclass to support sub-subcommands, like git remote add|remove|rename|.... See "examples/fauxgit.js" for an example.

  • CLI.prototype.do_<subcmd>.aliases = <array of strings>; to define one or more aliases for a command. These aliases are shown in the "Commands:" section of the generated help output.

  • CLI.prototype.do_<subcmd>.hiddenAliases = <array of strings>; to define one or more aliases for a command that are not shown in the generated help output. This can be useful when renaming a subcommand in a new version of a tool and still support the old name.

  • CLI.prototype.do_<subcmd>.options = <object>; is how to set the options (in dashdash format) for that subcommand.

  • CLI.prototype.do_<subcmd>.synopses = <array of strings>; Set to the synopsis string(s) for this command, i.e. the part typically in the "SYNOPSIS" section of a man page. See examples/conan.js. This supports some template variables:

    • {{name}} becomes (i.e. the tool name).
    • {{cmd}} becomes the sub-command name.

    Setting synopses can be used (a) for the {{usage}} template var in subcmd help (see below) and (b) for errHelp for UsageErrors (see below).

  • CLI.prototype.do_<subcmd>.helpOpts = <dashdash helpOpts object>; to override formatting settings for options help output for this command. By default the helpOpts passed into the CLI constructor are used. The set of supported helpOpts are defined by dashdash.

  • CLI.prototype.do_<subcmd>.help = <string>; to set the help string for a subcommand. This supports some template variables:

    • {{name}} becomes (i.e. the tool name).
    • {{cmd}} becomes the sub-command name.
    • {{usage}} becomes a "Usage:\n $synopses" block if synopses are defined (see above).
    • {{options}} becomes a "Options:\n $option-help" block if options are provided for the subcmd (see above).
  • CLI.prototype.do_<subcmd>.help = function (subcmd, opts, args, cb) is an alternate method to handle help for a subcommand. The given function will be run when tool help <subcmd> is called.

  • CLI.prototype.do_<subcmd>.desc = <string>; can be set to a short string to be used in the tool help output to summarize subcmd. If not provided, then the first line of do_<subcmd>.help will be used.

  • CLI.prototype.do_<subcmd>.hidden = <boolean>; Set to false to have tool help output not list this subcmd.

  • CLI.prototype.do_<subcmd>.interspersedOptions = <boolean>; Set to false to have tool <subcmd> ... not allow interspersed options (i.e. options after the first argument).

  • CLI.prototype.do_<subcmd>.allowUnknownOptions = <boolean>; Set to true to have tool <subcmd> ... allow unknown options.

  • CLI.prototype.do_<subcmd>.completionArgtypes = <array>; Set to an array of strings to define the Bash completion type for the corresponding positional arg. For example, the following: MyCLI.prototype.do_foo.completionArgtypes = ['fruit', 'file']; would mean that mycli foo <TAB> would complete "fruit" (using a complete_fruit bash function, typically provided via the specExtra arg to <cli>.bashCompletion()) and the second and subsequent positional args -- mycli foo banana <TAB> -- would use filename completion.

  • CLI.prototype.init(opts, args, cb) Hook run after option processing (this.opts is set), but before the subcommand handler is run.

  • CLI.prototype.fini(subcmd, err, cb) Hook run after the subcommand handler is run. Here err is the error returned by the invocation of the CLI. This allows a fini method to use or deal with that error, if necessary. To just pass that err on (to the calling main) do this:

      CLI.prototype.fini = function fini(subcmd, err, cb) {
          // Whatever finalization you want to do here (possibly with a
          // `finiErr`) ...
          cb(finiErr || err, subcmd);

    (Note: The call signature to fini changed in cmdln v3. See the changelog in

  • cli.showErrStack boolean. Set to true to have cmdln.main(), if used, print a full stack on a shown error. A common pattern of mine is to set this in the .init() method if a top-level -v,--verbose option is given.

  • cli.handlerFromSubcmd(<subcmd>) will return the appropriate do_<subcmd> method that handles the given sub-command. This resolves sub-command aliases.

  • cli.helpFromSubcmd(<subcmd>) will return the help string for that subcmd or, if defined, the help function defined for that subcmd. This is used by the default do_help implementation.

  • cli.bashCompletion() generates and returns bash completion for the CLI.


This is a convenience method for driving the mainline of your script using the your defined Cmdln subclass. There are a number of options to control how it works. Read the block comment on that function in "lib/cmdln.js" for the best docs.

errHelp and Errors

cmdln v4 introduced subcmd synopses, errHelp, and some related functionality to help provide brief automatic command help for some usage errors. errHelp is a brief message after a printed error, giving potentially helpful info. Some examples from familiar commands (marked here with >):

    $ ls -D
    ls: illegal option -- D
>   usage: ls [-ABCFGHLOPRSTUWabcdefghiklmnopqrstuwx1] [file ...]

    $ git foo
    git: 'foo' is not a git command. See 'git --help'.

>   Did you mean this?
>          fo

Use the following suggestions to get this kind of error-help for your commands:

  1. Optionally set synopses on your subcmd handlers. E.g.:

     do_list.synopses = ['{{name}} list [OPTIONS] FILTERS...'];

    Doing so allows two things: (a) the use of the {{usage}} template var in your command help, and (b) use of those synopses for errHelp.

  2. Optionally use the {{usage}} template var in your command help. E.g.: = [
         'List instances.',
  3. Optionally use the cmdln.UsageError error class for usage errors in your subcmds. E.g.:

     function do_list(subcmd, opts, args, callback) {
         // ...
         } else if (args.length < 1) {
             callback(new cmdln.UsageError('missing FILTER args'));
  4. Use cmdln.main() for your mainline

    This will now attempt to determine errHelp from any returned error and print it on stderr -- use options.showErrHelp=false to disable. Or if you are not using cmdln.main(), then you can use cmdln.errHelpFromErr(err) to get errHelp to print, if you like.

Error help is determined by calling err.cmdlnErrHelpFromErr(), which is implemented for cmdln's error classes:

  • cmdln.OptionError: Show a synopsis of the command's options.
  • cmdln.UsageError: Show the command's synopses, if available.
  • cmdln.UnknownCommandError: List possible fuzzy matches.

You can implement that method for custom error classes if you like.


This is a re-export of the dashdash option processing module that cmdln is using. This is exported so that calling code can add option types if wanted, via cmdln.dashdash.addOptionType. E.g.,

var cmdln = require('cmdln');

function parseCommaSepStringNoEmpties(option, optstr, arg) {
    return arg.trim().split(/\s*,\s*/g)
        .filter(function (part) { return part; });

    name: 'commaSepString',
    takesArg: true,
    helpArg: 'STRING',
    parseArg: parseCommaSepStringNoEmpties

// ...

See the node-dashdash documentation for details.