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πŸ’» A type-driven command line argument parser, with awesome error reporting 🀀

Not all command line arguments are strings, but for some reason, our CLI parsers force us to use strings everywhere. πŸ€” cmd-ts is a fully-fledged command line argument parser, influenced by Rust's clap and structopt:

🀩 Awesome autocomplete, awesome safeness

🎭 Decode your own custom types from strings with logic and context-aware error handling

🌲 Nested subcommands, composable API

Basic usage

import { command, run, string, number, positional, option } from 'cmd-ts';

const cmd = command({
  name: 'my-command',
  description: 'print something to the screen',
  version: '1.0.0',
  args: {
    number: positional({ type: number, displayName: 'num' }),
    message: option({
      long: 'greeting',
      type: string,
  handler: (args) => {
    args.message; // string
    args.number; // number

run(cmd, process.argv.slice(2));


Creates a CLI command.

Decoding custom types from strings

Not all command line arguments are strings. You sometimes want integers, UUIDs, file paths, directories, globs...

Note: this section describes the ReadStream type, implemented in ./src/example/test-types.ts

Let's say we're about to write a cat clone. We want to accept a file to read into stdout. A simple example would be something like:

// my-app.ts

import { command, run, positional, string } from 'cmd-ts';

const app = command({
  /// name: ...,
  args: {
    file: positional({ type: string, displayName: 'file' }),
  handler: ({ file }) => {
    // read the file to the screen

// parse arguments
run(app, process.argv.slice(2));

That works okay. But we can do better. In which ways?

  • Error handling is out of the command line argument parser context, and in userland, making things less consistent and pretty.
  • It shows we lack composability and encapsulation β€” and we miss a way to distribute shared "command line" behavior.

What if we had a way to get a Stream out of the parser, instead of a plain string? This is where cmd-ts gets its power from, custom type decoding:

// ReadStream.ts

import { Type } from 'cmd-ts';
import fs from 'fs';

// Type<string, Stream> reads as "A type from `string` to `Stream`"
const ReadStream: Type<string, Stream> = {
  async from(str) {
    if (!fs.existsSync(str)) {
      // Here is our error handling!
      throw new Error('File not found');

    return fs.createReadStream(str);

Now we can use (and share) this type and always get a Stream, instead of carrying the implementation detail around:

// my-app.ts

import { command, run, positional } from 'cmd-ts';

const app = command({
  // name: ...,
  args: {
    stream: positional({ type: ReadStream, displayName: 'file' }),
  handler: ({ stream }) => stream.pipe(process.stdout),

// parse arguments
run(app, process.argv.slice(2));

Encapsulating runtime behaviour and safe type conversions can help us with awesome user experience:

  • We can throw an error when the file is not found
  • We can try to parse the string as a URI and check if the protocol is HTTP, if so - make an HTTP request and return the body stream
  • We can see if the string is -, and when it happens, return process.stdin like many Unix applications

And the best thing about it β€” everything is encapsulated to an easily tested type definition, which can be easily shared and reused. Take a look at io-ts-types, for instance, which has types like DateFromISOString, NumberFromString and more, which is something we can totally do.


This project was previously called clio-ts, because it was based on io-ts. This is no longer the case, because I want to reduce the dependency count and mental overhead. I might have a function to migrate types between the two.