lets

name	lets
description	Use lets, the YAML-based CLI task runner. Activate when a repository has lets.yaml or the user asks to run, inspect, add, or debug lets tasks, commands, dependencies, options, or mixins.
license	MIT

lets Task Runner

Use this skill when working in a project that uses lets, usually indicated by a lets.yaml file.

Workflow

1. Inspect lets.yaml before changing or running tasks. Also check all files declared in mixins; those files extend or override the main config. Mixins prefixed with - are optional and may be gitignored.
2. Use lets --help to list commands and global flags.
3. Use lets help <command> to inspect a command's description, options, and usage before running it.
4. Prefer project-defined lets commands for build, test, lint, format, docs, and release workflows instead of invoking underlying tools directly.
5. After editing config, run the affected lets command or the repository's standard test/lint task.

Config Basics

The main config is usually lets.yaml. Create one with lets --init if the project does not have it.

Minimal config:

shell: bash

commands:
  test:
    description: Run tests
    cmd: go test ./...

Top-level fields:

• version: minimum required lets version.
• shell: default shell for commands; commonly bash.
• env: global environment available to every command.
• env_file: dotenv files loaded for every command.
• before: script prepended to every command and dependency invocation.
• init: script run once per lets invocation before the first command.
• mixins: additional lets config files or remote mixins.
• commands: map of task names to command definitions.

Writing Commands

Use short syntax for simple commands:

commands:
  fmt: go fmt ./...

Use long syntax when you need descriptions, dependencies, env, options, checksums, or cleanup:

commands:
  test:
    description: Run unit tests
    depends: [generate]
    env:
      GOFLAGS: -count=1
    cmd: go test ./...

Command fields:

• cmd: shell command to run.
• description: shown in command help.
• work_dir: run from a path relative to the config directory.
• shell: override shell for one command.
• after: cleanup script that runs even if cmd fails.
• depends: commands that must run first.
• env: command-specific environment.
• options: docopt-style CLI options exposed as LETSOPT_* and LETSCLI_* environment variables.
• env_file: command-specific dotenv files.
• checksum and persist_checksum: calculate file checksums and detect whether inputs changed.
• ref and args: reuse another command with arguments.
• group: organize commands in help output.

cmd can be a string, multiline string, array, or experimental map:

commands:
  build:
    cmd: |
      echo "Building"
      go build ./cmd/lets

  test:
    cmd:
      - go
      - test
      - ./...

  dev:
    cmd:
      api: go run ./cmd/api
      web: npm run dev

With array cmd, extra CLI args are appended. For example lets test -run TestName runs go test ./... -run TestName.

For map cmd, users can select entries with global flags before the command name, such as lets --only api dev or lets --exclude web dev.

Options And Arguments

Use options for user-facing command arguments. It is a docopt usage block.

commands:
  release:
    description: Create a release
    options: |
      Usage: lets release <version> [--dry-run] [--message=<message>]

      Options:
        <version>             Version to release
        --dry-run             Print actions without changing anything
        --message=<message>   Release message
    cmd: |
      if [[ -n "${LETSOPT_DRY_RUN}" ]]; then
        echo "Dry run for ${LETSOPT_VERSION}"
      fi
      echo "Message: ${LETSOPT_MESSAGE}"

Rules:

• Positional args become LETSOPT_<NAME>, for example <version> becomes LETSOPT_VERSION.
• Long flags become uppercase with - converted to _, for example --dry-run becomes LETSOPT_DRY_RUN.
• LETSOPT_* contains parsed values such as true, staging, or positional args.
• LETSCLI_* contains the raw CLI fragment, useful when forwarding flags to another command.
• Use kebab-case for CLI flags, not snake_case.

Environment

Global env applies to all commands. Command env extends or overrides it.

env:
  TARGET: dev
  IMAGE:
    sh: echo "app-${TARGET}"

commands:
  build:
    env:
      TAG:
        sh: git rev-parse --short HEAD
    cmd: docker build -t "${IMAGE}:${TAG}" .

Environment entries are evaluated in declaration order, so later entries can reference earlier ones. Command env can also reference global env.

Use env_file to load dotenv files:

env:
  TARGET: dev

env_file:
  - .env
  - -.env.local

commands:
  up:
    env_file:
      - .env.${TARGET}
      - name: .env.required
        required: true
    cmd: docker compose up

Rules:

• -filename means optional, equivalent to required: false.
• env_file paths are resolved relative to the config directory, not work_dir.
• Values from env files override values from env.
• File names are expanded after available env values are resolved.

Dependencies And Cleanup

Use depends for prerequisite commands. Dependencies run before the command.

commands:
  build:
    cmd: docker build -t app .

  test:
    depends: [build]
    cmd: go test ./...

Dependencies can pass args or env to the dependency command:

commands:
  test:
    depends:
      - name: build
        args: [--verbose]
        env:
          TARGET: test
    cmd: go test ./...

Use after for cleanup that must happen even when the command fails:

commands:
  redis:
    cmd: docker compose up redis
    after: docker compose stop redis

Use init for setup that should run once per lets invocation. Avoid heavy before scripts because before runs before each command and dependency.

Checksums

Use checksum when command behavior depends on file contents. lets calculates SHA1 checksums and exposes them as env vars.

commands:
  deps:
    checksum:
      deps:
        - package.json
        - package-lock.json
    persist_checksum: true
    cmd: |
      if [[ "${LETS_CHECKSUM_DEPS_CHANGED}" == "true" ]]; then
        npm install
      fi

Rules:

• A list checksum exposes LETS_CHECKSUM.
• A named checksum map exposes LETS_CHECKSUM_<NAME> plus combined LETS_CHECKSUM.
• persist_checksum: true exposes LETS_CHECKSUM_CHANGED and named LETS_CHECKSUM_<NAME>_CHANGED.
• Persisted checksums update only after a successful command exit.
• Glob patterns are supported in checksum file lists.

Mixins

Use mixins to split large configs or share common commands.

shell: bash

mixins:
  - lets.build.yaml
  - -lets.my.yaml

commands:
  test:
    cmd: go test ./...

Rules:

• Local mixins are paths to other lets config files.
• A mixin name prefixed with - is optional and may not exist; this is useful for gitignored personal config.
• Remote mixins use url and optional version; lets caches them under .lets/mixins.
• When editing a command, search mixins too because commands may be defined, extended, or overridden there.

Reusing Commands

Use experimental ref and args to create aliases with predefined arguments.

commands:
  hello:
    cmd: echo Hello $@

  hello-world:
    ref: hello
    args: World

ref is only compatible with args; do not combine it with normal command directives.

Use group to organize help output:

commands:
  build:
    group: Build
    cmd: npm run build

Common Pitfalls

• Do not commit .lets/, generated binaries, coverage files, dependency directories, or personal mixins such as lets.my.yaml unless the repository explicitly tracks them.
• Prefer one clear command over several near-duplicate commands. Use options, args, or ref when that keeps the config simpler.
• Keep description first-line concise; help output uses the first line.
• Use multiline cmd: | for scripts with conditionals or multiple shell statements.
• Quote env values that look like booleans or numbers when they must stay strings.
• Remember work_dir changes where cmd runs, but env_file remains relative to the config directory.
• Prefer project-defined lets tasks for validation, for example lets test, lets lint, or lets fmt when present.