devenv 1.6: Extensible Ad-Hoc Nix Environments

Apr 25, 2025

devenv 1.6 has been tagged, allowing you to:

Create temporary environments directly from the command line without requiring a devenv.nix file.
Temporarily modify existing environments.

Create Environments on the Fly

Developer environments on demand using the new --option (-O) flag:

$ devenv --option languages.python.enable:bool true \
         --option packages:pkgs "ncdu git ripgrep" \
         shell

This command creates a temporary Python environment without writing any configuration files.

Ad-hoc environments are ideal for quickly testing languages or tools without committing to a full project setup:

$ devenv -O languages.elixir.enable:bool true shell iex

Supported Option Types

The --option flag supports multiple data types, making it flexible for various use cases:

:string for text values
:int for integers
:float for decimal numbers
:bool for true/false values
:path for file paths
:pkgs for specifying Nix packages

GitHub Actions with Matrices

One of the most powerful applications of ad-hoc environments is in CI pipelines, where you can easily implement testing matrices across different configurations:

jobs:
  test:
    strategy:
      matrix:
        python-version: ['3.9', '3.10', '3.11']
    steps:
      - uses: actions/checkout@v3
      - uses: cachix/install-nix-action@v31
      - uses: cachix/cachix-action@v16
        with:
          name: devenv
      - name: Install devenv.sh
        run: nix profile install nixpkgs#devenv
      - name: Test with Python {{ '${{ matrix.python-version }}' }}
        run: |
          devenv --option languages.python.enable:bool true \
                 --option languages.python.version:string {{ '${{ matrix.python-version }}' }} \
                 test

This approach lets you validate your code across multiple language versions or dependency combinations without maintaining separate configuration files for each scenario.

Combining with Existing Configurations

When used with an existing devenv.nix file, --option values override the configuration settings in the file, making it easy to temporarily modify your environment.

Switching Between Environment Profiles

Ad-hoc options are perfect for switching between predefined profiles in your development environment:

$ devenv --option profile:string backend up

This enables you to switch between frontend, backend, or other custom profiles without modifying your configuration files.

See our Profiles guide for more details on setting up and using profiles.

For complete documentation on this feature, visit our Ad-hoc Developer Environments guide.

We’re excited to see how you’ll use ad-hoc environments to streamline your development workflow. Share your feedback on GitHub or join our Discord community!