Ad-hoc Developer Environments

Added in 1.6

Instead of creating and maintaining a devenv.nix file, you can create ad-hoc developer environments directly from the command line using the --option flag.

Basic Usage

You can specify any configuration option using the --option flag, allowing you to create temporary environments with specific tools and settings:

$ devenv --option languages.python.enable:bool true \
         --option languages.python.version:string "3.10" \
         shell

This creates a temporary Python development environment without needing any configuration files.

Option Types

The --option flag requires you to specify the inferred Nix type:

:string for string values
:int for integer values
:float for floating-point values
:bool for boolean values (true/false)
:path for file paths (interpreted as relative paths)
:pkg for a package
:pkgs for lists of packages (space-separated package names)

Installing Packages

To install packages from nixpkgs, use the special :pkgs type with the packages option:

$ devenv --option packages:pkgs "ncdu git ripgrep" shell

This creates an environment with ncdu, git, and ripgrep available, without needing a devenv.nix file.

Enabling Languages and Services

You can enable languages and services with ad-hoc environments:

$ devenv --option languages.rust.enable:bool true \
         --option languages.rust.channel:string nightly \
         shell

Running Ad-hoc Commands

You can also run specific commands directly in your ad-hoc environment:

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

This example launches an Elixir interactive shell (iex) immediately after creating the environment.

Replacing vs. Appending Lists

List types like :pkgs append to existing values by default. To replace the entire list instead, add a ! suffix to the type:

$ devenv -O packages:pkgs! "ncdu git" shell

This replaces all packages rather than appending to whatever devenv.nix already defines.

Combining with `devenv.nix`

When used with an existing devenv.nix file, --option values will override the configuration in the file.

Use Cases

Ad-hoc environments are particularly useful for:

One-off tasks that need specific dependencies
Trying different configurations before committing to a devenv.nix file
Creating lightweight environments without project setup
Creating a matrix of different options, e.g. testing different versions of Python

Limitations

While ad-hoc environments are convenient, they have some limitations:

Complex configurations are better managed in a devenv.nix file
Some complex options may be harder to express on the command line