# nixos-rebuild-ng

Work-in-Progress rewrite of
[`nixos-rebuild`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh).

## Why the rewrite?

The current state of `nixos-rebuild` is dire: it is one of the most critical
pieces of code we have in NixOS, but it has tons of issues:
- The code is written in Bash, and while this by itself is not necessary bad,
  it means that it is difficult to do refactorings due to the lack of tooling
  for the language
- The code itself is a hacky mess. Changing even one line of code can cause
  issues that affect dozens of people
- Lack of proper testing (we do have some integration tests, but no unit tests
  and coverage is probably pitiful)
- The code predates some of the improvements `nix` had over the years, e.g., it
  builds Flakes inside a temporary directory and reads the resulting symlink
  since the code seems to predate `--print-out-paths` flag

Given all of those above, improvements in the `nixos-rebuild` are difficult to
do. A full rewrite is probably the easier way to improve the situation since
this can be done in a separate package that will not break anyone. So this is
an attempt of the rewrite.

## Why Python?

- It is the language of choice for many critical things inside `nixpkgs`, like
  the `NixOSTest` and `systemd-boot-builder.py` activation scripts
- It is a language with great tooling, e.g.: `mypy` for type checking, `ruff`
  for linting, `pytest` for unit testing
- It is a scripting language that fits well with the scope of this project
- Python's standard library is great and it means we will need a low number of
  external dependencies for this project. For example, `nixos-rebuild`
  currently depends on `jq` for JSON parsing, while Python has `json` in
  standard library

## Do's and Don'ts

- Do: be as much of a drop-in replacement as possible
- Do: fix obvious bugs
- Do: improvements that are non-breaking
- Don't: change logic in breaking ways even if this would be an improvement

## How to use

If you want to use `nixos-rebuild-ng` without replacing `nixos-rebuild`, add the
following to your NixOS configuration:

```nix
{ pkgs, ... }:
{
  environment.systemPackages = [ pkgs.nixos-rebuild-ng ];
}
```

And use `nixos-rebuild-ng` instead of `nixos-rebuild`.

If you want to completely replace `nixos-rebuild` with `nixos-rebuild-ng`, add
the following to your NixOS configuration:

```nix
{ ... }:
{
  system.rebuild.enableNg = true;
}
```

This will set `config.system.build.nixos-rebuild` to `nixos-rebuild-ng`, so
all tools that expect it in that location should work.

## Development

Run:

```console
nix-build -A nixos-rebuild-ng -A nixos-rebuild-ng.tests.linters
```

The command above will build, run the unit tests and linters, and also check if
the code is formatted. However, sometimes is more convenient to run just a few
tests to debug, in this case you can run:

```console
nix-shell -A nixos-rebuild-ng.devShell
```

The command above should automatically put you inside `src` directory, and you
can run:

```console
# run program
python -m nixos_rebuild
# run tests
pytest
# check types
mypy .
# fix lint issues
ruff check --fix .
# format code
ruff format .
```

## Breaking changes