# Frequently Asked Questions

## What is the origin of the name Nix?

> The name *Nix* is derived from the Dutch word *niks*, meaning *nothing*;
> build actions do not see anything that has not been explicitly declared as an input.
>
> &mdash; <cite>[Nix: A Safe and Policy-Free System for Software Deployment](https://edolstra.github.io/pubs/nspfssd-lisa2004-final.pdf), LISA XVIII, 2004</cite>

The Nix logo is inspired by [an idea for the Haskell logo](https://wiki.haskell.org/File:Sgf-logo-blue.png) and the fact that [*nix* is Latin for *snow*](https://nix-dev.science.uu.narkive.com/VDaaP1BY/nix-logo).

## What are flakes?

See [](flakes-definition).

(channel-branches)=
## Which channel branch should I use?

Nixpkgs and NixOS have both stable and rolling releases.

### Stable

Stable releases receive conservative updates to fix bugs or security vulnerabilities; otherwise package versions are not changed.
A new stable release is made every six months.

- On Linux (including NixOS and WSL), use [`nixos-*`](https://github.com/NixOS/nixpkgs/branches/all?query=nixos-).

  These branches point to commits where most Linux packages got pre-built and can be fetched from the binary cache.
  Furthermore, these commits passed the full NixOS test suite.

- On macOS/Darwin, use [`nixpkgs-*-darwin`](https://github.com/NixOS/nixpkgs/branches/all?query=nixpkgs-)

  These branches point to commits where most Darwin packages got pre-built and can be fetched from the binary cache.

- On any other platform it doesn't matter which one of the above is used.

  Hydra doesn't pre-build any binaries for other platforms.

All of these "channel branches" follow the corresponding [`release-*`](https://github.com/NixOS/nixpkgs/branches/all?query=release-) branch.

:::{admonition} Example
`nixos-23.05` and `nixpkgs-23.05-darwin` are both based on `release-23.05`.
:::

### Rolling

Rolling releases follow [`master`](https://github.com/NixOS/nixpkgs/branches/all?query=master), the main development branch.

- On Linux (including NixOS and WSL), use [`nixos-unstable`](https://github.com/NixOS/nixpkgs/branches/all?query=nixos-unstable).
- On any other platform, use [`nixpkgs-unstable`](https://github.com/NixOS/nixpkgs/branches/all?query=nixpkgs-unstable).

[`*-small`](https://github.com/NixOS/nixpkgs/branches/all?query=-small) channel branches have passed a smaller test suite, which means they are more up-to-date with respect to their base branch, but offer fewer stability guarantees.

:::{tip}
Consult the [`nix-channel`](https://nixos.org/manual/nix/unstable/command-ref/nix-channel) entry in the Nix Reference Manual for more information on channels, and the [Nixpkgs contributing guide](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#branch-conventions) on the Nixpkgs branching strategy.
:::

## Are there any impurities left in sandboxed builds?

Yes. There is:

- CPU architecture—great effort being made to avoid compilation of native instructions in favour of hardcoded supported ones.
- System's current time/date.
- The filesystem used for building (see also [`TMPDIR`](https://nix.dev/manual/nix/2.18/command-ref/env-common.html#env-TMPDIR)).
- Linux kernel parameters, such as:
  - [IPv6 capabilities](https://github.com/NixOS/nix/issues/5615).
  - binfmt interpreters, e.g., those configured with [`boot.binfmt.emulatedSystems`](https://search.nixos.org/options?show=boot.binfmt.emulatedSystems).
- Timing behaviour of the build system—parallel Make build does not get the correct inputs in some cases.
- Insertion of random values, e.g., from `/dev/random` or `/dev/urandom`.
- Differences between Nix versions. For instance, a new Nix version might introduce a new environment variable. A statement like `env > $out` is not promised by Nix to result in the same output, going into the future.