(pinning-nixpkgs)=

# Towards reproducibility: pinning Nixpkgs

In various Nix examples, you'll often see the following:

```nix
{ pkgs ? import <nixpkgs> {} }:

...
```

:::{note}
`<nixpkgs>` points to the file system path of some revision of {term}`Nixpkgs`.
Find more information on [lookup paths](lookup-path-tutorial) in [](reading-nix-language).
:::

This is a **convenient** way to quickly demonstrate a Nix expression and get it working by importing Nix packages.

However, <ref-search-path>**the resulting Nix expression is not fully reproducible**.

## Pinning packages with URLs inside a Nix expression

To create **fully reproducible** Nix expressions, we can pin an exact version of Nixpkgs.

The simplest way to do this is to fetch the required Nixpkgs version as a tarball specified via the relevant Git commit hash:

```nix
{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/06278c77b5d162e62df170fec307e83f1812d94b.tar.gz") {}
}:

...
```

Picking the commit can be done via [status.nixos.org](https://status.nixos.org/),
which lists all the releases and the latest commit that has passed all tests.

When choosing a commit, it is recommended to follow either

- the **latest stable NixOS** release by using a specific version, such as `nixos-21.05`, **or**
- the latest **unstable release** via `nixos-unstable`.

## Next steps

- For more examples and details of the different ways to pin `nixpkgs`, see {ref}`ref-pinning-nixpkgs`.
- [](dependency-management-npins)