Nix 2.4 (2021-11-01) - Lix Reference Manual

## Keyboard shortcuts

Press `←` or `→` to navigate between chapters

Press `S` or `/` to search in the book

Press `?` to show this help

Press `Esc` to hide this help

# Release 2.4 (2021-11-01)

This is the first release in more than two years and is the result of more than 2800 commits from 195 contributors since release 2.3.

## Highlights

- Nix's error messages have been improved a lot. For instance, evaluation errors now point out the location of the error:

      $ nix build
      error: undefined variable 'bzip3'

             at /nix/store/449lv242z0zsgwv95a8124xi11sp419f-source/flake.nix:88:13:

                 87|           [ curl
                 88|             bzip3 xz brotli editline
                   |             ^
                 89|             openssl sqlite

- The `nix` command has seen a lot of work and is now almost at feature parity with the old command-line interface (the `nix-*` commands). It aims to be more modern, consistent and pleasant to use than the old CLI. It is still marked as experimental but its interface should not change much anymore in future releases.

- Flakes are a new format to package Nix-based projects in a more discoverable, composable, consistent and reproducible way. A flake is just a repository or tarball containing a file named `flake.nix` that specifies dependencies on other flakes and returns any Nix assets such as packages, Nixpkgs overlays, NixOS modules or CI tests. The new `nix` CLI is primarily based around flakes; for example, a command like `nix run nixpkgs#hello` runs the `hello` application from the `nixpkgs` flake.

  Flakes are currently marked as experimental. For an introduction, see this blog post. For detailed information about flake syntax and semantics, see the `nix flake` manual page.

- Nix's store can now be content-addressed, meaning that the hash component of a store path is the hash of the path's contents. Previously Nix could only build input-addressed store paths, where the hash is computed from the derivation dependency graph. Content-addressing allows deduplication, early cutoff in build systems, and unprivileged closure copying. This is still an experimental feature.

- The Nix manual has been converted into Markdown, making it easier to contribute. In addition, every `nix` subcommand now has a manual page, documenting every option.

- A new setting that allows experimental features to be enabled selectively. This allows us to merge unstable features into Nix more quickly and do more frequent releases.

## Other features

- There are many new `nix` subcommands:

  - `nix develop` is intended to replace `nix-shell`. It has a number of new features:

    - It automatically sets the output environment variables (such as `$out`) to writable locations (such as `./outputs/out`).

    - It can store the environment in a profile. This is useful for offline work.

    - It can run specific phases directly. For instance, `nix develop --build` runs `buildPhase`.

    - It allows dependencies in the Nix store to be "redirected" to arbitrary directories using the `--redirect` flag. This is useful if you want to hack on a package and some of its dependencies at the same time.

  - `nix print-dev-env` prints the environment variables and bash functions defined by a derivation. This is useful for users of other shells than bash (especially with `--json`).

  - `nix shell` was previously named `nix run` and is intended to replace `nix-shell -p`, but without the `stdenv` overhead. It simply starts a shell where some packages have been added to `$PATH`.

  - `nix run` (not to be confused with the old subcommand that has been renamed to `nix shell`) runs an "app", a flake output that specifies a command to run, or an eponymous program from a package. For example, `nix run nixpkgs#hello` runs the `hello` program from the `hello` package in `nixpkgs`.

  - `nix flake` is the container for flake-related operations, such as creating a new flake, querying the contents of a flake or updating flake lock files.

  - `nix registry` allows you to query and update the flake registry, which maps identifiers such as `nixpkgs` to concrete flake URLs.

  - `nix profile` is intended to replace `nix-env`. Its main advantage is that it keeps track of the provenance of installed packages (e.g. exactly which flake version a package came from). It also has some helpful subcommands:

    - `nix profile history` shows what packages were added, upgraded or removed between each version of a profile.

    - `nix profile diff-closures` shows the changes between the closures of each version of a profile. This allows you to discover the addition or removal of dependencies or size changes.

    Warning: after a profile has been updated using `nix profile`, it is no longer usable with `nix-env`.

  - `nix store diff-closures` shows the differences between the closures of two store paths in terms of the versions and sizes of dependencies in the closures.

  - `nix store make-content-addressable` rewrites an arbitrary closure to make it content-addressed. Such paths can be copied into other stores without requiring signatures.

  - `nix bundle` uses the `nix-bundle` program to convert a closure into a self-extracting executable.

  - Various other replacements for the old CLI, e.g. `nix store gc`, `nix store delete`, `nix store repair`, `nix nar dump-path`, `nix store prefetch-file`, `nix store prefetch-tarball`, `nix key` and `nix daemon`.

- Nix now has an evaluation cache for flake outputs. For example, a second invocation of the command `nix run nixpkgs#firefox` will not need to evaluate the `firefox` attribute because it's already in the evaluation cache. This is made possible by the hermetic evaluation model of flakes.

- The new `--offline` flag disables substituters and causes all locally cached tarballs and repositories to be considered up-to-date.

- The new `--refresh` flag causes all locally cached tarballs and repositories to be considered out-of-date.

- Many `nix` subcommands now have a `--json` option to produce machine-readable output.

- `nix repl` has a new `:doc` command to show documentation about builtin functions (e.g. `:doc builtins.map`).

- Binary cache stores now have an option `index-debug-info` to create an index of DWARF debuginfo files for use by `dwarffs`.

- To support flakes, Nix now has an extensible mechanism for fetching source trees. Currently it has the following backends:

  - Git repositories

  - Mercurial repositories

  - GitHub and GitLab repositories (an optimisation for faster fetching than Git)

  - Tarballs

  - Arbitrary directories

  The fetcher infrastructure is exposed via flake input specifications and via the `fetchTree` built-in.

- Languages changes: the only new language feature is that you can now have antiquotations in paths, e.g. `./${foo}` instead of `./. + foo`.

- New built-in functions:

  - `builtins.fetchTree` allows fetching a source tree using any backends supported by the fetcher infrastructure. It subsumes the functionality of existing built-ins like `fetchGit`, `fetchMercurial` and `fetchTarball`.

  - `builtins.getFlake` fetches a flake and returns its output attributes. This function should not be used inside flakes! Use flake inputs instead.

  - `builtins.floor` and `builtins.ceil` round a floating-point number down and up, respectively.

- Experimental support for recursive Nix. This means that Nix derivations can now call Nix to build other derivations. This is not in a stable state yet and not well documented.

- The new experimental feature `no-url-literals` disables URL literals. This helps to implement RFC 45.

- Nix now uses `libarchive` to decompress and unpack tarballs and zip files, so `tar` is no longer required.

- The priority of substituters can now be overridden using the `priority` substituter setting (e.g. `--substituters 'http://cache.nixos.org?priority=100 daemon?priority=10'`).

- `nix edit` now supports non-derivation attributes, e.g. `nix edit .#nixosConfigurations.bla`.

- The `nix` command now provides command line completion for `bash`, `zsh` and `fish`. Since the support for getting completions is built into `nix`, it's easy to add support for other shells.

- The new `--log-format` flag selects what Nix's output looks like. It defaults to a terse progress indicator. There is a new `internal-json` output format for use by other programs.

- `nix eval` has a new `--apply` flag that applies a function to the evaluation result.

- `nix eval` has a new `--write-to` flag that allows it to write a nested attribute set of string leaves to a corresponding directory tree.

- Memory improvements: many operations that add paths to the store or copy paths between stores now run in constant memory.

- Many `nix` commands now support the flag `--derivation` to operate on a `.drv` file itself instead of its outputs.

- There is a new store called `dummy://` that does not support building or adding paths. This is useful if you want to use the Nix evaluator but don't have a Nix store.

- The `ssh-ng://` store now allows substituting paths on the remote, as `ssh://` already did.

- When auto-calling a function with an ellipsis, all arguments are now passed.

- New `nix-shell` features:

  - It preserves the `PS1` environment variable if `NIX_SHELL_PRESERVE_PROMPT` is set.

  - With `-p`, it passes any `--arg`s as Nixpkgs arguments.

  - Support for structured attributes.

- `nix-prefetch-url` has a new `--executable` flag.

- On `x86_64` systems, `x86_64` microarchitecture levels are mapped to additional system types (e.g. `x86_64-v1-linux`).

- The new `--eval-store` flag allows you to use a different store for evaluation than for building or storing the build result. This is primarily useful when you want to query whether something exists in a read-only store, such as a binary cache:

      # nix path-info --json --store https://cache.nixos.org \
        --eval-store auto nixpkgs#hello

  (Here `auto` indicates the local store.)

- The Nix daemon has a new low-latency mechanism for copying closures. This is useful when building on remote stores such as `ssh-ng://`.

- Plugins can now register `nix` subcommands.

- The `--indirect` flag to `nix-store --add-root` has become a no-op. `--add-root` will always generate indirect GC roots from now on.

## Incompatible changes

- The `nix` command is now marked as an experimental feature. This means that you need to add

      experimental-features = nix-command

  to your `nix.conf` if you want to use it, or pass `--extra-experimental-features nix-command` on the command line.

- The `nix` command no longer has a syntax for referring to packages in a channel. This means that the following no longer works:

      nix build nixpkgs.hello # Nix 2.3

  Instead, you can either use the `#` syntax to select a package from a flake, e.g.

      nix build nixpkgs#hello

  Or, if you want to use the `nixpkgs` channel in the `NIX_PATH` environment variable:

      nix build -f '<nixpkgs>' hello

- The old `nix run` has been renamed to `nix shell`, while there is a new `nix run` that runs a default command. So instead of

      nix run nixpkgs.hello -c hello # Nix 2.3

  you should use

      nix shell nixpkgs#hello -c hello

  or just

      nix run nixpkgs#hello

  if the command you want to run has the same name as the package.

- It is now an error to modify the `plugin-files` setting via a command-line flag that appears after the first non-flag argument to any command, including a subcommand to `nix`. For example, `nix-instantiate default.nix --plugin-files ""` must now become `nix-instantiate --plugin-files "" default.nix`.

- We no longer release source tarballs. If you want to build from source, please build from the tags in the Git repository.

## Contributors

This release has contributions from Adam Höse, Albert Safin, Alex Kovar, Alex Zero, Alexander Bantyev, Alexandre Esteves, Alyssa Ross, Anatole Lucet, Anders Kaseorg, Andreas Rammhold, Antoine Eiche, Antoine Martin, Arnout Engelen, Arthur Gautier, aszlig, Ben Burdette, Benjamin Hipple, Bernardo Meurer, Björn Gohla, Bjørn Forsman, Bob van der Linden, Brian Leung, Brian McKenna, Brian Wignall, Bruce Toll, Bryan Richter, Calle Rosenquist, Calvin Loncaric, Carlo Nucera, Carlos D'Agostino, Chaz Schlarp, Christian Höppner, Christian Kampka, Chua Hou, Chuck, Cole Helbling, Daiderd Jordan, Dan Callahan, Dani, Daniel Fitzpatrick, Danila Fedorin, Daniël de Kok, Danny Bautista, DavHau, David McFarland, Dima, Domen Kožar, Dominik Schrempf, Dominique Martinet, dramforever, Dustin DeWeese, edef, Eelco Dolstra, Ellie Hermaszewska, Emilio Karakey, Emily, Eric Culp, Ersin Akinci, Fabian Möller, Farid Zakaria, Federico Pellegrin, Finn Behrens, Florian Franzen, Félix Baylac-Jacqué, Gabriella Gonzalez, Geoff Reedy, Georges Dubus, Graham Christensen, Greg Hale, Greg Price, Gregor Kleen, Gregory Hale, Griffin Smith, Guillaume Bouchard, Harald van Dijk, illustris, Ivan Zvonimir Horvat, Jade, Jake Waksbaum, jakobrs, James Ottaway, Jan Tojnar, Janne Heß, Jaroslavas Pocepko, Jarrett Keifer, Jeremy Schlatter, Joachim Breitner, Joe Pea, John Ericson, Jonathan Ringer, Josef Kemetmüller, Joseph Lucas, Jude Taylor, Julian Stecklina, Julien Tanguy, Jörg Thalheim, Kai Wohlfahrt, keke, Keshav Kini, Kevin Quick, Kevin Stock, Kjetil Orbekk, Krzysztof Gogolewski, kvtb, Lars Mühmel, Leonhard Markert, Lily Ballard, Linus Heckemann, Lorenzo Manacorda, Lucas Desgouilles, Lucas Franceschino, Lucas Hoffmann, Luke Granger-Brown, Madeline Haraj, Marwan Aljubeh, Mat Marini, Mateusz Piotrowski, Matthew Bauer, Matthew Kenigsberg, Mauricio Scheffer, Maximilian Bosch, Michael Adler, Michael Bishop, Michael Fellinger, Michael Forney, Michael Reilly, mlatus, Mykola Orliuk, Nathan van Doorn, Naïm Favier, ng0, Nick Van den Broeck, Nicolas Stig124 Formichella, Niels Egberts, Niklas Hambüchen, Nikola Knezevic, oxalica, p01arst0rm, Pamplemousse, Patrick Hilhorst, Paul Opiyo, Pavol Rusnak, Peter Kolloch, Philipp Bartsch, Philipp Middendorf, Piotr Szubiakowski, Profpatsch, Puck Meerburg, Ricardo M. Correia, Rickard Nilsson, Robert Hensing, Robin Gloster, Rodrigo, Rok Garbas, Ronnie Ebrin, Rovanion Luckey, Ryan Burns, Ryan Mulligan, Ryne Everett, Sam Doshi, Sam Lidder, Samir Talwar, Samuel Dionne-Riel, Sebastian Ullrich, Sergei Trofimovich, Sevan Janiyan, Shao Cheng, Shea Levy, Silvan Mosberger, Stefan Frijters, Stefan Jaax, sternenseemann, Steven Shaw, Stéphan Kochen, SuperSandro2000, Suraj Barkale, Taeer Bar-Yam, Thomas Churchman, Théophane Hufschmitt, Timothy DeHerrera, Timothy Klim, Tobias Möst, Tobias Pflug, Tom Bereknyei, Travis A. Everett, Ujjwal Jain, Vladimír Čunát, Wil Taylor, Will Dietz, Yaroslav Bolyukin, Yestin L. Harrison, YI, Yorick van Pelt, Yuriy Taraday and zimbatm.