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.

Intermediate results are not cached.
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 --args 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:
console
```
nix build nixpkgs.hello # Nix 2.3
```
Instead, you can either use the # syntax to select a package from a flake, e.g.
console
```
nix build nixpkgs#hello
```
Or, if you want to use the nixpkgs channel in the NIX_PATH environment variable:
console
```
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
console
```
nix run nixpkgs.hello -c hello # Nix 2.3
```
you should use
console
```
nix shell nixpkgs#hello -c hello
```
or just
console
```
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.