Nix Features: Transparent Deployment, Package Collection, Build Environments, Portability, NixOS, and License

functional language. A Nix expression describes everything that goes into a package build task (a “derivation”): other packages, sources, the build script, environment variables for the build script, etc. Nix tries very hard to ensure that Nix expressions are _deterministic_: building a Nix expression twice should yield the same result. Because it’s a functional language, it’s easy to support building variants of a package: turn the Nix expression into a function and call it any number of times with the appropriate arguments. Due to the hashing scheme, variants don’t conflict with each other in the Nix store. ## Transparent source/binary deployment Nix expressions generally describe how to build a package from source, so an installation action like ```console $ nix-env --install --attr nixpkgs.firefox ``` _could_ cause quite a bit of build activity, as not only Firefox but also all its dependencies (all the way up to the C library and the compiler) would have to be built, at least if they are not already in the Nix store. This is a _source deployment model_. For most users, building from source is not very pleasant as it takes far too long. However, Nix can automatically skip building from source and instead use a _binary cache_, a web server that provides pre-built binaries. For instance, when asked to build `/nix/store/b6gvzjyb2pg0…-firefox-33.1` from source, Nix would first check if the file `https://cache.nixos.org/b6gvzjyb2pg0….narinfo` exists, and if so, fetch the pre-built binary referenced from there; otherwise, it would fall back to building from source. ## Nix Packages collection We provide a large set of Nix expressions containing hundreds of existing Unix packages, the _Nix Packages collection_ (Nixpkgs). ## Managing build environments Nix is extremely useful for developers as it makes it easy to automatically set up the build environment for a package. Given a Nix expression that describes the dependencies of your package, the command `nix-shell` will build or download those dependencies if they’re not already in your Nix store, and then start a Bash shell in which all necessary environment variables (such as compiler search paths) are set. For example, the following command gets all dependencies of the Pan newsreader, as described by [its Nix expression](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix): ```console $ nix-shell '<nixpkgs>' --attr pan ``` You’re then dropped into a shell where you can edit, build and test the package: ```console [nix-shell]$ unpackPhase [nix-shell]$ cd pan-* [nix-shell]$ configurePhase [nix-shell]$ buildPhase [nix-shell]$ ./pan/gui/pan ``` ## Portability Nix runs on Linux and macOS. ## NixOS NixOS is a Linux distribution based on Nix. It uses Nix not just for package management but also to manage the system configuration (e.g., to build configuration files in `/etc`). This means, among other things, that it is easy to roll back the entire configuration of the system to an earlier state. Also, users can install software without root privileges. For more information and downloads, see the [NixOS homepage](https://nixos.org/). ## License Nix is released under the terms of the [GNU LGPLv2.1 or (at your option) any later version](http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html).

Nix expressions, written in a functional language, define package build tasks deterministically and support variants. Nix offers transparent source/binary deployment, using binary caches to avoid building from source. Nix provides a large package collection (Nixpkgs) and simplifies setting up build environments with `nix-shell`. Nix runs on Linux and macOS and is the base for NixOS, a Linux distribution that uses Nix for system configuration management. Nix is licensed under the GNU LGPLv2.1.