lix/src/nix/nix.md

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Examples

• Create a new flake:

  # nix flake new hello
  # cd hello
  

• Build the flake in the current directory:

  # nix build
  # ./result/bin/hello
  Hello, world!
  

• Run the flake in the current directory:

  # nix run
  Hello, world!
  

• Start a development shell for hacking on this flake:

  # nix develop
  # unpackPhase
  # cd hello-*
  # configurePhase
  # buildPhase
  # ./hello
  Hello, world!
  # installPhase
  # ../outputs/out/bin/hello
  Hello, world!
  

Description

Nix is a tool for building software, configurations and other artifacts in a reproducible and declarative way. For more information, see the Nix homepage or the Nix manual.

Installables

Many nix subcommands operate on one or more installables. These are command line arguments that represent something that can be built in the Nix store. Here are the recognised types of installables:

• Flake output attributes: nixpkgs#hello

  These have the form flakeref[#attrpath], where flakeref is a flake reference and attrpath is an optional attribute path. For more information on flakes, see the nix flake manual page. Flake references are most commonly a flake identifier in the flake registry (e.g. nixpkgs), or a raw path (e.g. /path/to/my-flake or . or ../foo), or a full URL (e.g. github:nixos/nixpkgs or path:.)

  When the flake reference is a raw path (a path without any URL scheme), it is interpreted in the following way:

  • If the supplied path does not contain flake.nix, then Nix searches for a directory containing flake.nix upwards of the supplied path (until a filesystem boundary or a git repository root). For example, if /foo/bar/flake.nix exists, then supplying /foo/bar/baz/ will find the directory /foo/bar/;
  • If flake.nix is in a Git repository, then this is essentially equivalent to git+file://<directory> (see the nix flake manual page), except that the dir parameter is derived automatically. For example, if /foo/bar is a Git repository and /foo/bar/baz contains flake.nix, then the flake reference /foo/bar/baz is equivalent to git+file:///foo/bar?dir=baz. Note that it will only include files indexed by git. In particular, files which are matched by .gitignore will not be available in the flake. If this is undesireable, specify path:<directory> explicitly;
  • If the directory is not inside a Git repository, then it is equivalent to path:<directory> (see the nix flake manual page), which includes the entire contents of the path.

  If attrpath is omitted, Nix tries some default values; for most subcommands, the default is defaultPackage.system (e.g. defaultPackage.x86_64-linux), but some subcommands have other defaults. If attrpath is specified, attrpath is interpreted as relative to one or more prefixes; for most subcommands, these are packages.system, legacyPackages.*system* and the empty prefix. Thus, on x86_64-linux nix build nixpkgs#hello will try to build the attributes packages.x86_64-linux.hello, legacyPackages.x86_64-linux.hello and hello.

• Store paths: /nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10

  These are paths inside the Nix store, or symlinks that resolve to a path in the Nix store.

• Store derivations: /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv

  Store derivations are store paths with extension .drv and are a low-level representation of a build-time dependency graph used internally by Nix. By default, if you pass a store derivation to a nix subcommand, it will operate on the output paths of the derivation. For example, nix path-info prints information about the output paths:

  # nix path-info --json /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv
  [{"path":"/nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10",…}]
  

  If you want to operate on the store derivation itself, pass the --derivation flag.

• Nix attributes: --file /path/to/nixpkgs hello

  When the -f / --file path option is given, installables are interpreted as attribute paths referencing a value returned by evaluating the Nix file path.

• Nix expressions: --expr '(import <nixpkgs> {}).hello.overrideDerivation (prev: { name = "my-hello"; })'.

  When the --expr option is given, all installables are interpreted as Nix expressions. You may need to specify --impure if the expression references impure inputs (such as <nixpkgs>).

For most commands, if no installable is specified, the default is ., i.e. Nix will operate on the default flake output attribute of the flake in the current directory.

Nix stores

Most nix subcommands operate on a Nix store.

TODO: list store types, options

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