nix-super/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

Warning

Installables are part of the unstable nix-command experimental feature, and subject to change without notice.

Many nix subcommands operate on one or more installables. These are command line arguments that represent something that can be realised in the Nix store.

The following types of installable are supported by most commands:

• Flake output attribute (experimental)
• Store path
• Nix file, optionally qualified by an attribute path
• Nix expression, optionally qualified by an attribute path

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

Flake output attribute

Warning

Flake output attribute installables depend on both the flakes and nix-command experimental features, and subject to change without notice.

Example: 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 as a path: or git+file: url in the following way:

• If the path is within a Git repository, then the url will be of the form git+file://[GIT_REPO_ROOT]?dir=[RELATIVE_FLAKE_DIR_PATH] where GIT_REPO_ROOT is the path to the root of the git repository, and RELATIVE_FLAKE_DIR_PATH is the path (relative to the directory root) of the closest parent of the given path that contains a flake.nix within the git repository. If no such directory exists, then Nix will error-out.

  Note that the search will only include files indexed by git. In particular, files which are matched by .gitignore or have never been git add-ed will not be available in the flake. If this is undesirable, specify path:<directory> explicitly;

  For example, if /foo/bar is a git repository with the following structure:

  .
  └── baz
      ├── blah
      │   └── file.txt
      └── flake.nix
  

  Then /foo/bar/baz/blah will resolve to git+file:///foo/bar?dir=baz

• If the supplied path is not a git repository, then the url will have the form path:FLAKE_DIR_PATH where FLAKE_DIR_PATH is the closest parent of the supplied path that contains a flake.nix file (within the same file-system). If no such directory exists, then Nix will error-out.

  For example, if /foo/bar/flake.nix exists, then /foo/bar/baz/ will resolve to path:/foo/bar

If attrpath is omitted, Nix tries some default values; for most subcommands, the default is packages.system.default (e.g. packages.x86_64-linux.default), 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.

If attrpath begins with . then no prefixes or defaults are attempted. This allows the form flakeref[#.attrpath], such as github:NixOS/nixpkgs#.lib.fakeSha256 to avoid a search of packages.*system*.lib.fakeSha256

Store path

Example: /nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10

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

A store derivation is also addressed by store path.

Example: /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv

If you want to refer to an output path of that store derivation, add the output name preceded by a caret (^).

Example: /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv^out

All outputs can be referred to at once with the special syntax ^*.

Example: /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv^*

Nix file

Example: --file /path/to/nixpkgs hello

When the option -f / --file path [attrpath...] is given, installables are interpreted as the value of the expression in the Nix file at path. If attribute paths are provided, commands will operate on the corresponding values accessible at these paths. The Nix expression in that file, or any selected attribute, must evaluate to a derivation.

Nix expression

Example: --expr 'import <nixpkgs> {}' hello

When the option --expr expression [attrpath...] is given, installables are interpreted as the value of the of the Nix expression. If attribute paths are provided, commands will operate on the corresponding values accessible at these paths. The Nix expression, or any selected attribute, must evaluate to a derivation.

You may need to specify --impure if the expression references impure inputs (such as <nixpkgs>).

Derivation output selection

Derivations can have multiple outputs, each corresponding to a different store path. For instance, a package can have a bin output that contains programs, and a dev output that provides development artifacts like C/C++ header files. The outputs on which nix commands operate are determined as follows:

• You can explicitly specify the desired outputs using the syntax installable^output1,...,outputN. For example, you can obtain the dev and static outputs of the glibc package:

  # nix build 'nixpkgs#glibc^dev,static'
  # ls ./result-dev/include/ ./result-static/lib/
  …
  

  and likewise, using a store path to a "drv" file to specify the derivation:

  # nix build '/nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev,static'
  …
  

• You can also specify that all outputs should be used using the syntax installable^*. For example, the following shows the size of all outputs of the glibc package in the binary cache:

  # nix path-info --closure-size --eval-store auto --store https://cache.nixos.org 'nixpkgs#glibc^*'
  /nix/store/g02b1lpbddhymmcjb923kf0l7s9nww58-glibc-2.33-123                 33208200
  /nix/store/851dp95qqiisjifi639r0zzg5l465ny4-glibc-2.33-123-bin             36142896
  /nix/store/kdgs3q6r7xdff1p7a9hnjr43xw2404z7-glibc-2.33-123-debug          155787312
  /nix/store/n4xa8h6pbmqmwnq0mmsz08l38abb06zc-glibc-2.33-123-static          42488328
  /nix/store/q6580lr01jpcsqs4r5arlh4ki2c1m9rv-glibc-2.33-123-dev             44200560
  

  and likewise, using a store path to a "drv" file to specify the derivation:

  # nix path-info --closure-size '/nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^*'
  …
  

• If you didn't specify the desired outputs, but the derivation has an attribute meta.outputsToInstall, Nix will use those outputs. For example, since the package nixpkgs#libxml2 has this attribute:

  # nix eval 'nixpkgs#libxml2.meta.outputsToInstall'
  [ "bin" "man" ]
  

  a command like nix shell nixpkgs#libxml2 will provide only those two outputs by default.

  Note that a store derivation (given by its .drv file store path) doesn't have any attributes like meta, and thus this case doesn't apply to it.

• Otherwise, Nix will use all outputs of the derivation.

Nix stores

Most nix subcommands operate on a Nix store. These are documented in nix help-stores.

Shebang interpreter

The nix command can be used as a #! interpreter. Arguments to Nix can be passed on subsequent lines in the script.

Verbatim strings may be passed in double backtick (``) quotes.

--file and --expr resolve relative paths based on the script location.

Examples:

#!/usr/bin/env nix
#! nix shell --file ``<nixpkgs>`` hello cowsay --command bash

hello | cowsay

or with flakes:

#!/usr/bin/env nix
#! nix shell nixpkgs#bash nixpkgs#hello nixpkgs#cowsay --command bash

hello | cowsay

or with an expression:

#! /usr/bin/env nix
#! nix shell --impure --expr ``
#! nix with (import (builtins.getFlake "nixpkgs") {});
#! nix terraform.withPlugins (plugins: [ plugins.openstack ])
#! nix ``
#! nix --command bash

terraform "$@"

or with cascading interpreters. Note that the #! nix lines don't need to follow after the first line, to accomodate other interpreters.

#!/usr/bin/env nix
//! ```cargo
//! [dependencies]
//! time = "0.1.25"
//! ```
/*
#!nix shell nixpkgs#rustc nixpkgs#rust-script nixpkgs#cargo --command rust-script
*/
fn main() {
    for argument in std::env::args().skip(1) {
        println!("{}", argument);
    };
    println!("{}", std::env::var("HOME").expect(""));
    println!("{}", time::now().rfc822z());
}
// vim: ft=rust

)""