path: root/src/nix/nix.md

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# Examples

* Create a new flake:

  ```console
  # nix flake new hello
  # cd hello
  ```

* Build the flake in the current directory:

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

* Run the flake in the current directory:

  ```console
  # nix run
  Hello, world!
  ```

* Start a development shell for hacking on this flake:

  ```console
  # 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](https://nixos.org/) or the [Nix
manual](https://nixos.org/manual/nix/stable/).

# Installables

> **Warning** \
> Installables are part of the unstable
> [`nix-command` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-nix-command),
> 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](#flake-output-attribute) (experimental)
  - This is the default
- [Store path](#store-path)
  - This is assumed if the argument is a Nix store path or a symlink to a Nix store path
- [Nix file](#nix-file), optionally qualified by an attribute path
  - Specified with `--file`/`-f`
- [Nix expression](#nix-expression), optionally qualified by an attribute path
  - Specified with `--expr`/`-E`

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`](@docroot@/contributing/experimental-features.md#xp-feature-flakes)
> and
> [`nix-command`](@docroot@/contributing/experimental-features.md#xp-feature-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](./nix3-flake.md#flake-references) and *attrpath* is an optional attribute path. For
more information on flakes, see [the `nix flake` manual
page](./nix3-flake.md).  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`.

### 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.

To emulate the `nix-build '<nixpkgs>' -A hello` pattern, use:

```console
$ nix build -f '<nixpkgs>' hello
```

### Nix expression

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

When the option `-E` / `--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>`).

To emulate the `nix-build -E 'with import <nixpkgs> { }; hello' pattern use:

```console
$ nix build --impure -E 'with import <nixpkgs> { }; hello'
```

## 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* — that is, a caret followed immediately by a comma-separated list of derivation outputs to select.
  For installables specified as [Flake output attributes](#flake-output-attribute) or [Store paths](#store-path), the output is specified in the same argument:

  For example, you can obtain the `dev` and `static` outputs of the `glibc` package:

  ```console
  # 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:

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

  For `-e`/`--expr` and `-f`/`--file`, the derivation output is specified as part of the attribute path:

  ```console
  $ nix build -f '<nixpkgs>' 'glibc^dev,static'
  $ nix build --impure -E 'import <nixpkgs> { }' 'glibc^dev,static'
  ```

  This syntax is the same even if the actual attribute path is empty:

  ```console
  $ nix build -E 'let pkgs = import <nixpkgs> { }; in pkgs.glibc' '^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:

  ```console
  # 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:

  ```console
  # 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:

  ```console
  # 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.

  [store derivation]: ../../glossary.md#gloss-store-derivation

* 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`](./nix3-help-stores.md).

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