nix

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Custom commands

Warning \ Custom commands are part of the unstable lix-custom-sub-commands experimental feature, and subject to change without notice.

Lix allows users to define custom subcommands by placing executables in the system's PATH that follow the naming convention lix-<command>. When a user runs lix <command>, Lix will attempt to locate and execute lix-<command> as a separate process.

Auto-completion of custom commands is not supported yet.

Usage

A custom Lix command must be an executable script or binary named lix-<command> and be accessible from the PATH. When the user invokes lix <command>, Lix will execute lix-<command> with the given arguments.

For example, if an executable named lix-example exists in the PATH, running:

$ lix example arg1 arg2

will be equivalent to running:

$ lix-example arg1 arg2

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

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

Lix is a fork of the original implementation CppNix.

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)
  • This is the default
• Store path
  • This is assumed if the argument is a Nix store path or a symlink to a Nix store path
• Fileish, optionally qualified by an attribute path
  • Specified with --file/-f
• Nix expression, optionally qualified by an attribute path
  • Specified with --expr/-E

For most commands, if no installable is specified, . is assumed. That is, Lix 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 manualpage. 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 Lix 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 Lix 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, Lix 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^*

Fileish

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

When the option -f / --file fileish [attrpath...] is given, installables are interpreted as the value of the Nix file specified by fileish. 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.

The fileish itself may take one of a few different forms, the first being a simple filesystem path, e.g. nix build -f /tmp/some-file.nix. Like the import builtin, specifying a directory is equivalent to specify default.nix within that directory. It may also be a search path (also known as a lookup path), like <nixpkgs>. Unlike using <nixpkgs> in a --expr argument, this does not require --impure.

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

$ nix build -f '<nixpkgs>' hello

If a fileish starts with http:// or https://, it is interpreted as the URL of a tarball which will be fetched and unpacked. Lix will then import the unpacked directory, so these tarballs must include at least a single top-level directory with a file called default.nix. For example, you could build from a specific version of Nixpkgs with something like:

$ nix build -f "https://github.com/NixOS/nixpkgs/archive/refs/heads/release-24.11.tar.gz" firefox

If a fileish starts with flake:, the rest of the argument is interpreted as a flakeref (see nix flake --help or man nix3-flake), which requires the "flakes" experimental feature to be enabled. This is is not quite the same as specifying a flake output attrpath. It does not access the flake directly and does not even consider the existence of flake.nix, but instead fetches it as if it is not a flake at all and imports the unpacked directory. In other words, it assumes that the flake has a default.nix file, and then interprets the attribute path relative to what default.nix evaluates to.

For many flakes — including Nixpkgs — this will end up evaluating to the same thing. These two commands build the same derivation, but one from the flake, and the other from default.nix:

$ nix build 'nixpkgs#firefox' # from flake.nix
$ nix build -f flake:nixpkgs firefox # from default.nix in the flake directory; ignores flake.nix altogether

Finally, for legacy reasons, if a fileish starts with channel:, the rest of the argument is interpreted as the name of a channel to fetch from https://nixos.org/channels/$CHANNEL_NAME/nixexprs.tar.gz. This is a hard coded URL pattern and is not related to the subscribed channels managed by the nix-channel command.

Note: any of the special syntaxes may always be disambiguated by prefixing the path. For example: a file in the current directory literally called <nixpkgs> can be addressed as ./<nixpkgs>, to escape the special interpretation.

In summary, a file path argument may be one of:

{{#include ../fileish-summary.md}}

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:

$ 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 or Store paths, the output is specified in the same argument:

  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'
  …

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

  $ 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:

  $ 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:

  # 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, Lix 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, Lix will use all outputs of the derivation.

Nix stores

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

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