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Rename doc/manual{src -> source}
This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
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John Ericson
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221 changed files with 75 additions and 74 deletions
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# Derivations
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The most important built-in function is `derivation`, which is used to describe a single derivation:
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a specification for running an executable on precisely defined input files to repeatably produce output files at uniquely determined file system paths.
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It takes as input an attribute set, the attributes of which specify the inputs to the process.
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It outputs an attribute set, and produces a [store derivation] as a side effect of evaluation.
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[store derivation]: @docroot@/glossary.md#gloss-store-derivation
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## Input attributes
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### Required
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- [`name`]{#attr-name} ([String](@docroot@/language/types.md#type-string))
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A symbolic name for the derivation.
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It is added to the [store path] of the corresponding [store derivation] as well as to its [output paths](@docroot@/glossary.md#gloss-output-path).
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[store path]: @docroot@/store/store-path.md
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> **Example**
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>
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> ```nix
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> derivation {
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> name = "hello";
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> # ...
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> }
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> ```
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>
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> The store derivation's path will be `/nix/store/<hash>-hello.drv`.
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> The [output](#attr-outputs) paths will be of the form `/nix/store/<hash>-hello[-<output>]`
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- [`system`]{#attr-system} ([String](@docroot@/language/types.md#type-string))
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The system type on which the [`builder`](#attr-builder) executable is meant to be run.
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A necessary condition for Nix to build derivations locally is that the `system` attribute matches the current [`system` configuration option].
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It can automatically [build on other platforms](@docroot@/language/derivations.md#attr-builder) by forwarding build requests to other machines.
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[`system` configuration option]: @docroot@/command-ref/conf-file.md#conf-system
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> **Example**
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>
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> Declare a derivation to be built on a specific system type:
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>
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> ```nix
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> derivation {
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> # ...
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> system = "x86_64-linux";
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> # ...
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> }
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> ```
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> **Example**
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>
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> Declare a derivation to be built on the system type that evaluates the expression:
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>
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> ```nix
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> derivation {
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> # ...
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> system = builtins.currentSystem;
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> # ...
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> }
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> ```
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>
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> [`builtins.currentSystem`](@docroot@/language/builtins.md#builtins-currentSystem) has the value of the [`system` configuration option], and defaults to the system type of the current Nix installation.
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- [`builder`]{#attr-builder} ([Path](@docroot@/language/types.md#type-path) | [String](@docroot@/language/types.md#type-string))
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Path to an executable that will perform the build.
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> **Example**
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>
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> Use the file located at `/bin/bash` as the builder executable:
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>
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> ```nix
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> derivation {
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> # ...
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> builder = "/bin/bash";
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> # ...
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> };
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> ```
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<!-- -->
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> **Example**
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>
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> Copy a local file to the Nix store for use as the builder executable:
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>
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> ```nix
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> derivation {
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> # ...
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> builder = ./builder.sh;
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> # ...
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> };
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> ```
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<!-- -->
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> **Example**
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>
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> Use a file from another derivation as the builder executable:
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>
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> ```nix
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> let pkgs = import <nixpkgs> {}; in
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> derivation {
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> # ...
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> builder = "${pkgs.python}/bin/python";
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> # ...
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> };
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> ```
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### Optional
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- [`args`]{#attr-args} ([List](@docroot@/language/types.md#type-list) of [String](@docroot@/language/types.md#type-string))
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Default: `[ ]`
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Command-line arguments to be passed to the [`builder`](#attr-builder) executable.
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> **Example**
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>
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> Pass arguments to Bash to interpret a shell command:
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>
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> ```nix
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> derivation {
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> # ...
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> builder = "/bin/bash";
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> args = [ "-c" "echo hello world > $out" ];
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> # ...
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> };
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> ```
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- [`outputs`]{#attr-outputs} ([List](@docroot@/language/types.md#type-list) of [String](@docroot@/language/types.md#type-string))
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Default: `[ "out" ]`
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Symbolic outputs of the derivation.
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Each output name is passed to the [`builder`](#attr-builder) executable as an environment variable with its value set to the corresponding [store path].
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By default, a derivation produces a single output called `out`.
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However, derivations can produce multiple outputs.
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This allows the associated [store objects](@docroot@/store/store-object.md) and their [closures](@docroot@/glossary.md#gloss-closure) to be copied or garbage-collected separately.
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> **Example**
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>
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> Imagine a library package that provides a dynamic library, header files, and documentation.
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> A program that links against such a library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time.
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> Thus, the library package could specify:
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>
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> ```nix
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> derivation {
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> # ...
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> outputs = [ "lib" "dev" "doc" ];
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> # ...
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> }
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> ```
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>
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> This will cause Nix to pass environment variables `lib`, `dev`, and `doc` to the builder containing the intended store paths of each output.
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> The builder would typically do something like
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>
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> ```bash
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> ./configure \
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> --libdir=$lib/lib \
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> --includedir=$dev/include \
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> --docdir=$doc/share/doc
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> ```
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>
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> for an Autoconf-style package.
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The name of an output is combined with the name of the derivation to create the name part of the output's store path, unless it is `out`, in which case just the name of the derivation is used.
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> **Example**
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>
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>
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> ```nix
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> derivation {
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> name = "example";
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> outputs = [ "lib" "dev" "doc" "out" ];
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> # ...
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> }
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> ```
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>
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> The store derivation path will be `/nix/store/<hash>-example.drv`.
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> The output paths will be
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> - `/nix/store/<hash>-example-lib`
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> - `/nix/store/<hash>-example-dev`
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> - `/nix/store/<hash>-example-doc`
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> - `/nix/store/<hash>-example`
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You can refer to each output of a derivation by selecting it as an attribute.
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The first element of `outputs` determines the *default output* and ends up at the top-level.
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> **Example**
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>
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> Select an output by attribute name:
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>
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> ```nix
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> let
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> myPackage = derivation {
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> name = "example";
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> outputs = [ "lib" "dev" "doc" "out" ];
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> # ...
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> };
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> in myPackage.dev
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> ```
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>
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> Since `lib` is the first output, `myPackage` is equivalent to `myPackage.lib`.
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<!-- FIXME: refer to the output attributes when we have one -->
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- See [Advanced Attributes](./advanced-attributes.md) for more, infrequently used, optional attributes.
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<!-- FIXME: This should be moved here -->
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- Every other attribute is passed as an environment variable to the builder.
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Attribute values are translated to environment variables as follows:
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- Strings are passed unchanged.
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- Integral numbers are converted to decimal notation.
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- Floating point numbers are converted to simple decimal or scientific notation with a preset precision.
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- A *path* (e.g., `../foo/sources.tar`) causes the referenced file
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to be copied to the store; its location in the store is put in
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the environment variable. The idea is that all sources should
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reside in the Nix store, since all inputs to a derivation should
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reside in the Nix store.
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- A *derivation* causes that derivation to be built prior to the
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present derivation. The environment variable is set to the [store path] of the derivation's default [output](#attr-outputs).
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- Lists of the previous types are also allowed. They are simply
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concatenated, separated by spaces.
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- `true` is passed as the string `1`, `false` and `null` are
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passed as an empty string.
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<!-- FIXME: add a section on output attributes -->
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## Builder execution
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The [`builder`](#attr-builder) is executed as follows:
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- A temporary directory is created under the directory specified by
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`TMPDIR` (default `/tmp`) where the build will take place. The
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current directory is changed to this directory.
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- The environment is cleared and set to the derivation attributes, as
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specified above.
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- In addition, the following variables are set:
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- `NIX_BUILD_TOP` contains the path of the temporary directory for
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this build.
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- Also, `TMPDIR`, `TEMPDIR`, `TMP`, `TEMP` are set to point to the
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temporary directory. This is to prevent the builder from
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accidentally writing temporary files anywhere else. Doing so
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might cause interference by other processes.
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- `PATH` is set to `/path-not-set` to prevent shells from
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initialising it to their built-in default value.
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- `HOME` is set to `/homeless-shelter` to prevent programs from
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using `/etc/passwd` or the like to find the user's home
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directory, which could cause impurity. Usually, when `HOME` is
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set, it is used as the location of the home directory, even if
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it points to a non-existent path.
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- `NIX_STORE` is set to the path of the top-level Nix store
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directory (typically, `/nix/store`).
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- `NIX_ATTRS_JSON_FILE` & `NIX_ATTRS_SH_FILE` if `__structuredAttrs`
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is set to `true` for the derivation. A detailed explanation of this
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behavior can be found in the
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[section about structured attrs](./advanced-attributes.md#adv-attr-structuredAttrs).
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- For each output declared in `outputs`, the corresponding
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environment variable is set to point to the intended path in the
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Nix store for that output. Each output path is a concatenation
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of the cryptographic hash of all build inputs, the `name`
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attribute and the output name. (The output name is omitted if
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it’s `out`.)
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- If an output path already exists, it is removed. Also, locks are
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acquired to prevent multiple Nix instances from performing the same
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build at the same time.
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- A log of the combined standard output and error is written to
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`/nix/var/log/nix`.
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- The builder is executed with the arguments specified by the
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attribute `args`. If it exits with exit code 0, it is considered to
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have succeeded.
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- The temporary directory is removed (unless the `-K` option was
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specified).
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- If the build was successful, Nix scans each output path for
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references to input paths by looking for the hash parts of the input
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paths. Since these are potential runtime dependencies, Nix registers
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them as dependencies of the output paths.
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- After the build, Nix sets the last-modified timestamp on all files
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in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to
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the default group, and sets the mode of the file to 0444 or 0555
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(i.e., read-only, with execute permission enabled if the file was
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originally executable). Note that possible `setuid` and `setgid`
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bits are cleared. Setuid and setgid programs are not currently
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supported by Nix. This is because the Nix archives used in
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deployment have no concept of ownership information, and because it
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makes the build result dependent on the user performing the build.
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