mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-22 05:56:15 +02:00
eb7d7780b1
This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
315 lines
10 KiB
Markdown
315 lines
10 KiB
Markdown
# Derivations
|
||
|
||
The most important built-in function is `derivation`, which is used to describe a single derivation:
|
||
a specification for running an executable on precisely defined input files to repeatably produce output files at uniquely determined file system paths.
|
||
|
||
It takes as input an attribute set, the attributes of which specify the inputs to the process.
|
||
It outputs an attribute set, and produces a [store derivation] as a side effect of evaluation.
|
||
|
||
[store derivation]: @docroot@/glossary.md#gloss-store-derivation
|
||
|
||
## Input attributes
|
||
|
||
### Required
|
||
|
||
- [`name`]{#attr-name} ([String](@docroot@/language/types.md#type-string))
|
||
|
||
A symbolic name for the derivation.
|
||
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).
|
||
|
||
[store path]: @docroot@/store/store-path.md
|
||
|
||
> **Example**
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> name = "hello";
|
||
> # ...
|
||
> }
|
||
> ```
|
||
>
|
||
> The store derivation's path will be `/nix/store/<hash>-hello.drv`.
|
||
> The [output](#attr-outputs) paths will be of the form `/nix/store/<hash>-hello[-<output>]`
|
||
|
||
- [`system`]{#attr-system} ([String](@docroot@/language/types.md#type-string))
|
||
|
||
The system type on which the [`builder`](#attr-builder) executable is meant to be run.
|
||
|
||
A necessary condition for Nix to build derivations locally is that the `system` attribute matches the current [`system` configuration option].
|
||
It can automatically [build on other platforms](@docroot@/language/derivations.md#attr-builder) by forwarding build requests to other machines.
|
||
|
||
[`system` configuration option]: @docroot@/command-ref/conf-file.md#conf-system
|
||
|
||
> **Example**
|
||
>
|
||
> Declare a derivation to be built on a specific system type:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> system = "x86_64-linux";
|
||
> # ...
|
||
> }
|
||
> ```
|
||
|
||
> **Example**
|
||
>
|
||
> Declare a derivation to be built on the system type that evaluates the expression:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> system = builtins.currentSystem;
|
||
> # ...
|
||
> }
|
||
> ```
|
||
>
|
||
> [`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.
|
||
|
||
- [`builder`]{#attr-builder} ([Path](@docroot@/language/types.md#type-path) | [String](@docroot@/language/types.md#type-string))
|
||
|
||
Path to an executable that will perform the build.
|
||
|
||
> **Example**
|
||
>
|
||
> Use the file located at `/bin/bash` as the builder executable:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> builder = "/bin/bash";
|
||
> # ...
|
||
> };
|
||
> ```
|
||
|
||
<!-- -->
|
||
|
||
> **Example**
|
||
>
|
||
> Copy a local file to the Nix store for use as the builder executable:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> builder = ./builder.sh;
|
||
> # ...
|
||
> };
|
||
> ```
|
||
|
||
<!-- -->
|
||
|
||
> **Example**
|
||
>
|
||
> Use a file from another derivation as the builder executable:
|
||
>
|
||
> ```nix
|
||
> let pkgs = import <nixpkgs> {}; in
|
||
> derivation {
|
||
> # ...
|
||
> builder = "${pkgs.python}/bin/python";
|
||
> # ...
|
||
> };
|
||
> ```
|
||
|
||
### Optional
|
||
|
||
- [`args`]{#attr-args} ([List](@docroot@/language/types.md#type-list) of [String](@docroot@/language/types.md#type-string))
|
||
|
||
Default: `[ ]`
|
||
|
||
Command-line arguments to be passed to the [`builder`](#attr-builder) executable.
|
||
|
||
> **Example**
|
||
>
|
||
> Pass arguments to Bash to interpret a shell command:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> builder = "/bin/bash";
|
||
> args = [ "-c" "echo hello world > $out" ];
|
||
> # ...
|
||
> };
|
||
> ```
|
||
|
||
- [`outputs`]{#attr-outputs} ([List](@docroot@/language/types.md#type-list) of [String](@docroot@/language/types.md#type-string))
|
||
|
||
Default: `[ "out" ]`
|
||
|
||
Symbolic outputs of the derivation.
|
||
Each output name is passed to the [`builder`](#attr-builder) executable as an environment variable with its value set to the corresponding [store path].
|
||
|
||
By default, a derivation produces a single output called `out`.
|
||
However, derivations can produce multiple outputs.
|
||
This allows the associated [store objects](@docroot@/store/store-object.md) and their [closures](@docroot@/glossary.md#gloss-closure) to be copied or garbage-collected separately.
|
||
|
||
> **Example**
|
||
>
|
||
> Imagine a library package that provides a dynamic library, header files, and documentation.
|
||
> A program that links against such a library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time.
|
||
> Thus, the library package could specify:
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> # ...
|
||
> outputs = [ "lib" "dev" "doc" ];
|
||
> # ...
|
||
> }
|
||
> ```
|
||
>
|
||
> This will cause Nix to pass environment variables `lib`, `dev`, and `doc` to the builder containing the intended store paths of each output.
|
||
> The builder would typically do something like
|
||
>
|
||
> ```bash
|
||
> ./configure \
|
||
> --libdir=$lib/lib \
|
||
> --includedir=$dev/include \
|
||
> --docdir=$doc/share/doc
|
||
> ```
|
||
>
|
||
> for an Autoconf-style package.
|
||
|
||
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.
|
||
|
||
> **Example**
|
||
>
|
||
>
|
||
> ```nix
|
||
> derivation {
|
||
> name = "example";
|
||
> outputs = [ "lib" "dev" "doc" "out" ];
|
||
> # ...
|
||
> }
|
||
> ```
|
||
>
|
||
> The store derivation path will be `/nix/store/<hash>-example.drv`.
|
||
> The output paths will be
|
||
> - `/nix/store/<hash>-example-lib`
|
||
> - `/nix/store/<hash>-example-dev`
|
||
> - `/nix/store/<hash>-example-doc`
|
||
> - `/nix/store/<hash>-example`
|
||
|
||
You can refer to each output of a derivation by selecting it as an attribute.
|
||
The first element of `outputs` determines the *default output* and ends up at the top-level.
|
||
|
||
> **Example**
|
||
>
|
||
> Select an output by attribute name:
|
||
>
|
||
> ```nix
|
||
> let
|
||
> myPackage = derivation {
|
||
> name = "example";
|
||
> outputs = [ "lib" "dev" "doc" "out" ];
|
||
> # ...
|
||
> };
|
||
> in myPackage.dev
|
||
> ```
|
||
>
|
||
> Since `lib` is the first output, `myPackage` is equivalent to `myPackage.lib`.
|
||
|
||
<!-- FIXME: refer to the output attributes when we have one -->
|
||
|
||
- See [Advanced Attributes](./advanced-attributes.md) for more, infrequently used, optional attributes.
|
||
|
||
<!-- FIXME: This should be moved here -->
|
||
|
||
- Every other attribute is passed as an environment variable to the builder.
|
||
Attribute values are translated to environment variables as follows:
|
||
|
||
- Strings are passed unchanged.
|
||
|
||
- Integral numbers are converted to decimal notation.
|
||
|
||
- Floating point numbers are converted to simple decimal or scientific notation with a preset precision.
|
||
|
||
- A *path* (e.g., `../foo/sources.tar`) causes the referenced file
|
||
to be copied to the store; its location in the store is put in
|
||
the environment variable. The idea is that all sources should
|
||
reside in the Nix store, since all inputs to a derivation should
|
||
reside in the Nix store.
|
||
|
||
- A *derivation* causes that derivation to be built prior to the
|
||
present derivation. The environment variable is set to the [store path] of the derivation's default [output](#attr-outputs).
|
||
|
||
- Lists of the previous types are also allowed. They are simply
|
||
concatenated, separated by spaces.
|
||
|
||
- `true` is passed as the string `1`, `false` and `null` are
|
||
passed as an empty string.
|
||
|
||
<!-- FIXME: add a section on output attributes -->
|
||
|
||
## Builder execution
|
||
|
||
The [`builder`](#attr-builder) is executed as follows:
|
||
|
||
- A temporary directory is created under the directory specified by
|
||
`TMPDIR` (default `/tmp`) where the build will take place. The
|
||
current directory is changed to this directory.
|
||
|
||
- The environment is cleared and set to the derivation attributes, as
|
||
specified above.
|
||
|
||
- In addition, the following variables are set:
|
||
|
||
- `NIX_BUILD_TOP` contains the path of the temporary directory for
|
||
this build.
|
||
|
||
- Also, `TMPDIR`, `TEMPDIR`, `TMP`, `TEMP` are set to point to the
|
||
temporary directory. This is to prevent the builder from
|
||
accidentally writing temporary files anywhere else. Doing so
|
||
might cause interference by other processes.
|
||
|
||
- `PATH` is set to `/path-not-set` to prevent shells from
|
||
initialising it to their built-in default value.
|
||
|
||
- `HOME` is set to `/homeless-shelter` to prevent programs from
|
||
using `/etc/passwd` or the like to find the user's home
|
||
directory, which could cause impurity. Usually, when `HOME` is
|
||
set, it is used as the location of the home directory, even if
|
||
it points to a non-existent path.
|
||
|
||
- `NIX_STORE` is set to the path of the top-level Nix store
|
||
directory (typically, `/nix/store`).
|
||
|
||
- `NIX_ATTRS_JSON_FILE` & `NIX_ATTRS_SH_FILE` if `__structuredAttrs`
|
||
is set to `true` for the derivation. A detailed explanation of this
|
||
behavior can be found in the
|
||
[section about structured attrs](./advanced-attributes.md#adv-attr-structuredAttrs).
|
||
|
||
- For each output declared in `outputs`, the corresponding
|
||
environment variable is set to point to the intended path in the
|
||
Nix store for that output. Each output path is a concatenation
|
||
of the cryptographic hash of all build inputs, the `name`
|
||
attribute and the output name. (The output name is omitted if
|
||
it’s `out`.)
|
||
|
||
- If an output path already exists, it is removed. Also, locks are
|
||
acquired to prevent multiple Nix instances from performing the same
|
||
build at the same time.
|
||
|
||
- A log of the combined standard output and error is written to
|
||
`/nix/var/log/nix`.
|
||
|
||
- The builder is executed with the arguments specified by the
|
||
attribute `args`. If it exits with exit code 0, it is considered to
|
||
have succeeded.
|
||
|
||
- The temporary directory is removed (unless the `-K` option was
|
||
specified).
|
||
|
||
- If the build was successful, Nix scans each output path for
|
||
references to input paths by looking for the hash parts of the input
|
||
paths. Since these are potential runtime dependencies, Nix registers
|
||
them as dependencies of the output paths.
|
||
|
||
- After the build, Nix sets the last-modified timestamp on all files
|
||
in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to
|
||
the default group, and sets the mode of the file to 0444 or 0555
|
||
(i.e., read-only, with execute permission enabled if the file was
|
||
originally executable). Note that possible `setuid` and `setgid`
|
||
bits are cleared. Setuid and setgid programs are not currently
|
||
supported by Nix. This is because the Nix archives used in
|
||
deployment have no concept of ownership information, and because it
|
||
makes the build result dependent on the user performing the build.
|