Glossary - Lix Reference Manual

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# Glossary

- derivation

  A description of a build task. The result of a derivation is a store object. Derivations are typically specified in Nix expressions using the `derivation` primitive. These are translated into low-level store derivations (implicitly by `nix-env` and `nix-build`, or explicitly by `nix-instantiate`).

- store derivation

  A derivation represented as a `.drv` file in the store. It has a store path, like any store object.

  Example: `/nix/store/g946hcz4c8mdvq2g8vxx42z51qb71rvp-git-2.38.1.drv`

  See `nix derivation show` (experimental) for displaying the contents of store derivations.

- instantiate, instantiation

  Translate a derivation into a store derivation.

  See `nix-instantiate`.

- realise, realisation

  Ensure a store path is valid.

  This means either running the `builder` executable as specified in the corresponding derivation, or fetching a pre-built store object from a substituter, or delegating to a remote builder and retrieving the outputs.

  See `nix-build` and `nix-store --realise`.

  See `nix build` (experimental).

- fixed-output derivation

  A derivation which includes the `outputHash` attribute.

- store

  The location in the file system where store objects live. Typically `/nix/store`.

  From the perspective of the location where Lix is invoked, the Nix store can be referred to as a "local" or a "remote" one:

  - A local store exists on the filesystem of the machine where Lix is invoked. You can use other local stores by passing the `--store` flag to the `nix` command. Local stores can be used for building derivations.

  - A remote store exists anywhere other than the local filesystem. One example is the `/nix/store` directory on another machine, accessed via `ssh` or served by the `nix-serve` Perl script.

- chroot store

  A local store whose canonical path is anything other than `/nix/store`.

- binary cache

  A binary cache is a Nix store which uses a different format: its metadata and signatures are kept in `.narinfo` files rather than in a Nix database. This different format simplifies serving store objects over the network, but cannot host builds. Examples of binary caches include S3 buckets and the NixOS binary cache.

- store path

  The location of a store object in the file system, i.e., an immediate child of the Nix store directory.

  Example: `/nix/store/a040m110amc4h71lds2jmr8qrkj2jhxd-git-2.38.1`

- file system object

  The Nix data model for representing simplified file system data.

  See File System Object for details.

- store object

  A store object consists of a file system object, references to other store objects, and other metadata. It can be referred to by a store path.

- input-addressed store object

  A store object produced by building a non-fixed-output derivation.

- output-addressed store object

  A store object whose store path is determined by its contents. This includes derivations and the outputs of fixed-output derivations.

- substitute

  A substitute is a command invocation stored in the Nix database that describes how to build a store object, bypassing the normal build mechanism (i.e., derivations). Typically, the substitute builds the store object by downloading a pre-built version of the store object from some server.

- substituter

  An additional store from which Lix can obtain store objects instead of building them. Often the substituter is a binary cache, but any store can serve as substituter.

  See the `substituters` configuration option for details.

- purity

  The assumption that equal Nix derivations when run always produce the same output. This cannot be guaranteed in general (e.g., a builder can rely on external inputs such as the network or the system time) but the Nix model assumes it.

- Nix database

  An SQlite database to track references between store objects. This is an implementation detail of the local store.

  Default location: `/nix/var/nix/db`.

- Nix expression

  A high-level description of software packages and compositions thereof. Deploying software using Lix entails writing Nix expressions for your packages. Nix expressions are translated to derivations that are stored in the Nix store. These derivations can then be built.

- reference

  A store object `O` is said to have a reference to a store object `P` if a store path to `P` appears in the contents of `O`.

  Store objects can refer to both other store objects and themselves. References from a store object to itself are called self-references. References other than a self-reference must not form a cycle.

- reachable

  A store path `Q` is reachable from another store path `P` if `Q` is in the closure of the references relation.

- closure

  The closure of a store path is the set of store paths that are directly or indirectly “reachable” from that store path; that is, it’s the closure of the path under the references relation. For a package, the closure of its derivation is equivalent to the build-time dependencies, while the closure of its output path is equivalent to its runtime dependencies. For correct deployment it is necessary to deploy whole closures, since otherwise at runtime files could be missing. The command `nix-store --query --requisites` prints out closures of store paths.

  As an example, if the store object at path `P` contains a reference to a store object at path `Q`, then `Q` is in the closure of `P`. Further, if `Q` references `R` then `R` is also in the closure of `P`.

- output

  A store object produced by a derivation.

- output path

  The store path to the output of a derivation.

- deriver

  The store derivation that produced an output path.

- validity

  A store path is valid if all store objects in its closure can be read from the store.

  For a local store, this means:

  - The store path leads to an existing store object in that store.
  - The store path is listed in the Nix database as being valid.
  - All paths in the store path's closure are valid.

- user environment

  An automatically generated store object that consists of a set of symlinks to “active” applications, i.e., other store paths. These are generated automatically by `nix-env`. See profiles.

- profile

  A symlink to the current user environment of a user, e.g., `/nix/var/nix/profiles/default`.

- installable

  Something that can be realised in the Nix store.

  See installables for `nix` commands (experimental) for details.

- NAR

  A Nix ARchive. This is a serialisation of a path in the Nix store. It can contain regular files, directories and symbolic links. NARs are generated and unpacked using `nix-store --dump` and `nix-store --restore`.

- `∅`

  The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.

- `ε`

  The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.

- string interpolation

  Expanding expressions enclosed in `${ }` within a string, path, or attribute name.

  See String interpolation for details.

- experimental feature

  Not yet stabilized functionality guarded by named experimental feature flags. These flags are enabled or disabled with the `experimental-features` setting.

  See the contribution guide on the purpose and lifecycle of experimental feaures.