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# Glossary
- [derivation]{#gloss-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](./language/derivations.md). These are
translated into low-level *store derivations* (implicitly by
`nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
[derivation]: #gloss-derivation
- [store derivation]{#gloss-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`](./command-ref/new-cli/nix3-derivation-show.md) (experimental) for displaying the contents of store derivations.
[store derivation]: #gloss-store-derivation
- [instantiate]{#gloss-instantiate}, instantiation
Translate a [derivation] into a [store derivation].
See [`nix-instantiate`](./command-ref/nix-instantiate.md).
[instantiate]: #gloss-instantiate
- [realise]{#gloss-realise}, realisation
Ensure a [store path] is [valid][validity].
This means either running the `builder` executable as specified in the corresponding [derivation] or fetching a pre-built [store object] from a [substituter].
See [`nix-build`](./command-ref/nix-build.md) and [`nix-store --realise`](@docroot@/command-ref/nix-store/realise.md).
See [`nix build`](./command-ref/new-cli/nix3-build.md) (experimental).
[realise]: #gloss-realise
- [content-addressed derivation]{#gloss-content-addressed-derivation}
A derivation which has the
[`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
attribute set to `true`.
- [fixed-output derivation]{#gloss-fixed-output-derivation}
A derivation which includes the
[`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute.
- [store]{#gloss-store}
The location in the file system where store objects live. Typically
`/nix/store`.
From the perspective of the location where Nix is
invoked, the Nix store can be referred to
as a "_local_" or a "_remote_" one:
+ A [local store]{#gloss-local-store} exists on the filesystem of
the machine where Nix 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.
[store]: #gloss-store
[local store]: #gloss-local-store
- [chroot store]{#gloss-chroot-store}
A [local store] whose canonical path is anything other than `/nix/store`.
- [binary cache]{#gloss-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](https://cache.nixos.org).
- [store path]{#gloss-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`
[store path]: #gloss-store-path
- [file system object]{#gloss-store-object}
The Nix data model for representing simplified file system data.
See [File System Object](@docroot@/architecture/file-system-object.md) for details.
[file system object]: #gloss-file-system-object
- [store object]{#gloss-store-object}
A store object consists of a [file system object], [reference]s to other store objects, and other metadata.
It can be referred to by a [store path].
[store object]: #gloss-store-object
- [input-addressed store object]{#gloss-input-addressed-store-object}
A store object produced by building a
non-[content-addressed](#gloss-content-addressed-derivation),
non-[fixed-output](#gloss-fixed-output-derivation)
derivation.
- [output-addressed store object]{#gloss-output-addressed-store-object}
A [store object] whose [store path] is determined by its contents.
This includes derivations, the outputs of [content-addressed derivations](#gloss-content-addressed-derivation), and the outputs of [fixed-output derivations](#gloss-fixed-output-derivation).
- [substitute]{#gloss-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]{#gloss-substituter}
An additional [store]{#gloss-store} from which Nix can obtain store objects instead of building them.
Often the substituter is a [binary cache](#gloss-binary-cache), but any store can serve as substituter.
See the [`substituters` configuration option](./command-ref/conf-file.md#conf-substituters) for details.
[substituter]: #gloss-substituter
- [purity]{#gloss-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]{#gloss-nix-database}
An SQlite database to track [reference]s between [store object]s.
This is an implementation detail of the [local store].
Default location: `/nix/var/nix/db`.
[Nix database]: #gloss-nix-database
- [Nix expression]{#gloss-nix-expression}
A high-level description of software packages and compositions
thereof. Deploying software using Nix 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]{#gloss-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.
[reference]: #gloss-reference
- [reachable]{#gloss-reachable}
A store path `Q` is reachable from another store path `P` if `Q`
is in the *closure* of the *references* relation.
- [closure]{#gloss-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`.
[closure]: #gloss-closure
- [output path]{#gloss-output-path}
A [store path] produced by a [derivation].
[output path]: #gloss-output-path
- [deriver]{#gloss-deriver}
The [store derivation] that produced an [output path].
- [validity]{#gloss-validity}
A store path is valid if all [store object]s 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.
[validity]: #gloss-validity
- [user environment]{#gloss-user-env}
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`](./command-ref/nix-env.md). See *profiles*.
- [profile]{#gloss-profile}
A symlink to the current *user environment* of a user, e.g.,
`/nix/var/nix/profiles/default`.
- [installable]{#gloss-installable}
Something that can be realised in the Nix store.
See [installables](./command-ref/new-cli/nix.md#installables) for [`nix` commands](./command-ref/new-cli/nix.md) (experimental) for details.
- [NAR]{#gloss-nar}
A *N*ix *AR*chive. 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`.
- [`∅`]{#gloss-emtpy-set}
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`]{#gloss-epsilon}
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]{#gloss-string-interpolation}
Expanding expressions enclosed in `${ }` within a [string], [path], or [attribute name].
See [String interpolation](./language/string-interpolation.md) for details.
[string]: ./language/values.md#type-string
[path]: ./language/values.md#type-path
[attribute name]: ./language/values.md#attribute-set
- [experimental feature]{#gloss-experimental-feature}
Not yet stabilized functionality guarded by named experimental feature flags.
These flags are enabled or disabled with the [`experimental-features`](./command-ref/conf-file.html#conf-experimental-features) setting.
See the contribution guide on the [purpose and lifecycle of experimental feaures](@docroot@/contributing/experimental-features.md).
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