Glossary

• content address

  A content address is a secure way to reference immutable data. The reference is calculated directly from the content of the data being referenced, which means the reference is tamper proof --- variations of the data should always calculate to distinct content addresses.

  For how Nix uses content addresses, see:

  • Content-Addressing File System Objects
  • Content-Addressing Store Objects
  • content-addressed derivation

  Software Heritage's writing on Intrinsic and Extrinsic identifiers is also a good introduction to the value of content-addressing over other referencing schemes.

  Besides content addressing, the Nix store also uses input addressing.

• derivation

  A description of a build task. The result of a derivation is a store object. Derivations declared in Nix expressions are specified using the derivation primitive. These are translated into low-level store derivations (implicitly by 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. It is the instantiated form of a derivation.

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

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

• instantiate, instantiation

  Save an evaluated derivation as a store derivation in the Nix store.

  See nix-instantiate, which produces a store derivation from a Nix expression that evaluates to a derivation.

• realise, realisation

  Ensure a store path is valid.

  This can be achieved by:

  • Fetching a pre-built store object from a substituter
  • Running the builder executable as specified in the corresponding derivation
  • Delegating to a remote machine and retrieving the outputs

  See nix-store --realise for a detailed description of the algorithm.

  See also nix-build and nix build (experimental).

• content-addressed derivation

  A derivation which has the __contentAddressed attribute set to true.

• fixed-output derivation

  A derivation which includes the outputHash attribute.

• store

  A collection of store objects, with operations to manipulate that collection. See Nix Store for details.

  There are many types of stores, see Store Types for details.

• 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

  See Store Path for details.

• file system object

  The Nix data model for representing simplified file system data.

  See File System Object for details.

• store object

  Part of the contents of a store.

  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.

  See Store Object for details.

• IFD

  Import From Derivation

• input-addressed store object

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

• content-addressed store object

  A store object which is content-addressed, i.e. whose store path is determined by its contents. This includes derivations, the outputs of content-addressed derivations, and the outputs of fixed-output derivations.

  See Content-Addressing Store Objects for details.

• 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 Nix 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.

• impure derivation

  An experimental feature that allows derivations to be explicitly marked as impure, so that they are always rebuilt, and their outputs not reused by subsequent calls to realise them.

• 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

  1. Commonly, a high-level description of software packages and compositions thereof. Deploying software using Nix entails writing Nix expressions for your packages. Nix expressions specify derivations, which are instantiated into the Nix store as store derivations. These derivations can then be realised to produce outputs.

  2. A syntactically valid use of the Nix language. For example, the contents of a .nix file form an expression.

• 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. See the outputs argument to the derivation function for details.

• output path

  The store path to the output of a derivation.

• output closure
  The closure of an output path. It only contains what is [reachable] from the output.

• deriving path

  Deriving paths are a way to refer to store objects that ar not yet realised. This is necessary because, in general and particularly for [content-addressed derivations][content-addressed derivation], the output path of an output is not known in advance. There are two forms:

  • constant: just a store path It can be made valid by copying it into the store: from the evaluator, command line interface or another store.

  • output: a pair of a store path to a derivation and an output name.

• deriver

  The store derivation that produced an output path.

  The deriver for an output path can be queried with the --deriver option to nix-store --query.

• 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.

• Nix Archive (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.

  See Nix Archive for details.

• ∅

  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.

• package

  1. A software package; a collection of files and other data.

  2. A package attribute set.

• package attribute set

  An attribute set containing the attribute type = "derivation"; (derivation for historical reasons), as well as other attributes, such as

  • attributes that refer to the files of a [package], typically in the form of derivation outputs,
  • attributes that declare something about how the package is supposed to be installed or used,
  • other metadata or arbitrary attributes.

• string interpolation

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

  See String interpolation for details.

• base directory

  The location from which relative paths are resolved.

  • For expressions in a file, the base directory is the directory containing that file. This is analogous to the directory of a base URL.
  • For expressions written in command line arguments with --expr, the base directory is the current working directory.

• 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.