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105 lines
4.7 KiB
Markdown
105 lines
4.7 KiB
Markdown
# Glossary
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- [derivation]{#gloss-derivation}\
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A description of a build action. The result of a derivation is a
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store object. Derivations are typically specified in Nix expressions
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using the [`derivation` primitive](expressions/derivations.md). These are
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translated into low-level *store derivations* (implicitly by
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`nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
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- [store]{#gloss-store}\
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The location in the file system where store objects live. Typically
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`/nix/store`.
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- [store path]{#gloss-store-path}\
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The location in the file system of a store object, i.e., an
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immediate child of the Nix store directory.
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- [store object]{#gloss-store-object}\
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A file that is an immediate child of the Nix store directory. These
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can be regular files, but also entire directory trees. Store objects
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can be sources (objects copied from outside of the store),
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derivation outputs (objects produced by running a build action), or
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derivations (files describing a build action).
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- [substitute]{#gloss-substitute}\
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A substitute is a command invocation stored in the Nix database that
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describes how to build a store object, bypassing the normal build
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mechanism (i.e., derivations). Typically, the substitute builds the
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store object by downloading a pre-built version of the store object
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from some server.
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- [purity]{#gloss-purity}\
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The assumption that equal Nix derivations when run always produce
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the same output. This cannot be guaranteed in general (e.g., a
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builder can rely on external inputs such as the network or the
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system time) but the Nix model assumes it.
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- [Nix expression]{#gloss-nix-expression}\
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A high-level description of software packages and compositions
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thereof. Deploying software using Nix entails writing Nix
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expressions for your packages. Nix expressions are translated to
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derivations that are stored in the Nix store. These derivations can
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then be built.
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- [reference]{#gloss-reference}\
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A store path `P` is said to have a reference to a store path `Q` if
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the store object at `P` contains the path `Q` somewhere. The
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*references* of a store path are the set of store paths to which it
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has a reference.
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A derivation can reference other derivations and sources (but not
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output paths), whereas an output path only references other output
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paths.
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- [reachable]{#gloss-reachable}\
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A store path `Q` is reachable from another store path `P` if `Q`
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is in the *closure* of the *references* relation.
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- [closure]{#gloss-closure}\
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The closure of a store path is the set of store paths that are
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directly or indirectly “reachable” from that store path; that is,
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it’s the closure of the path under the *references* relation. For
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a package, the closure of its derivation is equivalent to the
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build-time dependencies, while the closure of its output path is
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equivalent to its runtime dependencies. For correct deployment it
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is necessary to deploy whole closures, since otherwise at runtime
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files could be missing. The command `nix-store -qR` prints out
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closures of store paths.
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As an example, if the store object at path `P` contains a reference
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to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
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references `R` then `R` is also in the closure of `P`.
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- [output path]{#gloss-output-path}\
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A store path produced by a derivation.
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- [deriver]{#gloss-deriver}\
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The deriver of an *output path* is the store
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derivation that built it.
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- [validity]{#gloss-validity}\
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A store path is considered *valid* if it exists in the file system,
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is listed in the Nix database as being valid, and if all paths in
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its closure are also valid.
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- [user environment]{#gloss-user-env}\
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An automatically generated store object that consists of a set of
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symlinks to “active” applications, i.e., other store paths. These
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are generated automatically by
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[`nix-env`](command-ref/nix-env.md). See *profiles*.
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- [profile]{#gloss-profile}\
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A symlink to the current *user environment* of a user, e.g.,
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`/nix/var/nix/profiles/default`.
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- [NAR]{#gloss-nar}\
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A *N*ix *AR*chive. This is a serialisation of a path in the Nix
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store. It can contain regular files, directories and symbolic
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links. NARs are generated and unpacked using `nix-store --dump`
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and `nix-store --restore`.
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- [`∅`]{#gloss-emtpy-set}\
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The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.
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- [`ε`]{#gloss-epsilon}\
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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.
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