Store Path

Nix implements references to store objects as store paths.

Think of a store path as an opaque, unique identifier: The only way to obtain store path is by adding or building store objects. A store path will always reference exactly one store object.

Store paths are pairs of

  • A 20-byte digest for identification
  • A symbolic name for people to read

Example

  • Digest: b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z
  • Name: firefox-33.1

To make store objects accessible to operating system processes, stores have to expose store objects through the file system.

A store path is rendered to a file system path as the concatenation of

  • Store directory (typically /nix/store)
  • Path separator (/)
  • Digest rendered in a custom variant of Base32 (20 arbitrary bytes become 32 ASCII characters)
  • Hyphen (-)
  • Name

Example

  /nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1
  |--------| |------------------------------| |----------|
store directory            digest                 name

Store Directory

Every Nix store has a store directory.

Not every store can be accessed through the file system. But if the store has a file system representation, the store directory contains the store’s file system objects, which can be addressed by store paths.

This means a store path is not just derived from the referenced store object itself, but depends on the store the store object is in.

Note

The store directory defaults to /nix/store, but is in principle arbitrary.

It is important which store a given store object belongs to: Files in the store object can contain store paths, and processes may read these paths. Nix can only guarantee referential integrity if store paths do not cross store boundaries.

Therefore one can only copy store objects to a different store if

  • The source and target stores' directories match

    or

  • The store object in question has no references, that is, contains no store paths

One cannot copy a store object to a store with a different store directory. Instead, it has to be rebuilt, together with all its dependencies. It is in general not enough to replace the store directory string in file contents, as this may render executables unusable by invalidating their internal offsets or checksums.