Fetchurl

Fetchurl is a function in the Nix expression language used to acquire remote content during the build of a derivation. It downloads a file from a user-specified URL and yields a path to the downloaded file in the Nix store, which can be consumed as a source or input by other derivations. Fetchurl is one of several URL-fetching helpers in Nixpkgs and is commonly used to obtain tarballs, patches, or other release artifacts in a reproducible way.

Usage and syntax: fetchurl takes at least a url attribute and a sha256 attribute. Example: fetchurl { url = "https://example.org/foo-1.2.tar.gz"; sha256 = "0v..."; }. The function supports http and https URLs and stores the retrieved content under the store with a deterministic path. The sha256 hash binds the downloaded content to the derivation; if the file changes, the hash must be updated, otherwise the build fails. This mechanism underpins Nix’s content-addressable, reproducible builds and enables caching and pinning to specific sources.

Related functions include fetchzip, fetchpatch, fetchgit, and fetchFromGitHub, which fetch different types of resources. builtins.fetchurl is the language-level primitive used in newer Nix expressions. A common workflow uses a placeholder hash during initial setup to determine the correct sha256, after which the real hash is supplied.

See also: Nix, Nixpkgs, fixed-output derivations.