nixpkgs/doc/builders/fetchers.chapter.md

Fetchers

When using Nix, you will frequently need to download source code and other files from the internet. For this purpose, Nix provides the fixed output derivation feature and Nixpkgs provides various functions that implement the actual fetching from various protocols and services.

Caveats

Because fixed output derivations are identified by their hash, a common mistake is to update a fetcher's URL or a version parameter, without updating the hash. This will cause the old contents to be used. So remember to always invalidate the hash argument.

For those who develop and maintain fetchers, a similar problem arises with changes to the implementation of a fetcher. These may cause a fixed output derivation to fail, but won't normally be caught by tests because the supposed output is already in the store or cache. For the purpose of testing, you can use a trick that is embodied by the invalidateFetcherByDrvHash function. It uses the derivation name to create a unique output path per fetcher implementation, defeating the caching precisely where it would be harmful.

fetchurl and fetchzip

Two basic fetchers are fetchurl and fetchzip. Both of these have two required arguments, a URL and a hash. The hash is typically sha256, although many more hash algorithms are supported. Nixpkgs contributors are currently recommended to use sha256. This hash will be used by Nix to identify your source. A typical usage of fetchurl is provided below.

{ stdenv, fetchurl }:

stdenv.mkDerivation {
  name = "hello";
  src = fetchurl {
    url = "http://www.example.org/hello.tar.gz";
    sha256 = "1111111111111111111111111111111111111111111111111111";
  };
}

The main difference between fetchurl and fetchzip is in how they store the contents. fetchurl will store the unaltered contents of the URL within the Nix store. fetchzip on the other hand will decompress the archive for you, making files and directories directly accessible in the future. fetchzip can only be used with archives. Despite the name, fetchzip is not limited to .zip files and can also be used with any tarball.

fetchpatch works very similarly to fetchurl with the same arguments expected. It expects patch files as a source and performs normalization on them before computing the checksum. For example it will remove comments or other unstable parts that are sometimes added by version control systems and can change over time.

Most other fetchers return a directory rather than a single file.

fetchsvn

Used with Subversion. Expects url to a Subversion directory, rev, and sha256.

fetchgit

Used with Git. Expects url to a Git repo, rev, and sha256. rev in this case can be full the git commit id (SHA1 hash) or a tag name like refs/tags/v1.0.

Additionally the following optional arguments can be given: fetchSubmodules = true makes fetchgit also fetch the submodules of a repository. If deepClone is set to true, the entire repository is cloned as opposing to just creating a shallow clone. deepClone = true also implies leaveDotGit = true which means that the .git directory of the clone won't be removed after checkout.

If only parts of the repository are needed, sparseCheckout can be used. This will prevent git from fetching unnecessary blobs from server, see git sparse-checkout and git clone --filter for more infomation:

{ stdenv, fetchgit }:

stdenv.mkDerivation {
  name = "hello";
  src = fetchgit {
    url = "https://...";
    sparseCheckout = ''
      path/to/be/included
      another/path
    '';
    sha256 = "0000000000000000000000000000000000000000000000000000";
  };
}

fetchfossil

Used with Fossil. Expects url to a Fossil archive, rev, and sha256.

fetchcvs

Used with CVS. Expects cvsRoot, tag, and sha256.

fetchhg

Used with Mercurial. Expects url, rev, and sha256.

A number of fetcher functions wrap part of fetchurl and fetchzip. They are mainly convenience functions intended for commonly used destinations of source code in Nixpkgs. These wrapper fetchers are listed below.

fetchFromGitHub

fetchFromGitHub expects four arguments. owner is a string corresponding to the GitHub user or organization that controls this repository. repo corresponds to the name of the software repository. These are located at the top of every GitHub HTML page as owner/repo. rev corresponds to the Git commit hash or tag (e.g v1.0) that will be downloaded from Git. Finally, sha256 corresponds to the hash of the extracted directory. Again, other hash algorithms are also available but sha256 is currently preferred.

fetchFromGitHub uses fetchzip to download the source archive generated by GitHub for the specified revision. If leaveDotGit, deepClone or fetchSubmodules are set to true, fetchFromGitHub will use fetchgit instead. Refer to its section for documentation of these options.

fetchFromGitLab

This is used with GitLab repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromGitiles

This is used with Gitiles repositories. The arguments expected are similar to fetchgit.

fetchFromBitbucket

This is used with BitBucket repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromSavannah

This is used with Savannah repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromRepoOrCz

This is used with repo.or.cz repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromSourcehut

This is used with sourcehut repositories. Similar to fetchFromGitHub above, it expects owner, repo, rev and sha256, but don't forget the tilde (~) in front of the username! Expected arguments also include vc ("git" (default) or "hg"), domain and fetchSubmodules.

If fetchSubmodules is true, fetchFromSourcehut uses fetchgit or fetchhg with fetchSubmodules or fetchSubrepos set to true, respectively. Otherwise the fetcher uses fetchzip.