Fetchers

Building software with Nix often requires downloading source code and other files from the internet. nixpkgs provides fetchers for different protocols and services. Fetchers are functions that simplify downloading files.

Caveats

Fetchers create fixed output derivations from downloaded files. Nix can reuse the downloaded files via the hash of the resulting derivation.

The fact that the hash belongs to the Nix derivation output and not the file itself can lead to confusion. For example, consider the following fetcher:

fetchurl {
  url = "http://www.example.org/hello-1.0.tar.gz";
  sha256 = "0v6r3wwnsk5pdjr188nip3pjgn1jrn5pc5ajpcfy6had6b3v4dwm";
};

A common mistake is to update a fetcher’s URL, or a version parameter, without updating the hash.

fetchurl {
  url = "http://www.example.org/hello-1.1.tar.gz";
  sha256 = "0v6r3wwnsk5pdjr188nip3pjgn1jrn5pc5ajpcfy6had6b3v4dwm";
};

This will reuse the old contents. Remember to invalidate the hash argument, in this case by setting the sha256 attribute to an empty string.

fetchurl {
  url = "http://www.example.org/hello-1.1.tar.gz";
  sha256 = "";
};

Use the resulting error message to determine the correct hash.

error: hash mismatch in fixed-output derivation '/path/to/my.drv':
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-RApQUm78dswhBLC/rfU9y0u6pSAzHceIJqgmetRD24E=

A similar problem arises while testing changes to a fetcher's implementation. If the output of the derivation already exists in the Nix store, test failures can go undetected. The invalidateFetcherByDrvHash function helps prevent reusing cached derivations.

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

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.

  • relative: Similar to using git-diff's --relative flag, only keep changes inside the specified directory, making paths relative to it.
  • stripLen: Remove the first stripLen components of pathnames in the patch.
  • extraPrefix: Prefix pathnames by this string.
  • excludes: Exclude files matching these patterns (applies after the above arguments).
  • includes: Include only files matching these patterns (applies after the above arguments).
  • revert: Revert the patch.

Note that because the checksum is computed after applying these effects, using or modifying these arguments will have no effect unless the sha256 argument is changed as well.

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 for more information:

{ stdenv, fetchgit }:

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

fetchFromGitea

fetchFromGitea expects five arguments. domain is the gitea server name. owner is a string corresponding to the Gitea user or organization that controls this repository. repo corresponds to the name of the software repository. These are located at the top of every Gitea 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

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.