When using Nix, you will frequently need to download source code and other files from the internet. Nixpkgs comes with a few helper functions that allow you to fetch fixed-output derivations in a structured way. The two fetcher primitives 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. 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 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. Other fetcher functions allow you to add source code directly from a VCS such as subversion or git. These are mostly straightforward names based on the name of the command used with the VCS system. Because they give you a working repository, they act most like fetchzip. 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. 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. fetchFromGitLab This is used with GitLab repositories. The arguments expected are very similar to fetchFromGitHub above. 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.