nix-prefetch-url copy a file from a URL into the store and print its MD5 hash nix-prefetch-url url The command nix-prefetch-url downloads the file referenced by the URL url, prints its MD5 cryptographic hash code, and copies it into the Nix store. The file name in the store is hash-basename, where basename is everything following the final slash in url. This command is just a convenience to Nix expression writers. Often a Nix expressions fetch some source distribution from the network using the fetchurl expression contained in nixpkgs. However, fetchurl requires an MD5 hash. If you don't know the hash, you would have to download the file first, and then fetchurl would download it again when you build your Nix expression. Since fetchurl uses the same name for the downloaded file as nix-prefetch-url, the redundant download can be avoided. $ nix-prefetch-url ftp://ftp.nluug.nl/pub/gnu/make/make-3.80.tar.bz2 ... file has hash 0bbd1df101bc0294d440471e50feca71 ...