nix-prefetch-url - copy a file from a URL into the store and print its hash


nix-prefetch-url url [hash] [--type hashAlgo] [--print-path] [--unpack] [--name name]


The command nix-prefetch-url downloads the file referenced by the URL url, prints its cryptographic hash, 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 for Nix expression writers. Often a Nix expression fetches some source distribution from the network using the fetchurl expression contained in Nixpkgs. However, fetchurl requires a cryptographic 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.

If hash is specified, then a download is not performed if the Nix store already contains a file with the same hash and base name. Otherwise, the file is downloaded, and an error is signaled if the actual hash of the file does not match the specified hash.

This command prints the hash on standard output. Additionally, if the option --print-path is used, the path of the downloaded file in the Nix store is also printed.


  • --type hashAlgo
    Use the specified cryptographic hash algorithm, which can be one of md5, sha1, sha256, and sha512.

  • --print-path
    Print the store path of the downloaded file on standard output.

  • --unpack
    Unpack the archive (which must be a tarball or zip file) and add the result to the Nix store. The resulting hash can be used with functions such as Nixpkgs’s fetchzip or fetchFromGitHub.

  • --executable
    Set the executable bit on the downloaded file.

  • --name name
    Override the name of the file in the Nix store. By default, this is hash-basename, where basename is the last component of url. Overriding the name is necessary when basename contains characters that are not allowed in Nix store paths.


$ nix-prefetch-url
$ nix-prefetch-url --print-path mirror://gnu/hello/hello-2.10.tar.gz
$ nix-prefetch-url --unpack --print-path