DIRENV-FETCHURL 1 “2019” direnv “User Manuals”


direnv fetchurl - Fetch a URL to disk


direnv fetchurl []


This command downloads the given URL into a fixed disk location, based on the content of the retrieved file.

This has been introduced to avoid a dependency on curl or wget, while also promoting a more secure way to fetch data from the Internet. Use this instead of curl <url> | sh.

This command has two modes of operation:

  1. Just pass the URL to discover the integrity hash.
  2. Pass the URL and the integrity hash to get back the on-disk location.

Since the on-disk location is based on the hash, it also acts as a cache. One implication of this design is that URLs must be stable and always return the same content.

Downloaded files are marked as read-only and executable so it can also be used to fetch and execute static binaries.



direnv fetchurl outputs different content based on the argument.

If the integrity-hash is being passed, it will output the path to the on-disk location, if the retrieval was successful.

If only the url is being passed, it will output the hash as well as some human-readable instruction. If stdout is not a tty, only the hash will be displayed.


$ ./direnv fetchurl https://releases.nixos.org/nix/nix-2.3.7/install
Found hash: sha256-7Gxl5GzI9juc/U30Igh/pY+p6+gj5Waohfwql3jHIds=

Invoke fetchurl again with the hash as an argument to get the disk location:

  direnv fetchurl "https://releases.nixos.org/nix/nix-2.3.7/install" "sha256-7Gxl5GzI9juc/U30Igh/pY+p6+gj5Waohfwql3jHIds="
  #=> /home/zimbatm/.cache/direnv/cas/sha256-7Gxl5GzI9juc_U30Igh_pY+p6+gj5Waohfwql3jHIds=


XDG_CACHE_HOME This variable is used to select the on-disk location of the fetched URLs as $XDG_CACHE_HOME/direnv/cas. If XDG_CACHE_HOME is unset or empty, defaults to $HOME/.cache.

MIT licence - Copyright (C) 2019 @zimbatm and contributors