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.


A HTTP URL that returns content on HTTP GET requests. 301 and other redirects are followed. When passed, the integrity of the retrieved content will be validated against the given hash. The hash encoding is based on the SRI W3C specificiation (see https://www.w3.org/TR/SRI/ ).


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