This provides a replacement/improvement for the Nix support built into Direnv.
Direnv provides a nice way to manage environment variables by project directory. By placing a file in any directory called .envrc with a simple entry, we can then use direnv to associate different environment settings for any file in the directory. Direnv performs this association with scripts, and a standard installation provides a use_nix function to pull environment settings from a nix-shell invocation of a Nix file.
Many tools are already Direnv-aware, including a variety of shell programs, and also text editors. Because Direnv is more well-known and integrated with than Nix, we can use this integration to relieve the burden of having to integrate all these tools with Nix. Also, because Direnv can pull environment variables from much more than Nix, we only have to configure these tools for Direnv.
As an alternative, consider a tool like nix-buffer which performs a functionality similar to Direnv, but only for the Emacs editor, and only pulling environment variables with Nix. By separating more concerns, Direnv gives us more flexibility/options.
Unfortunately, there's a few problems with the use_nix function that comes standard with Direnv:
- If we invoke
nix-collect-garbage, the files under/nix/storethat
various environment settings use (like PATH or PYTHONPATH) can be deleted.
use_nixcallsnix-shellevery time we ask Direnv what the environment
settings are for a given file. This might take a few seconds, which can be annoying when programming.
Our replacement provides a new function use_nix_gcrooted that addresses both of these problems.
As you may have guessed, for this integration to work you must perform the following steps first:
If you have a vanilla installation of Direnv, copy or link the provided direnvrc file to the configuration directory for Direnv (typically ~/.config/direnv).
If you already have a direnvrc file, edit it to source the one provided.
Now you'll have a new function use_nix_gcrooted available for use in .envrc files.
In any directory you can create .envrc files that use the use_nix_gcrooted function with the following options:
use_nix_gcrooted [-c] [-C CACHE_FILE]... [NIX_FILE]
NIX_FILE Nix file to use with nix-shell
-c cache environment settings
-C extra files to watch for cache invalidation
If NIX_FILE is provided, nix-shell is called on it, otherwise shell.nix is used if it exists in the same directory as .envrc, otherwise default.nix. Unlike use_nix, use_nix_gcrooted doesn't work with nix-shell's -p option, and only support Nix files.
use_nix_gcrooted creates a .direnv directory next to .envrc. In this directory, a .direnv/nix directory has symlinks that keep generated Nix artifacts from being garbage collected by nix-collect-garbage. You can delete this directory if you'd like to free them for collection.
If you use the -c option, then environment settings are cached in .direnv/env.cache. This cache is invalidated with timestamps. If you make changes and want to recalculate the cache, just touch the Nix file used by the use_nix_gcrooted entry in .envrc (usually touch shell.nix).
Additionally with the -C switch, you can specify more files to watch for cache invalidation.
Once you create the .envrc file, you need to activate the directory by changing into it and running
direnv allowHaving scripts dynamically modify environment variables has security implications, so Direnv forces users to manually elect into giving this ability to Direnv per-directory. You have to rerun direnv allow if the .envrc file changes.