[WIP] Nix overlay for making programs xdg compliant

This repo contains a nixos module (also usable with home-manager) for making programs xdg compliant, by wrapping the binaries with certain environment variables set or certains flags passed. Created the first draft in early 2017, but only just got it to a usable state, motivated by nix-community/home-manager#95.

All of it is done with a manual specification for each program, which looks something like this:

{
  wget.flags = { cache }: "--hsts-file=${cache}/hsts";
  less.env.ZDOTDIR = { config }: config;
}

This spec defines that all binaries in the wget package be wrapped with the "--hsts-file" flag set to a file in the packages cache directory, which is ${XDG_CACHE_HOME:-$HOME/.cache}/wget in this case, evaluating to ~/.cache/wget at runtime if XDG_CACHE_HOME isn't set. Second, all binaries in the zsh package be wrapped with ZDOTDIR set to the packages config directory, which is ${XDG_CONFIG_HOME:-$HOME/.config}/zsh. Similarly, you can use a data argument for getting ${XDG_DATA_HOME:-$HOME/.local/share}/<name>.

The specs.nix file contains some more examples. Also supported is wrapping specific binaries (for packages containing multiple binaries) and changing the folder name of the package directories (maybe useful for different versions). By default, the top-level attributes name ("wget" and "less" in the above example is used to get the package to wrap, this can be made explicit with package attribute. The following example demonstrates these features (along with some others):

{
  myHello = {
    # Use the hello package, since `pkgs.myHello` doesn't exist
    package = pkgs.hello;

    # This will make the config argument be
    # "${XDG_CONFIG_HOME:-$HOME/.config}/hithere", and similarily for cache and
    # data
    xdgName = "hithere";

    # Wrapping the hello binary only
    bin.hello.env.HELLO_DATA = { data }: data;

    # Such a function can occur anywhere _within_ the packages spec, and the
    arguments will contain the packages directories
    env = { config }: {
      # This will apply for all binaries except the hello one, since HELLO_DATA
      # is explicitly specified for it already.
      HELLO_DATA = "$HOME/.hello";
      HELLO_CONFIG = config;
    };
  };
}

How to use

Import the module.nix file in your NixOS or home-manager configuration, by either cloning this repository and adding

{
  imports = [
    ~/path/to/module.nix
  ];
}

or with nix directly:

{
  imports = [
    (builtins.fetchTarBall https://github.com/Infinisil/xdg-nix/archive/master.tar.gz)
  ];
}

By default no specs are added.

Adding specs

You can add specifications by declaring them in the xdgSpec option like

{
  xdgSpec = {
    wget.flags = { cache }: "--hsts-file=${cache}/hsts";
  };
}

All attributes in the specs.nix file are available through an optional argument. For example you can include the base set like this:

{
  xdgSpec = specs: specs.base;
}

Using packages

To actually use the wrapped package, you'll have to specify it somewhere in your nixos / home-manager config, such as

{
  # NixOS
  environment.systemPackages = with pkgs; [ wget ];

  # home-manager
  home.packages = with pkgs; [ wget ];
}

Avoiding rebuild of dependent packages

Since the wrapped packages differ, other packages dependent on these will be rebuilt when adding/changing the spec, which is unavoidable if you really want to use overlays and have the changes propagate. To avoid having to do that, you can include the package in your packages directly, which is mostly just what you need:

{ config, ... }:
let
  xdg = import <nixpkgs> {
    overlays = [ (config.lib.xdg.xdgOverlay {
      specs = foo: foo.all;
    })];
  };
in {
  # NixOS
  environment.systemPackages = with xdg; [ curl ];

  # home-manager
  home.packages = with xdg; [ curl ];
}

ToDo

• Split into more files, add default.nix, support for just importing the functions, add overlay.nix
• Overlay rebuilds a lot, but most often you just want changes in the user profiles packages anyways -> what to use instead of an overlay to not make it ugly?
• How to use only specific packages wrapped easily?
• Potential home-manager integration, see nix-community/home-manager#95
• Go through https://wiki.archlinux.org/index.php/XDG_Base_Directory_support and check if there are any problems
• Maybe add XDG_RUNTIME_DIR