nixpkgs/modules/system/etc/etc.nix

# Produce a script to generate /etc.
{ config, pkgs, ... }:

with pkgs.lib;

###### interface
let

  option = {
    environment.etc = mkOption {
      default = [];
      example = [
        { source = "/nix/store/.../etc/dir/file.conf.example";
          target = "dir/file.conf";
          mode = "0440";
        }
      ];
      description = ''
        List of files that have to be linked in /etc.
      '';
    };
  };
in

###### implementation
let

  etc = pkgs.stdenv.mkDerivation {
    name = "etc";

    builder = ./make-etc.sh;

    /* !!! Use toXML. */
    sources = map (x: x.source) config.environment.etc;
    targets = map (x: x.target) config.environment.etc;
    modes = map (x: if x ? mode then x.mode else "symlink") config.environment.etc;
  };

in

{
  require = [option];

  system.build.etc = etc;

  system.activationScripts.etc = stringAfter [ "stdio" ]
    ''
      # Set up the statically computed bits of /etc.
      echo "setting up /etc..."
      ${pkgs.perl}/bin/perl ${./setup-etc.pl} ${etc}/etc
    '';

}