nixpkgs/nixos/modules/config/system-environment.nix

# This module defines a system-wide environment that will be
# initialised by pam_env (that is, not only in shells).
{ config, lib, pkgs, ... }:

with lib;

let

  cfg = config.environment;

in

{

  options = {

    environment.sessionVariables = mkOption {
      default = {};
      description = ''
        A set of environment variables used in the global environment.
        These variables will be set by PAM early in the login process.

        The value of each session variable can be either a string or a
        list of strings. The latter is concatenated, interspersed with
        colon characters.

        Note, due to limitations in the PAM format values may not
        contain the <literal>"</literal> character.

        Also, these variables are merged into
        <xref linkend="opt-environment.variables"/> and it is
        therefore not possible to use PAM style variables such as
        <code>@{HOME}</code>.
      '';
      type = with types; attrsOf (either str (listOf str));
      apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
    };

    environment.profileRelativeSessionVariables = mkOption {
      type = types.attrsOf (types.listOf types.str);
      example = { PATH = [ "/bin" ]; MANPATH = [ "/man" "/share/man" ]; };
      description = ''
        Attribute set of environment variable used in the global
        environment. These variables will be set by PAM early in the
        login process.

        Variable substitution is available as described in
        <citerefentry>
          <refentrytitle>pam_env.conf</refentrytitle>
          <manvolnum>5</manvolnum>
        </citerefentry>.

        Each attribute maps to a list of relative paths. Each relative
        path is appended to the each profile of
        <option>environment.profiles</option> to form the content of
        the corresponding environment variable.

        Also, these variables are merged into
        <xref linkend="opt-environment.profileRelativeEnvVars"/> and it is
        therefore not possible to use PAM style variables such as
        <code>@{HOME}</code>.
      '';
    };

  };

  config = {

    system.build.pamEnvironment =
      let
        suffixedVariables =
          flip mapAttrs cfg.profileRelativeSessionVariables (envVar: suffixes:
            flip concatMap cfg.profiles (profile:
              map (suffix: "${profile}${suffix}") suffixes
            )
          );

        # We're trying to use the same syntax for PAM variables and env variables.
        # That means we need to map the env variables that people might use to their
        # equivalent PAM variable.
        replaceEnvVars = replaceStrings ["$HOME" "$USER"] ["@{HOME}" "@{PAM_USER}"];

        pamVariable = n: v:
          ''${n}   DEFAULT="${concatStringsSep ":" (map replaceEnvVars (toList v))}"'';

        pamVariables =
          concatStringsSep "\n"
          (mapAttrsToList pamVariable
          (zipAttrsWith (n: concatLists)
            [
              (mapAttrs (n: toList) cfg.sessionVariables)
              suffixedVariables
            ]));
      in
        pkgs.writeText "pam-environment" "${pamVariables}\n";

  };

}