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 = lib.mdDoc ''
        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 `"` character.

        Also, these variables are merged into
        [](#opt-environment.variables) and it is
        therefore not possible to use PAM style variables such as
        `@{HOME}`.
      '';
      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 = lib.mdDoc ''
        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
        {manpage}`pam_env.conf(5)`.

        Each attribute maps to a list of relative paths. Each relative
        path is appended to the each profile of
        {option}`environment.profiles` to form the content of
        the corresponding environment variable.

        Also, these variables are merged into
        [](#opt-environment.profileRelativeEnvVars) and it is
        therefore not possible to use PAM style variables such as
        `@{HOME}`.
      '';
    };

  };

  config = {
    environment.etc."pam/environment".text = 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)
          [
            # Make sure security wrappers are prioritized without polluting
            # shell environments with an extra entry. Sessions which depend on
            # pam for its environment will otherwise have eg. broken sudo. In
            # particular Gnome Shell sometimes fails to source a proper
            # environment from a shell.
            { PATH = [ config.security.wrapperDir ]; }

            (mapAttrs (n: toList) cfg.sessionVariables)
            suffixedVariables
          ]));
    in ''
      ${pamVariables}
    '';
  };

}