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,
  options,
  pkgs,
  ...
}:
let

  cfg = config.environment;

in

{

  options = {

    environment.sessionVariables = lib.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 `"` character.

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

    environment.profileRelativeSessionVariables = lib.mkOption {
      type = lib.types.attrsOf (lib.types.listOf lib.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
        {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 = lib.flip lib.mapAttrs cfg.profileRelativeSessionVariables (
          envVar: suffixes:
          lib.flip lib.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 = lib.replaceStrings [ "$HOME" "$USER" ] [ "@{HOME}" "@{PAM_USER}" ];

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

        pamVariables = lib.concatStringsSep "\n" (
          lib.mapAttrsToList pamVariable (
            lib.zipAttrsWith (n: lib.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 ]; }

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

}