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

# This module defines a global environment configuration and
# a common configuration for all shells.

{ config, lib, utils, pkgs, ... }:

with lib;

let

  cfg = config.environment;

  exportedEnvVars =
    let
      absoluteVariables =
        mapAttrs (n: toList) cfg.variables;

      suffixedVariables =
        flip mapAttrs cfg.profileRelativeEnvVars (envVar: listSuffixes:
          concatMap (profile: map (suffix: "${profile}${suffix}") listSuffixes) cfg.profiles
        );

      allVariables =
        zipAttrsWith (n: concatLists) [ absoluteVariables suffixedVariables ];

      exportVariables =
        mapAttrsToList (n: v: ''export ${n}="${concatStringsSep ":" v}"'') allVariables;
    in
      concatStringsSep "\n" exportVariables;
in

{

  options = {

    environment.variables = mkOption {
      default = {};
      description = ''
        A set of environment variables used in the global environment.
        These variables will be set on shell initialisation.
        The value of each variable can be either a string or a list of
        strings.  The latter is concatenated, interspersed with colon
        characters.
      '';
      type = with types; attrsOf (either str (listOf str));
      apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
    };

    environment.profiles = mkOption {
      default = [];
      description = ''
        A list of profiles used to setup the global environment.
      '';
      type = types.listOf types.str;
    };

    environment.profileRelativeEnvVars = mkOption {
      type = types.attrsOf (types.listOf types.str);
      example = { PATH = [ "/bin" ]; MANPATH = [ "/man" "/share/man" ]; };
      description = ''
        Attribute set of environment variable.  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.
      '';
    };

    # !!! isn't there a better way?
    environment.extraInit = mkOption {
      default = "";
      description = ''
        Shell script code called during global environment initialisation
        after all variables and profileVariables have been set.
        This code is asumed to be shell-independent, which means you should
        stick to pure sh without sh word split.
      '';
      type = types.lines;
    };

    environment.shellInit = mkOption {
      default = "";
      description = ''
        Shell script code called during shell initialisation.
        This code is asumed to be shell-independent, which means you should
        stick to pure sh without sh word split.
      '';
      type = types.lines;
    };

    environment.loginShellInit = mkOption {
      default = "";
      description = ''
        Shell script code called during login shell initialisation.
        This code is asumed to be shell-independent, which means you should
        stick to pure sh without sh word split.
      '';
      type = types.lines;
    };

    environment.interactiveShellInit = mkOption {
      default = "";
      description = ''
        Shell script code called during interactive shell initialisation.
        This code is asumed to be shell-independent, which means you should
        stick to pure sh without sh word split.
      '';
      type = types.lines;
    };

    environment.shellAliases = mkOption {
      default = {};
      example = { ll = "ls -l"; };
      description = ''
        An attribute set that maps aliases (the top level attribute names in
        this option) to command strings or directly to build outputs. The
        aliases are added to all users' shells.
      '';
      type = types.attrs; # types.attrsOf types.stringOrPath;
    };

    environment.binsh = mkOption {
      default = "${config.system.build.binsh}/bin/sh";
      defaultText = "\${config.system.build.binsh}/bin/sh";
      example = literalExample ''
        "''${pkgs.dash}/bin/dash"
      '';
      type = types.path;
      visible = false;
      description = ''
        The shell executable that is linked system-wide to
        <literal>/bin/sh</literal>. Please note that NixOS assumes all
        over the place that shell to be Bash, so override the default
        setting only if you know exactly what you're doing.
      '';
    };

    environment.shells = mkOption {
      default = [];
      example = literalExample "[ pkgs.bashInteractive pkgs.zsh ]";
      description = ''
        A list of permissible login shells for user accounts.
        No need to mention <literal>/bin/sh</literal>
        here, it is placed into this list implicitly.
      '';
      type = types.listOf (types.either types.shellPackage types.path);
    };

  };

  config = {

    system.build.binsh = pkgs.bashInteractive;

    # Set session variables in the shell as well. This is usually
    # unnecessary, but it allows changes to session variables to take
    # effect without restarting the session (e.g. by opening a new
    # terminal instead of logging out of X11).
    environment.variables = config.environment.sessionVariables;

    environment.etc."shells".text =
      ''
        ${concatStringsSep "\n" (map utils.toShellPath cfg.shells)}
        /bin/sh
      '';

    system.build.setEnvironment = pkgs.writeText "set-environment"
       ''
         ${exportedEnvVars}

         ${cfg.extraInit}

         # ~/bin if it exists overrides other bin directories.
         export PATH="$HOME/bin:$PATH"
       '';

    system.activationScripts.binsh = stringAfter [ "stdio" ]
      ''
        # Create the required /bin/sh symlink; otherwise lots of things
        # (notably the system() function) won't work.
        mkdir -m 0755 -p /bin
        ln -sfn "${cfg.binsh}" /bin/.sh.tmp
        mv /bin/.sh.tmp /bin/sh # atomically replace /bin/sh
      '';

  };

}