nixpkgs/nixos/modules/programs/shell.nix

# This module defines a standard configuration for NixOS shells.

{ config, lib, ... }:

with lib;

{

  config = {

    environment.shellInit =
      ''
        # Set up the per-user profile.
        mkdir -m 0755 -p "$NIX_USER_PROFILE_DIR"
        if [ "$(stat --printf '%u' "$NIX_USER_PROFILE_DIR")" != "$(id -u)" ]; then
            echo "WARNING: the per-user profile dir $NIX_USER_PROFILE_DIR should belong to user id $(id -u)" >&2
        fi

        if [ -w "$HOME" ]; then
          if ! [ -L "$HOME/.nix-profile" ]; then
              if [ "$USER" != root ]; then
                  ln -s "$NIX_USER_PROFILE_DIR/profile" "$HOME/.nix-profile"
              else
                  # Root installs in the system-wide profile by default.
                  ln -s /nix/var/nix/profiles/default "$HOME/.nix-profile"
              fi
          fi

          # Subscribe the root user to the NixOS channel by default.
          if [ "$USER" = root -a ! -e "$HOME/.nix-channels" ]; then
              echo "${config.system.defaultChannel} nixos" > "$HOME/.nix-channels"
          fi

          # Create the per-user garbage collector roots directory.
          NIX_USER_GCROOTS_DIR="/nix/var/nix/gcroots/per-user/$USER"
          mkdir -m 0755 -p "$NIX_USER_GCROOTS_DIR"
          if [ "$(stat --printf '%u' "$NIX_USER_GCROOTS_DIR")" != "$(id -u)" ]; then
              echo "WARNING: the per-user gcroots dir $NIX_USER_GCROOTS_DIR should belong to user id $(id -u)" >&2
          fi

          # Set up a default Nix expression from which to install stuff.
          if [ ! -e "$HOME/.nix-defexpr" -o -L "$HOME/.nix-defexpr" ]; then
              rm -f "$HOME/.nix-defexpr"
              mkdir -p "$HOME/.nix-defexpr"
              if [ "$USER" != root ]; then
                  ln -s /nix/var/nix/profiles/per-user/root/channels "$HOME/.nix-defexpr/channels_root"
              fi
          fi
        fi
      '';

  };

}