The NixOS command line: old and new interfaces and everyday maintenance

Background: Historical baggage has left Nix with two command-line interfaces: nix-xxx, the older standalone commands, and nix xxx, the newer unified subcommands. Enabling nix-command and flakes makes the new CLI available, but the old commands are still used for older profiles, channels, and specific administrative operations. Parts of the new CLI remain experimental; consult the help for your installed version.

1. The everyday survival kit: just remember these 4#

For everyday use, start by understanding these four types of operation and their effects:

2. Old and new CLI counterparts#

After enabling experimental-features = [ "nix-command" "flakes" ], the new subcommands provide corresponding functionality. Profile formats and dependency sources differ, however, so the commands in this table are not unconditional drop-in replacements.

** Note the exceptions**: nixos-rebuild (system management) and nix-collect-garbage (garbage collection) handle system rebuilding and profile-history cleanup combined with GC. nix-collect-garbage also works on non-NixOS systems, while nix store gc handles only unreferenced store objects.

3. Common new commands for development and testing#

• Run a program temporarily, without installing it:
  EXCERPTCopy code

  nix run nixpkgs#cowsay -- "Hello NixOS"

• Enter a temporary shell environment:
  EXCERPTCopy code

  nix shell nixpkgs#python3 nixpkgs#postgresql

• Enter the project's development environment (requires a flake.nix in the project root with devShells configured):
  EXCERPTCopy code

  nix develop

4. Advanced generation management#

A generation is like a game's "save point." switch/boot operations that update the system profile can record a new save point; build/dry-build alone do not. Old generations are not deleted automatically.

Store GC and deleting profile history

The new nix CLI includes nix store gc to clean the underlying store directory, but it does not automatically delete old generations for you. If you use only nix store gc, the old generations remain, so their packages are not considered "unreferenced" and cannot be removed.
nix-collect-garbage collects only unreachable objects by default. Adding -d or --delete-older-than first deletes old history from the profiles it finds. Running it as root may affect multiple users' profiles; you must understand the scope of its scan.

History interfaces for the system profile

The new nix profile provides interfaces for the new profile workflow. When handling NixOS system-level configurations at /nix/var/nix/profiles/system, the old and new commands differ as follows:

• If you want to clean up with the new commands, this is an alternative:
  You can use nix profile wipe-history in place of nix-env.

    # 新命令：删除 30 天前的非当前历史（先确认本机版本支持此 profile）
    sudo nix profile wipe-history --profile /nix/var/nix/profiles/system --older-than 30d

• Why keep the traditional history interfaces and GC tools:
  Retaining generations by age or by count is a maintenance-policy choice. nix-collect-garbage --delete-older-than 30d combines "delete generations" and "reclaim space" in one operation. With the new commands, these are two separate steps: first delete generations with nix profile wipe-history, then reclaim space with nix store gc. This makes it easier to control history-deletion scope and space reclamation separately.

5. Declarative automatic maintenance: put it in your configuration#

To avoid running cleanup commands manually, enable automatic maintenance directly in configuration.nix:

{ config, pkgs, ... }:
{
  # 1. 限制 GRUB 启动菜单中保留的世代数量（避免启动列表过长）
  boot.loader.grub.configurationLimit = 20;
  # 2. 开启新版命令与 Flakes
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
  # 3. 自动垃圾回收（每周清理 30 天前的世代）
  nix.gc = {
    automatic = true;
    dates = "weekly";
    options = "--delete-older-than 30d";
  };
  # 4. 自动优化存储（硬链接去重，节省空间）
  nix.settings.auto-optimise-store = true;
  # 5. (可选) 构建期间尝试 GC 的可用空间阈值，不是容量硬限制
  nix.extraOptions = ''
    min-free = ${toString (5 * 1024 * 1024 * 1024)}
    max-free = ${toString (10 * 1024 * 1024 * 1024)}
  '';
}

6. Remember commands by responsibility#

1. Everyday system management: nixos-rebuild. Temporary programs and development environments: nix run, nix shell, and nix develop.
2. Profile, channel, and older-expression maintenance: nix-env, nix-build, nix-shell, and nix-channel still have uses, depending on your workflow.
3. Installation and recovery: nixos-install, nixos-enter, and nixos-generate-config may still be useful for recovery after installation.

Configuration files are a good fit for stable system and user environments; imperative environments are useful for temporary exploration. Garbage collection does not automatically remove all installed software, nor does it preserve rollback points that have already been removed.

The command table uses the current Nix 2.34 interface. nix profile install is a compatibility alias for add; older versions may have only the old name. A package output being in the store does not mean it is permanently installed. Outputs may remain in the store after a temporary environment ends and later be reclaimed by GC.