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https://github.com/ryan4yin/nixos-and-flakes-book.git
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feat: update contents about lib.mkBefore & lib.mkAfter
This commit is contained in:
Ryan Yin
parent
883393eeb9
commit
50ef281137
@ -185,6 +185,8 @@ Then, for my desktop machine, I override the value in [ryan4yin/nix-config/blob/
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In addition to `lib.mkDefault` and `lib.mkForce`, there are also `lib.mkBefore` and `lib.mkAfter`, which are used to set the merge order of **list-type options**. These functions further contribute to the modularization of the configuration.
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> I haven't found the official documentation for list-type options, but I simply understand that they are types whose merge results are related to the order of merging. According to this understanding, both `list` and `string` types are list-type options, and these functions can indeed be used on these two types in practice.
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As mentioned earlier, when you define multiple values with the same **override priority**, Nix will throw an error. However, by using `lib.mkOrder`, `lib.mkBefore`, or `lib.mkAfter`, you can define multiple values with the same override priority, and they will be merged in the order you specify.
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To examine the source code of `lib.mkBefore`, you can run `nix repl -f '<nixpkgs>'` and then enter `:e lib.mkBefore`. To learn more about `nix repl`, type `:?` for the help information:
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@ -210,69 +212,86 @@ Therefore, `lib.mkBefore` is a shorthand for `lib.mkOrder 500`, and `lib.mkAfter
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To test the usage of `lib.mkBefore` and `lib.mkAfter`, let's create a simple Flake project:
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```shell{16-29}
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# Create flake.nix with the following content
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› cat <<EOF | sudo tee flake.nix
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```nix{10-38}
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# flake.nix
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{
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description = "Ryan's NixOS Flake";
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inputs = {
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nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
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};
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outputs = { self, nixpkgs, ... }@inputs: {
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inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
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outputs = {nixpkgs, ...}: {
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nixosConfigurations = {
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"nixos-test" = nixpkgs.lib.nixosSystem {
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system = "x86_64-linux";
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modules = [
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# Demo module 1: insert 'git' at the head of the list
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkBefore [pkgs.git];
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({lib, ...}: {
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programs.bash.shellInit = lib.mkBefore ''
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echo 'insert before default'
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'';
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programs.zsh.shellInit = lib.mkBefore "echo 'insert before default';";
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nix.settings.substituters = lib.mkBefore [
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"https://nix-community.cachix.org"
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];
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})
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# Demo module 2: insert 'vim' at the tail of the list
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkAfter [pkgs.vim];
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({lib, ...}: {
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programs.bash.shellInit = lib.mkAfter ''
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echo 'insert after default'
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'';
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programs.zsh.shellInit = lib.mkAfter "echo 'insert after default';";
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nix.settings.substituters = lib.mkAfter [
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"https://ryan4yin.cachix.org"
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];
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})
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# Demo module 3: simply add 'curl' to the list
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({lib, pkgs, ...}: {
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environment.systemPackages = with pkgs; [curl];
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({lib, ...}: {
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programs.bash.shellInit = ''
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echo 'this is default'
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'';
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programs.zsh.shellInit = "echo 'this is default';";
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nix.settings.substituters = [
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"https://nix-community.cachix.org"
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];
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})
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];
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};
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};
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};
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}
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EOF
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# Create flake.lock
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› nix flake update
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# Enter the nix repl environment
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› nix repl
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Welcome to Nix 2.13.3. Type :? for help.
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# Load the flake we just created
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nix-repl> :lf .
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Added 9 variables.
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# Check the order of systemPackages
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nix-repl> outputs.nixosConfigurations.nixos-test.config.environment.systemPackages
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[ «derivation /nix/store/0xvn7ssrwa0ax646gl4hwn8cpi05zl9j-git-2.40.1.drv»
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«derivation /nix/store/7x8qmbvfai68sf73zq9szs5q78mc0kny-curl-8.1.1.drv»
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«derivation /nix/store/bly81l03kh0dfly9ix2ysps6kyn1hrjl-nixos-container.drv»
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......
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......
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«derivation /nix/store/qpmpv
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q5azka70lvamsca4g4sf55j8994-vim-9.0.1441.drv» ]
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```
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As you can see, the order of `systemPackages` is `git -> curl -> default packages -> vim`, which matches the order we defined in `flake.nix`.
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The flake above contains the usage of `lib.mkBefore` and `lib.mkAfter` on multiline strings, single-line strings, and lists. Let's test the results:
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```bash
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# Example 1: multiline string merging
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› echo $(nix eval .#nixosConfigurations.nixos-test.config.programs.bash.shellInit)
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trace: warning: system.stateVersion is not set, defaulting to 23.11. Read why this matters on https://nixos.org/manual/nixos/stable/options.html#opt-system.stateVersio
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n.
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"echo 'insert before default'
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echo 'this is default'
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if [ -z \"$__NIXOS_SET_ENVIRONMENT_DONE\" ]; then
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. /nix/store/60882lm9znqdmbssxqsd5bgnb7gybaf2-set-environment
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fi
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echo 'insert after default'
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"
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# example 2: single-line string merging
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› echo $(nix eval .#nixosConfigurations.nixos-test.config.programs.zsh.shellInit)
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"echo 'insert before default';
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echo 'this is default';
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echo 'insert after default';"
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# Example 3: list merging
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› nix eval .#nixosConfigurations.nixos-test.config.nix.settings.substituters
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[ "https://nix-community.cachix.org" "https://nix-community.cachix.org" "https://cache.nixos.org/" "https://ryan4yin.cachix.org" ]
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```
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As you can see, `lib.mkBefore` and `lib.mkAfter` can define the order of merging of multiline strings, single-line strings, and lists. The order of merging is the same as the order of definition.
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> Although adjusting the order of `systemPackages` may not be useful in practice, it can be helpful in other scenarios.
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> For a deeper introduction to the module system, see [Module System & Custom Options](../other-usage-of-flakes/module-system.md).
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@ -162,6 +162,8 @@ Nix Flakes 对目录结构没有任何要求，你可以参考上面的例子，
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`lib.mkBefore` 跟 `lib.mkAfter` 用于设置**列表类型**的合并顺序，它们跟 `lib.mkDefault` 和 `lib.mkForce` 一样，也被用于模块化配置。
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> 列表类型的定义我没找到官方文档，但我简单理解，应该就是合并结果与合并先后顺序有关的类型。按这个理解，list 跟 string 类型都是列表类型，实际测试这几个函数也确实能用在这两个类型上。
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前面说了如果你定义了多个优先级相同的值，Nix 会报错说存在参数冲突，需要你手动解决。
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但是如果你定义的是**列表类型**的值，Nix 就不会报错了，因为 Nix 会把你定义的多个值合并成一个列表，而 `lib.mkBefore` 跟 `lib.mkAfter` 就是用于设置这个列表的合并顺序的。
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@ -189,67 +191,86 @@ Nix Flakes 对目录结构没有任何要求，你可以参考上面的例子，
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为了更直观地理解这两个函数，现在来创建一个 flake 测试下：
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```shell{16-29}
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# 使用如下内容创建一个 flake.nix 文件
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› cat <<EOF | sudo tee flake.nix
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```nix{10-38}
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# flake.nix
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{
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description = "Ryan's NixOS Flake";
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inputs = {
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nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
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};
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outputs = { self, nixpkgs, ... }@inputs: {
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inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
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outputs = {nixpkgs, ...}: {
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nixosConfigurations = {
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"nixos-test" = nixpkgs.lib.nixosSystem {
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system = "x86_64-linux";
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modules = [
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# demo module 1, 在列表头插入 git
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkBefore [pkgs.git];
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({lib, ...}: {
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programs.bash.shellInit = lib.mkBefore ''
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echo 'insert before default'
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'';
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programs.zsh.shellInit = lib.mkBefore "echo 'insert before default';";
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nix.settings.substituters = lib.mkBefore [
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"https://nix-community.cachix.org"
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];
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})
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# demo module 2, 在列表尾插入 vim
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({lib, pkgs, ...}: {
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environment.systemPackages = lib.mkAfter [pkgs.vim];
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({lib, ...}: {
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programs.bash.shellInit = lib.mkAfter ''
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echo 'insert after default'
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'';
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programs.zsh.shellInit = lib.mkAfter "echo 'insert after default';";
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nix.settings.substituters = lib.mkAfter [
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"https://ryan4yin.cachix.org"
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];
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})
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# demo module 3, 添加 curl，但是不设置优先级
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({lib, pkgs, ...}: {
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environment.systemPackages = with pkgs; [curl];
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({lib, ...}: {
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programs.bash.shellInit = ''
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echo 'this is default'
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'';
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programs.zsh.shellInit = "echo 'this is default';";
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nix.settings.substituters = [
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"https://nix-community.cachix.org"
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];
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})
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];
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};
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};
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};
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}
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EOF
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# 生成 flake.lock
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› nix flake update
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# 进入 nix REPL 解释器
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› nix repl
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Welcome to Nix 2.13.3. Type :? for help.
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# 将我们刚刚创建好的 flake 加载到当前作用域中
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nix-repl> :lf .
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Added 9 variables.
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# 检查下 systemPackages 的顺序，看看跟我们预期的是否一致
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nix-repl> outputs.nixosConfigurations.nixos-test.config.environment.systemPackages
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[ «derivation /nix/store/0xvn7ssrwa0ax646gl4hwn8cpi05zl9j-git-2.40.1.drv»
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«derivation /nix/store/7x8qmbvfai68sf73zq9szs5q78mc0kny-curl-8.1.1.drv»
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«derivation /nix/store/bly81l03kh0dfly9ix2ysps6kyn1hrjl-nixos-container.drv»
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......
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......
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«derivation /nix/store/qpmpvq5azka70lvamsca4g4sf55j8994-vim-9.0.1441.drv» ]
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```
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能看到 `systemPackages` 的顺序是 `git -> curl -> default packages -> vim`，跟我们预期的一致。`lib.mkBefore [pkgs.git]` 确实是将 `git` 插入到了列表头，而 `lib.mkAfter [pkgs.vim]` 则是将 `vim` 插入到了列表尾。
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上面的例子包含了在多行字符串、单行字符串，以及列表三种类型上应用 `lib.mkBefore` 和 `lib.mkAfter`，下面测试下结果：
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```bash
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# 示例一：多行字符串合并
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› echo $(nix eval .#nixosConfigurations.nixos-test.config.programs.bash.shellInit)
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trace: warning: system.stateVersion is not set, defaulting to 23.11. Read why this matters on https://nixos.org/manual/nixos/stable/options.html#opt-system.stateVersio
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n.
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"echo 'insert before default'
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echo 'this is default'
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if [ -z \"$__NIXOS_SET_ENVIRONMENT_DONE\" ]; then
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. /nix/store/60882lm9znqdmbssxqsd5bgnb7gybaf2-set-environment
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fi
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echo 'insert after default'
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"
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# 示例二：单行字符串合并
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› echo $(nix eval .#nixosConfigurations.nixos-test.config.programs.zsh.shellInit)
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"echo 'insert before default';
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echo 'this is default';
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echo 'insert after default';"
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# 示例三：列表合并
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› nix eval .#nixosConfigurations.nixos-test.config.nix.settings.substituters
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[ "https://nix-community.cachix.org" "https://nix-community.cachix.org" "https://cache.nixos.org/" "https://ryan4yin.cachix.org" ]
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```
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可以看到，`lib.mkBefore` 会将后面的值插入到前面，而 `lib.mkAfter` 会将后面的值插入到前面。
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> 虽然单纯调整 `systemPackages` 的顺序没什么用，但是在其他地方可能会有用...
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> 对模块系统更深入的介绍，参见 [模块系统与自定义 options](../other-usage-of-flakes/module-system.md).
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