Add 'nix' manpage

src/nix/main.cc

@@ -184,6 +184,13 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
     {
         return "a tool for reproducible and declarative configuration management";
     }
+
+    std::string doc() override
+    {
+        return
+          #include "nix.md"
+          ;
+    }
 };
 
 static void showHelp(std::vector<std::string> subcommand)

src/nix/nix.md

@@ -0,0 +1,119 @@
+R""(
+
+# Examples
+
+* Create a new flake:
+
+  ```console
+  # nix flake new hello
+  # cd hello
+  ```
+
+* Build the flake in the current directory:
+
+  ```console
+  # nix build
+  # ./result/bin/hello
+  Hello, world!
+  ```
+
+* Run the flake in the current directory:
+
+  ```console
+  # nix run
+  Hello, world!
+  ```
+
+* Start a development shell for hacking on this flake:
+
+  ```console
+  # nix develop
+  # unpackPhase
+  # cd hello-*
+  # configurePhase
+  # buildPhase
+  # ./hello
+  Hello, world!
+  # installPhase
+  # ../outputs/out/bin/hello
+  Hello, world!
+  ```
+
+# Description
+
+Nix is a tool for building software, configurations and other
+artifacts in a reproducible and declarative way. For more information,
+see the [Nix homepage](https://nixos.org/) or the [Nix
+manual](https://nixos.org/manual/nix/stable/).
+
+# Installables
+
+Many `nix` subcommands operate on one or more *installables*. These are
+command line arguments that represent something that can be built in
+the Nix store. Here are the recognised types of installables:
+
+* **Flake output attributes**: `nixpkgs#hello`
+
+  These have the form *flakeref*[`#`*attrpath*], where *flakeref* is a
+  flake reference and *attrpath* is an optional attribute path. For
+  more information on flakes, see [the `nix flake` manual
+  page](./nix3-flake.md). Flake references are most commonly a flake
+  identifier in the flake registry (e.g. `nixpkgs`) or a path
+  (e.g. `/path/to/my-flake` or `.`).
+
+  If *attrpath* is omitted, Nix tries some default values; for most
+  subcommands, the default is `defaultPackage.`*system*
+  (e.g. `defaultPackage.x86_64-linux`), but some subcommands have
+  other defaults. If *attrpath* *is* specified, *attrpath* is
+  interpreted as relative to one or more prefixes; for most
+  subcommands, these are `packages.`*system*,
+  `legacyPackages.*system*` and the empty prefix. Thus, on
+  `x86_64-linux` `nix build nixpkgs#hello` will try to build the
+  attributes `packages.x86_64-linux.hello`,
+  `legacyPackages.x86_64-linux.hello` and `hello`.
+
+* **Store paths**: `/nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10`
+
+  These are paths inside the Nix store, or symlinks that resolve to a
+  path in the Nix store.
+
+* **Store derivations**: `/nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv`
+
+  Store derivations are store paths with extension `.drv` and are a
+  low-level representation of a build-time dependency graph used
+  internally by Nix. By default, if you pass a store derivation to a
+  `nix` subcommand, it will operate on the *output paths* of the
+  derivation. For example, `nix path-info` prints information about
+  the output paths:
+
+  ```console
+  # nix path-info --json /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv
+  [{"path":"/nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10",…}]
+  ```
+
+  If you want to operate on the store derivation itself, pass the
+  `--derivation` flag.
+
+* **Nix attributes**: `--file /path/to/nixpkgs hello`
+
+  When the `-f` / `--file` *path* option is given, installables are
+  interpreted as attribute paths referencing a value returned by
+  evaluating the Nix file *path*.
+
+* **Nix expressions**: `--expr '(import <nixpkgs> {}).hello.overrideDerivation (prev: { name = "my-hello"; })'`.
+
+  When the `--expr` option is given, all installables are interpreted
+  as Nix expressions. You may need to specify `--impure` if the
+  expression references impure inputs (such as `<nixpkgs>`).
+
+For most commands, if no installable is specified, the default is `.`,
+i.e. Nix will operate on the default flake output attribute of the
+flake in the current directory.
+
+# Nix stores
+
+Most `nix` subcommands operate on a *Nix store*.
+
+TODO: list store types, options
+
+)""