Merge pull request #4336 from NixOS/manpages

Documentation for nix subcommands

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+R""(
+
+# Description
+
+`nix flake` provides subcommands for managing *flake
+registries*. Flake registries are a convenience feature that allows
+you to refer to flakes using symbolic identifiers such as `nixpkgs`,
+rather than full URLs such as `git://github.com/NixOS/nixpkgs`. You
+can use these identifiers on the command line (e.g. when you do `nix
+run nixpkgs#hello`) or in flake input specifications in `flake.nix`
+files. The latter are automatically resolved to full URLs and recorded
+in the flake's `flake.lock` file.
+
+In addition, the flake registry allows you to redirect arbitrary flake
+references (e.g. `github:NixOS/patchelf`) to another location, such as
+a local fork.
+
+There are multiple registries. These are, in order from lowest to
+highest precedence:
+
+* The global registry, which is a file downloaded from the URL
+  specified by the setting `flake-registry`. It is cached locally and
+  updated automatically when it's older than `tarball-ttl`
+  seconds. The default global registry is kept in [a GitHub
+  repository](https://github.com/NixOS/flake-registry).
+
+* The system registry, which is shared by all users. The default
+  location is `/etc/nix/registry.json`. On NixOS, the system registry
+  can be specified using the NixOS option `nix.registry`.
+
+* The user registry `~/.config/nix/registry.json`. This registry can
+  be modified by commands such as `nix flake pin`.
+
+* Overrides specified on the command line using the option
+  `--override-flake`.
+
+# Registry format
+
+A registry is a JSON file with the following format:
+
+```json
+{
+  "version": 2,
+  [
+    {
+      "from": {
+        "type": "indirect",
+        "id": "nixpkgs"
+      },
+      "to": {
+        "type": "github",
+        "owner": "NixOS",
+        "repo": "nixpkgs"
+      }
+    },
+    ...
+  ]
+}
+```
+
+That is, it contains a list of objects with attributes `from` and
+`to`, both of which contain a flake reference in attribute
+representation. (For example, `{"type": "indirect", "id": "nixpkgs"}`
+is the attribute representation of `nixpkgs`, while `{"type":
+"github", "owner": "NixOS", "repo": "nixpkgs"}` is the attribute
+representation of `github:NixOS/nixpkgs`.)
+
+Given some flake reference *R*, a registry entry is used if its
+`from` flake reference *matches* *R*. *R* is then replaced by the
+*unification* of the `to` flake reference with *R*.
+
+# Matching
+
+The `from` flake reference in a registry entry *matches* some flake
+reference *R* if the attributes in `from` are the same as the
+attributes in `R`. For example:
+
+* `nixpkgs` matches with `nixpkgs`.
+
+* `nixpkgs` matches with `nixpkgs/nixos-20.09`.
+
+* `nixpkgs/nixos-20.09` does not match with `nixpkgs`.
+
+* `nixpkgs` does not match with `git://github.com/NixOS/patchelf`.
+
+# Unification
+
+The `to` flake reference in a registry entry is *unified* with some flake
+reference *R* by taking `to` and applying the `rev` and `ref`
+attributes from *R*, if specified. For example:
+
+* `github:NixOS/nixpkgs` unified with `nixpkgs` produces `github:NixOS/nixpkgs`.
+
+* `github:NixOS/nixpkgs` unified with `nixpkgs/nixos-20.09` produces `github:NixOS/nixpkgs/nixos-20.09`.
+
+* `github:NixOS/nixpkgs/master` unified with `nixpkgs/nixos-20.09` produces `github:NixOS/nixpkgs/nixos-20.09`.
+
+)""