From 09660b855778531be14968b720308d092af4dd2e Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 8 Dec 2020 17:16:23 +0100
Subject: [PATCH] Add 'nix run' and 'nix shell' manpages

---
 src/nix/run.cc   | 42 +++++------------------
 src/nix/run.md   | 87 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/nix/shell.md | 48 ++++++++++++++++++++++++++
 3 files changed, 143 insertions(+), 34 deletions(-)
 create mode 100644 src/nix/run.md
 create mode 100644 src/nix/shell.md

diff --git a/src/nix/run.cc b/src/nix/run.cc
index 92a52c6cd..c2400f9ee 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -86,26 +86,11 @@ struct CmdShell : InstallablesCommand, RunCommon, MixEnvironment
         return "run a shell in which the specified packages are available";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To start a shell providing GNU Hello from NixOS 20.03:",
-                "nix shell nixpkgs/nixos-20.03#hello"
-            },
-            Example{
-                "To start a shell providing youtube-dl from your 'nixpkgs' channel:",
-                "nix shell nixpkgs#youtube-dl"
-            },
-            Example{
-                "To run GNU Hello:",
-                "nix shell nixpkgs#hello -c hello --greeting 'Hi everybody!'"
-            },
-            Example{
-                "To run GNU Hello in a chroot store:",
-                "nix shell --store ~/my-nix nixpkgs#hello -c hello"
-            },
-        };
+        return
+          #include "shell.md"
+          ;
     }
 
     void run(ref<Store> store) override
@@ -168,22 +153,11 @@ struct CmdRun : InstallableCommand, RunCommon
         return "run a Nix application";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To run Blender:",
-                "nix run blender-bin"
-            },
-            Example{
-                "To run vim from nixpkgs:",
-                "nix run nixpkgs#vim"
-            },
-            Example{
-                "To run vim from nixpkgs with arguments:",
-                "nix run nixpkgs#vim -- --help"
-            },
-        };
+        return
+          #include "run.md"
+          ;
     }
 
     Strings getDefaultFlakeAttrPaths() override
diff --git a/src/nix/run.md b/src/nix/run.md
new file mode 100644
index 000000000..c178e8b13
--- /dev/null
+++ b/src/nix/run.md
@@ -0,0 +1,87 @@
+R""(
+
+# Examples
+
+* Run the default app from the `blender-bin` flake:
+
+  ```console
+  # nix run blender-bin
+  ```
+
+* Run a non-default app from the `blender-bin` flake:
+
+  ```console
+  # nix run blender-bin#blender_2_83
+  ```
+
+  Tip: you can find apps provided by this flake by running `nix flake
+  show blender-bin`.
+
+* Run `vim` from the `nixpkgs` flake:
+
+  ```console
+  # nix run nixpkgs#vim
+  ```
+
+  Note that `vim` (as of the time of writing of this page) is not an
+  app but a package. Thus, Nix runs the eponymous file from the `vim`
+  package.
+
+* Run `vim` with arguments:
+
+  ```console
+  # nix run nixpkgs#vim -- --help
+  ```
+
+# Description
+
+`nix run` builds and runs *installable*, which must evaluate to an
+*app* or a regular Nix derivation.
+
+If *installable* evaluates to an *app* (see below), it executes the
+program specified by the app definition.
+
+If *installable* evaluates to a derivation, it will try to execute the
+program `<out>/bin/<name>`, where *out* is the primary output store
+path of the derivation and *name* is the name part of the value of the
+`name` attribute of the derivation (e.g. if `name` is set to
+`hello-1.10`, it will run `$out/bin/hello`).
+
+# Flake output attributes
+
+If no flake output attribute is given, `nix run` tries the following
+flake output attributes:
+
+* `defaultApp.<system>`
+
+* `defaultPackage.<system>`
+
+If an attribute *name* is given, `nix run` tries the following flake
+output attributes:
+
+* `apps.<system>.<name>`
+
+* `packages.<system>.<name>`
+
+* `legacyPackages.<system>.<name>`
+
+# Apps
+
+An app is specified by a flake output attribute named
+`apps.<system>.<name>` or `defaultApp.<system>`. It looks like this:
+
+```nix
+apps.x86_64-linux.blender_2_79 = {
+  type = "app";
+  program = "${self.packages.x86_64-linux.blender_2_79}/bin/blender";
+};
+```
+
+The only supported attributes are:
+
+* `type` (required): Must be set to `app`.
+
+* `program` (required): The full path of the executable to run. It
+  must reside in the Nix store.
+
+)""
diff --git a/src/nix/shell.md b/src/nix/shell.md
new file mode 100644
index 000000000..2a379e03f
--- /dev/null
+++ b/src/nix/shell.md
@@ -0,0 +1,48 @@
+R""(
+
+# Examples
+
+* Start a shell providing `youtube-dl` from the `nixpkgs` flake:
+
+  ```console
+  # nix shell nixpkgs#youtube-dl
+  # youtube-dl --version
+  2020.11.01.1
+  ```
+
+* Start a shell providing GNU Hello from NixOS 20.03:
+
+  ```console
+  # nix shell nixpkgs/nixos-20.03#hello
+  ```
+
+* Run GNU Hello:
+
+  ```console
+  # nix shell nixpkgs#hello -c hello --greeting 'Hi everybody!'
+  Hi everybody!
+  ```
+
+* Run GNU Hello in a chroot store:
+
+  ```console
+  # nix shell --store ~/my-nix nixpkgs#hello -c hello
+  ```
+
+* Start a shell providing GNU Hello in a chroot store:
+
+  ```console
+  # nix shell --store ~/my-nix nixpkgs#hello nixpkgs#bashInteractive -c bash
+  ```
+
+  Note that it's necessary to specify `bash` explicitly because your
+  default shell (e.g. `/bin/bash`) generally will not exist in the
+  chroot.
+
+# Description
+
+`nix shell` runs a command in an environment in which the `$PATH`
+variable provides the specified *installables*. If not command is
+specified, it starts the default shell of your user account.
+
+)""