Merge pull request #4336 from NixOS/manpages

Documentation for nix subcommands

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+R""(
+
+# Examples
+
+* Show the store derivation that results from evaluating the Hello
+  package:
+
+  ```console
+  # nix show-derivation nixpkgs#hello
+  {
+    "/nix/store/s6rn4jz1sin56rf4qj5b5v8jxjm32hlk-hello-2.10.drv": {
+      …
+    }
+  }
+  ```
+
+* Show the full derivation graph (if available) that produced your
+  NixOS system:
+
+  ```console
+  # nix show-derivation -r /run/current-system
+  ```
+
+* Print all files fetched using `fetchurl` by Firefox's dependency
+  graph:
+
+  ```console
+  # nix show-derivation -r nixpkgs#firefox \
+    | jq -r '.[] | select(.outputs.out.hash and .env.urls) | .env.urls' \
+    | uniq | sort
+  ```
+
+  Note that `.outputs.out.hash` selects *fixed-output derivations*
+  (derivations that produce output with a specified content hash),
+  while `.env.urls` selects derivations with a `urls` attribute.
+
+# Description
+
+This command prints on standard output a JSON representation of the
+store derivations to which *installables* evaluate. Store derivations
+are used internally by Nix. They are store paths with extension `.drv`
+that represent the build-time dependency graph to which a Nix
+expression evaluates.
+
+By default, this command only shows top-level derivations, but with
+`--recursive`, it also shows their dependencies.
+
+The JSON output is a JSON object whose keys are the store paths of the
+derivations, and whose values are a JSON object with the following
+fields:
+
+* `outputs`: Information about the output paths of the
+  derivation. This is a JSON object with one member per output, where
+  the key is the output name and the value is a JSON object with these
+  fields:
+
+  * `path`: The output path.
+  * `hashAlgo`: For fixed-output derivations, the hashing algorithm
+    (e.g. `sha256`), optionally prefixed by `r:` if `hash` denotes a
+    NAR hash rather than a flat file hash.
+  * `hash`: For fixed-output derivations, the expected content hash in
+    base-16.
+
+  Example:
+
+  ```json
+  "outputs": {
+    "out": {
+      "path": "/nix/store/2543j7c6jn75blc3drf4g5vhb1rhdq29-source",
+      "hashAlgo": "r:sha256",
+      "hash": "6fc80dcc62179dbc12fc0b5881275898f93444833d21b89dfe5f7fbcbb1d0d62"
+    }
+  }
+  ```
+
+* `inputSrcs`: A list of store paths on which this derivation depends.
+
+* `inputDrvs`: A JSON object specifying the derivations on which this
+  derivation depends, and what outputs of those derivations. For
+  example,
+
+  ```json
+  "inputDrvs": {
+    "/nix/store/6lkh5yi7nlb7l6dr8fljlli5zfd9hq58-curl-7.73.0.drv": ["dev"],
+    "/nix/store/fn3kgnfzl5dzym26j8g907gq3kbm8bfh-unzip-6.0.drv": ["out"]
+  }
+  ```
+
+  specifies that this derivation depends on the `dev` output of
+  `curl`, and the `out` output of `unzip`.
+
+* `system`: The system type on which this derivation is to be built
+  (e.g. `x86_64-linux`).
+
+* `builder`: The absolute path of the program to be executed to run
+  the build. Typically this is the `bash` shell
+  (e.g. `/nix/store/r3j288vpmczbl500w6zz89gyfa4nr0b1-bash-4.4-p23/bin/bash`).
+
+* `args`: The command-line arguments passed to the `builder`.
+
+* `env`: The environment passed to the `builder`.
+
+)""