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diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..d645695
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,202 @@
+
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diff --git a/README.md b/README.md
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index 0000000..cfb1773
--- /dev/null
+++ b/README.md
@@ -0,0 +1,167 @@
+# Justbuild on Nix
+
+This repository demonstrates a way to build locally with
+[justbuild](https://github.com/just-buildsystem/justbuild)
+using dependencies from [Nix](https://nixos.org/) in a clean and
+versioned way.
+
+## Background: Standard Paths on non-Nix Distributions
+
+On many non-Nix Linux distributions the local tools are installed to
+a small number of "standard" paths (like `/bin`, `/sbin`, `/usr/bin`,
+`/usr/sbin`). Moreover, tools like `sh` (used by default by the
+built-in `"generic"` rule) and `env` (the default action launcher)
+are configured to take those standard paths into account if the
+`PATH` environment variable is not set on their startup. Hence
+the tools installed in those standard paths are always available
+for local builds, without the need of setting `PATH` in action
+definitions.
+
+Such an approach obviously has the disadvantage that the cache has
+to be cleared, whenever the tools installed in those standard
+paths are updated. For users following a stable release with only
+security fixes, this can be acceptable.
+
+While not the idiomatic way of building on Nix, such a
+behavior can be simulated by using as launcher a tool
+like [withExtendedPath](./src/withExtendedPath/extend-path.cc)
+extending the value of the environment variable `PATH` by
+a given string, e.g., your `~/.nix-profile` controlled by
+the [home-manager](https://rycee.gitlab.io/home-manager/"). To
+avoid interfering with the clean Nix-idiomatic builds described
+in the following sections, it is recommended to also set the local
+build root to a different directory; remember to `just gc` twice
+whenever updating your `~/.nix-profile`. Your `~/.just-mrrc` thus
+would look something like
+```
+{ "local launcher":
+  [ "/home/YOURUSERNAME/.nix-profile/bin/withExtendedPath"
+  , "/home/YOURUSERNAME/.nix-profile/bin"
+  ]
+, "local build root": {"root": "home", "path": ".cache/just-nix-home"}
+}
+```
+Such an approach can be useful when dealing with a large number of
+project repositories all requiring basically the same tool chain.
+This discussion also shows how to easily provide a well-defined
+remote-execution environment on Nix: local launcher and local build
+root can also be set on the command line when starting `just execute`.
+
+## Getting Nix Paths into Justbuild Actions
+
+On Nix, the usual "standard" paths are pretty empty. Instead, all
+packages are installed into paths in the nix store containing a recursive
+hash of the full build description. So a path to the nix store brings
+a well-defined dependency and we're only left with the problem of
+getting the right paths into the actions.
+
+Justbuild deliberately ignores any environment variables of the invocation;
+the environment of an actions has to be provided through the build description.
+When using [`rules-cc`](https://github.com/just-buildsystem/rules-cc),
+the `"defaults"` targets can be used to set tools and paths for
+targets defined by a particular rule. We link those targets to
+the [`nixpkgs`](https://github.com/NixOS/nixpkgs) in a maintable
+way as follows.
+ - All the `"defaults"` targets simply take their values from appropriate
+   parts of the `"TOOLCHAIN_CONFIG"` just configuration variable.
+ - The just configuration file is generated by a
+   nix [derivation](./nix-dependencies/dependencies.nix) using that
+   nix derivations have easy access to the needed paths in the nix
+   store. That derivation also generates an rc-file pointing to
+   that configuration.
+ - The precise [sources](./nix/sources.json) of the `nixpkgs` are
+   pinned using [niv](https://github.com/nmattia/niv).
+ - A [nix shell](./shell.nix) uses the derivation at the pinned
+   snapshot of the `nixpkgs` and sets an alias for `just-mr` to
+   use the derived rc-file.
+
+So to build with the correct dependencies for the checked out version,
+simply start a `nix-shell` at the top level of this repository and
+use `just-mr build` as usual. It should be noted that the `nix-shell`
+does not pull in justbuild itself and instead inherits it from the
+invoking shell; in that way, when going back to an old snapshot
+it is still built using the currect justbuild. The reason is that,
+while newer versions of justbuild can work with a local build root
+generated by older versions, this is not necessarily the case the
+other way round (e.g., versions before `1.3.0` are not aware of
+the large-object CAS introduced in that release, and hence will
+not find certain artifacts referenced in the action cache). So,
+by using the current justbuild when checking out older snapshots,
+we can reconstruct the old actions without the need of cleaning up
+the local build root.
+
+## Shell commands
+
+The built-in rule `"generic"` allows to define a target by
+executing a shell command (via `sh -c ...`). This is convenient
+on systems where the shell is configured to have "the standard
+tools" available by default. On Nix, one would have to set
+the `"env"` appropriately on every invocation. As would be
+quite cumbersome, preference is given to the
+rule [`["shell", "cmds"]`](https://github.com/just-buildsystem/rules-cc?tab=readme-ov-file#rule-shell-cmds)
+that is a replacement for `"generic"` honoring the shell toolchain (as
+defined in the appropriate `"defaults"` target).
+
+
+## Repository Overview
+
+### Version pinning and updating
+
+There are two files pinning dependencies
+ - The `nixpkgs` are pinned in [nix/sources.json](./nix/sources.json). They can
+   be updated using [update-nix-dependencies.sh](./update-nix-dependencies.sh)
+   which simply calls `niv update` in our `nix-shell`.
+ - The dependencies on other other justbuild projects are
+   pinned in [etc/repos.json](etc/repos.json). As, at the
+   moment, there is only one external justbuild dependency,
+   the [rules-cc](https://github.com/just-buildsystem/rules-cc),
+   this is updated by hand. For larger
+   projects [just-import-git](https://github.com/just-buildsystem/justbuild/blob/master/share/man/just-import-git.1.md)
+   would be used to generate this file out of a description of the
+   local repositories and the dependencies to import.
+
+### Logical repositories
+
+This project uses several logical repositories.
+
+ - The directories `src` and `test` provide standard main and test
+   repositories. The logical structure is, as usual, that the test
+   repository has access to the main repository, but not the other
+   way round. In this way, the main repository can be imported
+   without pulling in the test dependencies. Note that in those
+   directories (where the main project-development work will happen)
+   there is nothing Nix-specific, except maybe for the choice to
+   refrain from using the built-in rule `"generic"`.
+
+ - The rules `rules/nix` and `rules/nix-test` are derived from
+   the [rules-cc](https://github.com/just-buildsystem/rules-cc) by
+   setting the target-file layer to the directory `etc/defaults`.
+   Here the toolchains are defined in a fine-granular way. The toolchains
+   of `rules/nix-test` inherit from the ones in `rules/nix`. In this
+   way, we can bring in additional tools only for tests. Those extra
+   paths are set in `TOOLCHAIN_CONFIG["test"]["PATH"]`.
+
+This fine-granular setting of the toolchains has the effect that actions only
+have the paths necessary set and in that way are not unnecessarily affected
+by changes of the depedencies. A good example for this granularity can be
+seen by building verbosely
+```
+[nix-shell]$  just-mr --main test build hello '' --log-limit 5
+```
+and looking at the differnt values of the environment: `PKG_CONFIG_PATH`
+is only set in the actions calling `pkg-config`, only the test
+action has the additional test tools in `PATH`.
+
+### Nix dependencies
+
+As explained, the nix-dependencies are declared in
+a [derivation](./nix-dependencies/dependencies.nix). For
+libraries, which probably form the majority of dependencies
+added, they way they become available is via `PKG_CONFIG_PATH`
+which makes them discoverable
+as [`["CC/pkgconfig", "system_library"]`](https://github.com/just-buildsystem/rules-cc?tab=readme-ov-file#rule-ccpkgconfig-system_library).
+As in the `buildPhase` of a Nix derivation, the environment variable
+`PKC_CONFIG_PATH` is already set appropriately, it is sufficient
+to simply add new libraries to `buildInputs`. As an example, note
+that `fmt` is available without being explicitly mentioned in
+`buildPhase`.
diff --git a/etc/defaults/CC/TARGETS b/etc/defaults/CC/TARGETS
new file mode 100644
index 0000000..4ac5673
--- /dev/null
+++ b/etc/defaults/CC/TARGETS
@@ -0,0 +1,51 @@
+{ "defaults":
+  { "type": "configure"
+  , "target": "defaults (unconfigured)"
+  , "arguments_config": ["TOOLCHAIN_CONFIG"]
+  , "config":
+    { "type": "let*"
+    , "bindings":
+      [ [ "CC toolchain"
+        , { "type": "lookup"
+          , "map":
+            { "type": "var"
+            , "name": "TOOLCHAIN_CONFIG"
+            , "default": {"type": "empty_map"}
+            }
+          , "key": "CC"
+          , "default": {"type": "empty_map"}
+          }
+        ]
+      , [ "CC"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "CC toolchain"}
+          , "key": "CC"
+          , "default": "cc"
+          }
+        ]
+      , [ "CXX"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "CC toolchain"}
+          , "key": "CXX"
+          , "default": "c++"
+          }
+        ]
+      , [ "PATH"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "CC toolchain"}
+          , "key": "PATH"
+          , "default": []
+          }
+        ]
+      ]
+    , "body": {"type": "env", "vars": ["CC", "CXX", "PATH"]}
+    }
+  }
+, "defaults (unconfigured)":
+  { "type": "defaults"
+  , "arguments_config": ["CC", "CXX", "PATH"]
+  , "CC": [{"type": "var", "name": "CC"}]
+  , "CXX": [{"type": "var", "name": "CXX"}]
+  , "PATH": {"type": "var", "name": "PATH"}
+  }
+}
diff --git a/etc/defaults/CC/pkgconfig/TARGETS b/etc/defaults/CC/pkgconfig/TARGETS
new file mode 100644
index 0000000..63be252
--- /dev/null
+++ b/etc/defaults/CC/pkgconfig/TARGETS
@@ -0,0 +1,51 @@
+{ "defaults":
+  { "type": "configure"
+  , "target": "defaults (unconfigured)"
+  , "arguments_config": ["TOOLCHAIN_CONFIG"]
+  , "config":
+    { "type": "let*"
+    , "bindings":
+      [ [ "PKGCONFIG toolchain"
+        , { "type": "lookup"
+          , "map":
+            { "type": "var"
+            , "name": "TOOLCHAIN_CONFIG"
+            , "default": {"type": "empty_map"}
+            }
+          , "key": "PKGCONFIG"
+          , "default": {"type": "empty_map"}
+          }
+        ]
+      , [ "pkg-config"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "PKGCONFIG toolchain"}
+          , "key": "pkg-config"
+          , "default": "pkg-config"
+          }
+        ]
+      , [ "PATH"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "PKGCONFIG toolchain"}
+          , "key": "PATH"
+          , "default": []
+          }
+        ]
+      , [ "PKG_CONFIG_PATH"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "PKGCONFIG toolchain"}
+          , "key": "PKG_CONFIG_PATH"
+          , "default": []
+          }
+        ]
+      ]
+    , "body": {"type": "env", "vars": ["pkg-config", "PATH", "PKG_CONFIG_PATH"]}
+    }
+  }
+, "defaults (unconfigured)":
+  { "type": "defaults"
+  , "arguments_config": ["pkg-config", "PATH", "PKG_CONFIG_PATH"]
+  , "pkg-config": [{"type": "var", "name": "pkg-config"}]
+  , "PATH": {"type": "var", "name": "PATH"}
+  , "PKG_CONFIG_PATH": {"type": "var", "name": "PKG_CONFIG_PATH"}
+  }
+}
diff --git a/etc/defaults/CC/pkgconfig/test.TARGETS b/etc/defaults/CC/pkgconfig/test.TARGETS
new file mode 100644
index 0000000..993fea3
--- /dev/null
+++ b/etc/defaults/CC/pkgconfig/test.TARGETS
@@ -0,0 +1,3 @@
+{ "defaults":
+  {"type": "defaults", "base": [["@", "base", "CC/pkgconfig", "defaults"]]}
+}
diff --git a/etc/defaults/CC/proto/TARGETS b/etc/defaults/CC/proto/TARGETS
new file mode 100644
index 0000000..7f98293
--- /dev/null
+++ b/etc/defaults/CC/proto/TARGETS
@@ -0,0 +1,70 @@
+{ "defaults":
+  { "type": "configure"
+  , "target": "configure"
+  , "config":
+    {"type": "singleton_map", "key": "deps", "value": ["libprotobuf"]}
+  }
+, "service defaults":
+  { "type": "configure"
+  , "target": "configure"
+  , "config":
+    { "type": "singleton_map"
+    , "key": "deps"
+    , "value": ["libgrpc++", "libprotobuf"]
+    }
+  }
+, "configure":
+  { "type": "configure"
+  , "target": "defaults (unconfigured)"
+  , "arguments_config": ["TOOLCHAIN_CONFIG"]
+  , "config":
+    { "type": "let*"
+    , "bindings":
+      [ [ "proto toolchain"
+        , { "type": "lookup"
+          , "map":
+            { "type": "var"
+            , "name": "TOOLCHAIN_CONFIG"
+            , "default": {"type": "empty_map"}
+            }
+          , "key": "PROTO"
+          , "default": {"type": "empty_map"}
+          }
+        ]
+      , [ "PROTOC"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "proto toolchain"}
+          , "key": "PROTOC"
+          , "default": "protoc"
+          }
+        ]
+      , [ "GRPC_PLUGIN"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "proto toolchain"}
+          , "key": "GRPC_PLUGIN"
+          , "default": "grpc_cpp_plugin"
+          }
+        ]
+      , [ "PATH"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "proto toolchain"}
+          , "key": "PATH"
+          , "default": []
+          }
+        ]
+      ]
+    , "body": {"type": "env", "vars": ["PROTOC", "GRPC_PLUGIN", "PATH"]}
+    }
+  }
+, "defaults (unconfigured)":
+  { "type": "defaults"
+  , "arguments_config": ["PROTOC", "GRPC_PLUGIN", "PATH", "deps"]
+  , "PROTOC": [{"type": "var", "name": "PROTOC"}]
+  , "GRPC_PLUGIN": [{"type": "var", "name": "GRPC_PLUGIN"}]
+  , "PATH": {"type": "var", "name": "PATH"}
+  , "deps": {"type": "var", "name": "deps"}
+  }
+, "libprotobuf":
+  {"type": ["CC/pkgconfig", "system_library"], "name": ["protobuf"]}
+, "libgrpc++": {"type": ["CC/pkgconfig", "system_library"], "name": ["grpc++"]}
+}
diff --git a/etc/defaults/CC/proto/test.TARGETS b/etc/defaults/CC/proto/test.TARGETS
new file mode 100644
index 0000000..9f2fcae
--- /dev/null
+++ b/etc/defaults/CC/proto/test.TARGETS
@@ -0,0 +1,5 @@
+{ "defaults":
+  {"type": "defaults", "base": [["@", "base", "CC/proto", "defaults"]]}
+, "service defaults":
+  {"type": "defaults", "base": [["@", "base", "CC/proto", "defaults"]]}
+}
diff --git a/etc/defaults/CC/test.TARGETS b/etc/defaults/CC/test.TARGETS
new file mode 100644
index 0000000..8a876f1
--- /dev/null
+++ b/etc/defaults/CC/test.TARGETS
@@ -0,0 +1 @@
+{"defaults": {"type": "defaults", "base": [["@", "base", "CC", "defaults"]]}}
diff --git a/etc/defaults/shell/TARGETS b/etc/defaults/shell/TARGETS
new file mode 100644
index 0000000..378611c
--- /dev/null
+++ b/etc/defaults/shell/TARGETS
@@ -0,0 +1,43 @@
+{ "defaults":
+  { "type": "configure"
+  , "target": "defaults (unconfigured)"
+  , "arguments_config": ["TOOLCHAIN_CONFIG"]
+  , "config":
+    { "type": "let*"
+    , "bindings":
+      [ [ "shell toolchain"
+        , { "type": "lookup"
+          , "map":
+            { "type": "var"
+            , "name": "TOOLCHAIN_CONFIG"
+            , "default": {"type": "empty_map"}
+            }
+          , "key": "shell"
+          , "default": {"type": "empty_map"}
+          }
+        ]
+      , [ "sh"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "shell toolchain"}
+          , "key": "sh"
+          , "default": "/bin/sh"
+          }
+        ]
+      , [ "PATH"
+        , { "type": "lookup"
+          , "map": {"type": "var", "name": "shell toolchain"}
+          , "key": "PATH"
+          , "default": []
+          }
+        ]
+      ]
+    , "body": {"type": "env", "vars": ["sh", "PATH"]}
+    }
+  }
+, "defaults (unconfigured)":
+  { "type": "defaults"
+  , "arguments_config": ["sh", "PATH"]
+  , "sh": [{"type": "var", "name": "sh"}]
+  , "PATH": {"type": "var", "name": "PATH"}
+  }
+}
diff --git a/etc/defaults/shell/test.TARGETS b/etc/defaults/shell/test.TARGETS
new file mode 100644
index 0000000..7c099ff
--- /dev/null
+++ b/etc/defaults/shell/test.TARGETS
@@ -0,0 +1,21 @@
+{ "defaults":
+  { "type": "defaults"
+  , "arguments_config": ["TOOLCHAIN_CONFIG"]
+  , "PATH":
+    { "type": "lookup"
+    , "key": "PATH"
+    , "default": []
+    , "map":
+      { "type": "lookup"
+      , "key": "test"
+      , "default": {"type": "empty_map"}
+      , "map":
+        { "type": "var"
+        , "name": "TOOLCHAIN_CONFIG"
+        , "default": {"type": "empty_map"}
+        }
+      }
+    }
+  , "base": [["@", "base", "shell", "defaults"]]
+  }
+}
diff --git a/etc/defaults/shell/test/test.TARGETS b/etc/defaults/shell/test/test.TARGETS
new file mode 100644
index 0000000..0967ef4
--- /dev/null
+++ b/etc/defaults/shell/test/test.TARGETS
@@ -0,0 +1 @@
+{}
diff --git a/etc/repos.json b/etc/repos.json
new file mode 100644
index 0000000..d1537db
--- /dev/null
+++ b/etc/repos.json
@@ -0,0 +1,30 @@
+{ "repositories":
+  { "":
+    { "repository": {"type": "file", "path": "src"}
+    , "bindings": {"rules": "rules/nix"}
+    }
+  , "test":
+    { "repository": {"type": "file", "path": "test"}
+    , "bindings": {"rules": "rules/nix-test", "src": ""}
+    }
+  , "defaults": {"repository": {"type": "file", "path": "etc/defaults"}}
+  , "rules":
+    { "repository":
+      { "type": "git"
+      , "branch": "master"
+      , "commit": "0d436f26134d3fcaa695e4e6e87249d7fa381e44"
+      , "repository": "https://github.com/just-buildsystem/rules-cc.git"
+      , "subdir": "rules"
+      }
+    }
+  , "rules/nix":
+    {"repository": "rules", "target_root": "defaults", "rule_root": "rules"}
+  , "rules/nix-test":
+    { "repository": "rules"
+    , "target_root": "defaults"
+    , "rule_root": "rules"
+    , "target_file_name": "test.TARGETS"
+    , "bindings": {"base": "rules/nix"}
+    }
+  }
+}
diff --git a/nix-dependencies/default.nix b/nix-dependencies/default.nix
new file mode 100644
index 0000000..9adde33
--- /dev/null
+++ b/nix-dependencies/default.nix
@@ -0,0 +1,2 @@
+{ nixpkgs ? import <nixpkgs> {} }:
+nixpkgs.callPackage ./dependencies.nix {}
diff --git a/nix-dependencies/dependencies.nix b/nix-dependencies/dependencies.nix
new file mode 100644
index 0000000..99280cc
--- /dev/null
+++ b/nix-dependencies/dependencies.nix
@@ -0,0 +1,58 @@
+{ stdenv
+, jo
+, pkg-config
+, coreutils
+, protobuf_25
+, grpc
+, clang
+, fmt
+
+  # for tests
+, gnugrep
+, unixtools
+}:
+
+stdenv.mkDerivation rec {
+  name = "nix-dependencies";
+
+  unpackPhase=''true'';
+
+  nativeBuildInputs = [
+    pkg-config
+    protobuf_25
+    grpc
+    jo
+  ];
+
+  buildInputs = [
+    clang
+    fmt
+    coreutils
+    gnugrep
+    unixtools.xxd
+  ];
+
+  buildPhase = ''
+    echo PKG_CONFIG_PATH=$PKG_CONFIG_PATH
+    jo TOOLCHAIN_CONFIG=$(jo \
+          CC=$(jo PATH=$(jo -a ${clang}/bin ${coreutils}/bin)) \
+          PROTO=$(jo PATH=$(jo -a ${protobuf_25}/bin ${grpc}/bin) \
+                     PROTOC=${protobuf_25}/bin/protoc \
+                     GRPC_PLUGIN=${grpc}/bin/grpc_cpp_plugin \
+             ) \
+          shell=$(jo PATH=$(jo -a ${coreutils}/bin)) \
+          test=$(jo PATH=$(jo -a ${gnugrep}/bin ${unixtools.xxd}/bin)) \
+          PKGCONFIG=$(jo pkg-config=${pkg-config}/bin/pkg-config \
+                         PKG_CONFIG_PATH=$(jo -a $PKG_CONFIG_PATH)) \
+          ) > config.json
+    cat config.json
+    jo "just files"=$(jo config=$(jo -a $(jo root=system path=$out/share/config.json))) > rc.json
+  '';
+
+  installPhase = ''
+    mkdir -p $out/share
+    cp config.json $out/share
+    cp rc.json $out/share
+  '';
+
+}
diff --git a/nix/sources.json b/nix/sources.json
new file mode 100644
index 0000000..8dfabc6
--- /dev/null
+++ b/nix/sources.json
@@ -0,0 +1,26 @@
+{
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "f7c538837892dd2eb83567c9f380a11efb59b53f",
+        "sha256": "0xl33k24vfc29cg9lnp95kvcq69qbq5fzb7jk9ig4lgrhaarh651",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/f7c538837892dd2eb83567c9f380a11efb59b53f.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs-unstable": {
+        "branch": "nixpkgs-unstable",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "6132b0f6e344ce2fe34fc051b72fb46e34f668e0",
+        "sha256": "0xh6fmkn2p8ivwqymb9gymfz07lw16fycyipgiydhxzgx6y9j7gd",
+        "type": "tarball",
+        "url": "https://github.com/nixos/nixpkgs/archive/6132b0f6e344ce2fe34fc051b72fb46e34f668e0.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}
diff --git a/nix/sources.nix b/nix/sources.nix
new file mode 100644
index 0000000..9a01c8a
--- /dev/null
+++ b/nix/sources.nix
@@ -0,0 +1,194 @@
+# This file has been generated by Niv.
+
+let
+
+  #
+  # The fetchers. fetch_<type> fetches specs of type <type>.
+  #
+
+  fetch_file = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
+      if spec.builtin or true then
+        builtins_fetchurl { inherit (spec) url sha256; name = name'; }
+      else
+        pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
+
+  fetch_tarball = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
+      if spec.builtin or true then
+        builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
+      else
+        pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
+
+  fetch_git = name: spec:
+    let
+      ref =
+        if spec ? ref then spec.ref else
+          if spec ? branch then "refs/heads/${spec.branch}" else
+            if spec ? tag then "refs/tags/${spec.tag}" else
+              abort "In git source '${name}': Please specify `ref`, `tag` or `branch`!";
+      submodules = if spec ? submodules then spec.submodules else false;
+      submoduleArg =
+        let
+          nixSupportsSubmodules = builtins.compareVersions builtins.nixVersion "2.4" >= 0;
+          emptyArgWithWarning =
+            if submodules == true
+            then
+              builtins.trace
+                (
+                  "The niv input \"${name}\" uses submodules "
+                  + "but your nix's (${builtins.nixVersion}) builtins.fetchGit "
+                  + "does not support them"
+                )
+                {}
+            else {};
+        in
+          if nixSupportsSubmodules
+          then { inherit submodules; }
+          else emptyArgWithWarning;
+    in
+      builtins.fetchGit
+        ({ url = spec.repo; inherit (spec) rev; inherit ref; } // submoduleArg);
+
+  fetch_local = spec: spec.path;
+
+  fetch_builtin-tarball = name: throw
+    ''[${name}] The niv type "builtin-tarball" is deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=tarball -a builtin=true'';
+
+  fetch_builtin-url = name: throw
+    ''[${name}] The niv type "builtin-url" will soon be deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=file -a builtin=true'';
+
+  #
+  # Various helpers
+  #
+
+  # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
+  sanitizeName = name:
+    (
+      concatMapStrings (s: if builtins.isList s then "-" else s)
+        (
+          builtins.split "[^[:alnum:]+._?=-]+"
+            ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
+        )
+    );
+
+  # The set of packages used when specs are fetched using non-builtins.
+  mkPkgs = sources: system:
+    let
+      sourcesNixpkgs =
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };
+      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
+      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
+    in
+      if builtins.hasAttr "nixpkgs" sources
+      then sourcesNixpkgs
+      else if hasNixpkgsPath && ! hasThisAsNixpkgsPath then
+        import <nixpkgs> {}
+      else
+        abort
+          ''
+            Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+            add a package called "nixpkgs" to your sources.json.
+          '';
+
+  # The actual fetching function.
+  fetch = pkgs: name: spec:
+
+    if ! builtins.hasAttr "type" spec then
+      abort "ERROR: niv spec ${name} does not have a 'type' attribute"
+    else if spec.type == "file" then fetch_file pkgs name spec
+    else if spec.type == "tarball" then fetch_tarball pkgs name spec
+    else if spec.type == "git" then fetch_git name spec
+    else if spec.type == "local" then fetch_local spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball name
+    else if spec.type == "builtin-url" then fetch_builtin-url name
+    else
+      abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
+
+  # If the environment variable NIV_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  replace = name: drv:
+    let
+      saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
+      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
+    in
+      if ersatz == "" then drv else
+        # this turns the string into an actual Nix path (for both absolute and
+        # relative paths)
+        if builtins.substring 0 1 ersatz == "/" then /. + ersatz else /. + builtins.getEnv "PWD" + "/${ersatz}";
+
+  # Ports of functions for older nix versions
+
+  # a Nix version of mapAttrs if the built-in doesn't exist
+  mapAttrs = builtins.mapAttrs or (
+    f: set: with builtins;
+    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
+  );
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/lists.nix#L295
+  range = first: last: if first > last then [] else builtins.genList (n: first + n) (last - first + 1);
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L257
+  stringToCharacters = s: map (p: builtins.substring p 1 s) (range 0 (builtins.stringLength s - 1));
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
+  stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
+  concatMapStrings = f: list: concatStrings (map f list);
+  concatStrings = builtins.concatStringsSep "";
+
+  # https://github.com/NixOS/nixpkgs/blob/8a9f58a375c401b96da862d969f66429def1d118/lib/attrsets.nix#L331
+  optionalAttrs = cond: as: if cond then as else {};
+
+  # fetchTarball version that is compatible between all the versions of Nix
+  builtins_fetchTarball = { url, name ? null, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchTarball;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchTarball ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
+      else
+        fetchTarball attrs;
+
+  # fetchurl version that is compatible between all the versions of Nix
+  builtins_fetchurl = { url, name ? null, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchurl;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchurl ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
+      else
+        fetchurl attrs;
+
+  # Create the final "sources" from the config
+  mkSources = config:
+    mapAttrs (
+      name: spec:
+        if builtins.hasAttr "outPath" spec
+        then abort
+          "The values in sources.json should not have an 'outPath' attribute"
+        else
+          spec // { outPath = replace name (fetch config.pkgs name spec); }
+    ) config.sources;
+
+  # The "config" used by the fetchers
+  mkConfig =
+    { sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
+    , sources ? if isNull sourcesFile then {} else builtins.fromJSON (builtins.readFile sourcesFile)
+    , system ? builtins.currentSystem
+    , pkgs ? mkPkgs sources system
+    }: rec {
+      # The sources, i.e. the attribute set of spec name to spec
+      inherit sources;
+
+      # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
+      inherit pkgs;
+    };
+
+in
+mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }
diff --git a/shell.nix b/shell.nix
new file mode 100644
index 0000000..b89488b
--- /dev/null
+++ b/shell.nix
@@ -0,0 +1,23 @@
+let
+
+  sources = import ./nix/sources.nix;
+
+  nixpkgs = sources."nixpkgs-unstable";
+
+  pkgs = import nixpkgs {};
+
+  nix-dependencies = import ./nix-dependencies { nixpkgs = pkgs; } ;
+
+in pkgs.mkShell rec {
+
+  name = "hello-nix";
+
+  buildInputs = with pkgs; [
+     niv
+  ];
+
+  shellHook = ''
+     alias just-mr="just-mr --rc ${nix-dependencies}/share/rc.json"
+  '';
+
+}
diff --git a/src/hello/TARGETS b/src/hello/TARGETS
new file mode 100644
index 0000000..80380c7
--- /dev/null
+++ b/src/hello/TARGETS
@@ -0,0 +1,9 @@
+{ "":
+  { "type": ["@", "rules", "CC", "binary"]
+  , "name": ["hello"]
+  , "srcs": ["hello.cpp"]
+  , "private-deps": ["fmt"]
+  }
+, "fmt":
+  {"type": ["@", "rules", "CC/pkgconfig", "system_library"], "name": ["fmt"]}
+}
diff --git a/src/hello/hello.cpp b/src/hello/hello.cpp
new file mode 100644
index 0000000..0c9df99
--- /dev/null
+++ b/src/hello/hello.cpp
@@ -0,0 +1,11 @@
+#include <iostream>
+
+#include <fmt/format.h>
+#include <string>
+
+int main(int argc, char **argv) {
+  std::cout << fmt::format("Hello {}!",
+                           argc > 1 ? std::string{argv[1]} : "World")
+            << std::endl;
+  return 0;
+}
diff --git a/src/proto/TARGETS b/src/proto/TARGETS
new file mode 100644
index 0000000..512d7c2
--- /dev/null
+++ b/src/proto/TARGETS
@@ -0,0 +1,18 @@
+{ "example":
+  { "type": ["@", "rules", "proto", "library"]
+  , "name": ["example"]
+  , "srcs": ["example.proto"]
+  }
+, "write":
+  { "type": ["@", "rules", "CC", "binary"]
+  , "name": ["write"]
+  , "srcs": ["write.cc"]
+  , "private-proto": ["example"]
+  }
+, "read":
+  { "type": ["@", "rules", "CC", "binary"]
+  , "name": ["read"]
+  , "srcs": ["read.cc"]
+  , "private-proto": ["example"]
+  }
+}
diff --git a/src/proto/example.proto b/src/proto/example.proto
new file mode 100644
index 0000000..69b5953
--- /dev/null
+++ b/src/proto/example.proto
@@ -0,0 +1,7 @@
+syntax = "proto3";
+package sample;
+
+message Example {
+  string foo = 1;
+  int32 bar = 2;
+}
diff --git a/src/proto/read.cc b/src/proto/read.cc
new file mode 100644
index 0000000..f97a39a
--- /dev/null
+++ b/src/proto/read.cc
@@ -0,0 +1,18 @@
+#include <fstream>
+#include <iostream>
+
+#include "example.pb.h"
+
+int main(int argc, char **argv) {
+  sample::Example example;
+
+  {
+    std::fstream input(argv[1], std::ios::in | std::ios::binary);
+    example.ParseFromIstream(&input);
+  }
+
+  std::cout << "foo=" << example.foo() << "\n";
+  std::cout << "bar=" << example.bar() << std::endl;
+
+  return 0;
+}
diff --git a/src/proto/write.cc b/src/proto/write.cc
new file mode 100644
index 0000000..2e206e5
--- /dev/null
+++ b/src/proto/write.cc
@@ -0,0 +1,17 @@
+#include <fstream>
+#include <string>
+
+#include "example.pb.h"
+
+int main(int argc, char **argv) {
+  sample::Example example;
+  example.set_foo(std::string{argv[1]});
+  example.set_bar(atoi(argv[2]));
+  {
+    std::fstream output(argv[3],
+                        std::ios::out | std::ios::trunc | std::ios::binary);
+    example.SerializeToOstream(&output);
+  }
+
+  return 0;
+}
diff --git a/src/shell/TARGETS b/src/shell/TARGETS
new file mode 100644
index 0000000..f4b2dbf
--- /dev/null
+++ b/src/shell/TARGETS
@@ -0,0 +1,13 @@
+{ "out":
+  { "type": ["@", "rules", "shell", "cmds"]
+  , "outs": ["out.txt"]
+  , "cmds": ["./hello > out.txt"]
+  , "deps": [["hello", ""]]
+  }
+, "":
+  { "type": ["@", "rules", "shell", "cmds"]
+  , "outs": ["wc.txt", "tr.txt"]
+  , "cmds": ["wc out.txt > wc.txt", "cat out.txt | tr a-z A-Z > tr.txt"]
+  , "deps": ["out"]
+  }
+}
diff --git a/src/withExtendedPath/TARGETS b/src/withExtendedPath/TARGETS
new file mode 100644
index 0000000..e226541
--- /dev/null
+++ b/src/withExtendedPath/TARGETS
@@ -0,0 +1,6 @@
+{ "":
+  { "type": ["@", "rules", "CC", "binary"]
+  , "name": ["withExtendedPath"]
+  , "srcs": ["extend-path.cc"]
+  }
+}
diff --git a/src/withExtendedPath/extend-path.cc b/src/withExtendedPath/extend-path.cc
new file mode 100644
index 0000000..a466728
--- /dev/null
+++ b/src/withExtendedPath/extend-path.cc
@@ -0,0 +1,37 @@
+#include <algorithm>
+#include <cstdlib>
+#include <string>
+#include <unistd.h>
+#include <vector>
+
+int main(int argc, char* argv[]) {
+  constexpr int kForkFailed = 65;
+  constexpr int kPrerequisiteError = 97;
+  auto kEnv = std::string{"/usr/bin/env"};
+
+  if (argc < 3) {
+    return kPrerequisiteError;
+  }
+
+  // compute PATH
+  auto path = std::string{argv[1]};
+  auto const* env_path = std::getenv("PATH");
+  if (env_path && (*env_path != '\0')) {
+    path = std::string{env_path} + std::string{":"} + path;
+  }
+  auto path_arg = std::string{"PATH="} + path;
+
+  // create new argument vector
+  std::vector<char*> nargv{};
+  nargv.reserve(argc+3);
+  nargv.push_back(kEnv.data());
+  nargv.push_back(path_arg.data());
+  for (int i=2; i < argc; i++) {
+    nargv.push_back(argv[i]);
+  }
+  nargv.push_back(nullptr);
+
+  // exec
+  (void) execvp(nargv[0], static_cast<char* const*>(nargv.data()));
+  return kForkFailed;
+}
diff --git a/test/hello/TARGETS b/test/hello/TARGETS
new file mode 100644
index 0000000..d01c382
--- /dev/null
+++ b/test/hello/TARGETS
@@ -0,0 +1,7 @@
+{ "":
+  { "type": ["@", "rules", "shell/test", "script"]
+  , "name": ["hello"]
+  , "test": ["test_hello.sh"]
+  , "deps": [["@", "src", "hello", ""]]
+  }
+}
diff --git a/test/hello/test_hello.sh b/test/hello/test_hello.sh
new file mode 100644
index 0000000..08347e8
--- /dev/null
+++ b/test/hello/test_hello.sh
@@ -0,0 +1,5 @@
+set -e
+
+./hello | grep -i world
+./hello Universe | grep Universe
+./hello Universe | grep -i world && exit 1 || :
diff --git a/test/proto/TARGETS b/test/proto/TARGETS
new file mode 100644
index 0000000..50d5195
--- /dev/null
+++ b/test/proto/TARGETS
@@ -0,0 +1,7 @@
+{ "":
+  { "type": ["@", "rules", "shell/test", "script"]
+  , "name": ["proto"]
+  , "test": ["test.sh"]
+  , "deps": [["@", "src", "proto", "write"], ["@", "src", "proto", "read"]]
+  }
+}
diff --git a/test/proto/test.sh b/test/proto/test.sh
new file mode 100644
index 0000000..fbd946b
--- /dev/null
+++ b/test/proto/test.sh
@@ -0,0 +1,12 @@
+set -e
+
+./write FOO 42 data.bin 2>&1
+
+echo
+echo Binary encoding
+xxd data.bin
+
+echo
+echo Verifying read
+./read data.bin | grep foo=FOO
+./read data.bin | grep bar=42
diff --git a/test/test_hello.sh b/test/test_hello.sh
new file mode 100644
index 0000000..08347e8
--- /dev/null
+++ b/test/test_hello.sh
@@ -0,0 +1,5 @@
+set -e
+
+./hello | grep -i world
+./hello Universe | grep Universe
+./hello Universe | grep -i world && exit 1 || :
diff --git a/test/withExtendedPath/TARGETS b/test/withExtendedPath/TARGETS
new file mode 100644
index 0000000..73a5a9e
--- /dev/null
+++ b/test/withExtendedPath/TARGETS
@@ -0,0 +1,7 @@
+{"withExtendedPath":
+  { "type": ["@", "rules", "shell/test", "script"]
+  , "name": ["find-in-extended-path"]
+  , "test": ["test.sh"]
+  , "deps": [["@", "src", "withExtendedPath", ""]]
+  }
+}
diff --git a/test/withExtendedPath/test.sh b/test/withExtendedPath/test.sh
new file mode 100644
index 0000000..de51742
--- /dev/null
+++ b/test/withExtendedPath/test.sh
@@ -0,0 +1,12 @@
+set -e
+
+TOOLS_DIR=`pwd`/additionalTools
+mkdir -p ${TOOLS_DIR}
+cat > ${TOOLS_DIR}/mytool <<'EOF'
+#!/bin/sh
+
+echo using my CuStOm tool
+EOF
+chmod 755 ${TOOLS_DIR}/mytool
+
+./withExtendedPath ${TOOLS_DIR} mytool | grep CuStOm
diff --git a/update-nix-dependencies.sh b/update-nix-dependencies.sh
new file mode 100755
index 0000000..d4953a8
--- /dev/null
+++ b/update-nix-dependencies.sh
@@ -0,0 +1 @@
+nix-shell --run "niv update"