Dependencies

Previously we have built toy derivations with dummy output. In practice, Nix derivations are used for building programs, with build artifacts such as compiled binaries being the derivation output.

We can demonstrate this property by "building" a greet program. First we have to import a pinned version of nixpkgs as before. To simplify the process we abstract it out into a nixpkgs.nix at the root directory.

Now we build a greet program in greet.nix:

let
  nixpkgs = import ../../nixpkgs.nix;

  greet = nixpkgs.stdenv.mkDerivation {
    name = "greet";
    unpackPhase = "true";

    buildPhase = ''
      echo "building greet..."
      sleep 3
    '';

    installPhase = ''
      mkdir -p $out/bin

      cat <<'EOF' > $out/bin/greet
      #!/usr/bin/env bash
      echo "Hello, $1!"
      EOF

      chmod +x $out/bin/greet
    '';
  };
in
greet

$ nix-build code/04-derivations/02-dependencies/greet.nix
these derivations will be built:
  /nix/store/97lmyym0isl0ism7pfnv1b0ls4cahpi8-greet.drv
building '/nix/store/97lmyym0isl0ism7pfnv1b0ls4cahpi8-greet.drv'...
unpacking sources
patching sources
configuring
no configure script, doing nothing
building
building greet...
installing
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet
strip is /nix/store/bnjps68g8ax6abzvys2xpx12imrx8949-binutils-2.31.1/bin/strip
stripping (with command strip and flags -S) in /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet/bin
patching script interpreter paths in /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet
/nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet/bin/greet: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23/bin/bash"
checking for references to /build/ in /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet...
/nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet

Now we can run greet:

$ result/bin/greet John
Hello, John!

Let's try to see what's inside the produced greet script:

$ cat result/bin/greet
#!/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23/bin/bash
echo "Hello, $1!"

The shebang to the bash shell has been modified to pin to the Nix version of Bash.

Upper Greet

Our greet program can now be used as a dependency to other derivations. Let's create an upper-greet derivation that convert any greet result to upper case.

upper-greet.nix:

let
  nixpkgs = import ../../nixpkgs.nix;

  inherit (nixpkgs) coreutils;

  greet = import ./greet.nix;
in
nixpkgs.stdenv.mkDerivation {
  name = "upper-greet";

  unpackPhase = "true";

  buildPhase = ''
    echo "building upper-greet..."
    sleep 3
  '';

  installPhase = ''
    mkdir -p $out/bin

    cat <<'EOF' > $out/bin/upper-greet
    #!/usr/bin/env bash
    ${greet}/bin/greet "$@" | ${coreutils}/bin/tr [a-z] [A-Z]
    EOF

    chmod +x $out/bin/upper-greet
  '';
}

Show Derivation

First we instantiate upper-greet.drv without building it yet:

drv=$(nix-instantiate 04-derivations/02-dependencies/upper-greet.nix)

We can use nix show-derivation to find out the dependency graph of the derivation of upper-greet:

$ nix show-derivation $drv
{
  "/nix/store/n61g8616l7g7zv32q52yrzmzr850mjp0-upper-greet.drv": {
    "outputs": {
      "out": {
        "path": "/nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet"
      }
    },
    "inputSrcs": [
      "/nix/store/9krlzvny65gdc8s7kpb6lkx8cd02c25b-default-builder.sh"
    ],
    "inputDrvs": {
      "/nix/store/7gby8zic1p851ap63q1vdpwy7z1db85c-coreutils-8.32.drv": [
        "out"
      ],
      "/nix/store/97lmyym0isl0ism7pfnv1b0ls4cahpi8-greet.drv": [
        "out"
      ],
      "/nix/store/l54djrh1n7d8zdfn26w7v6zjh5wp7faa-bash-4.4-p23.drv": [
        "out"
      ],
      "/nix/store/x9why09hwx2pcnmw0fw7hhh1511hyskl-stdenv-linux.drv": [
        "out"
      ]
    },
    ...
    "env": {
      "buildInputs": "",
      "buildPhase": "echo \"building upper-greet...\"\nsleep 3\n",
      "builder": "/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23/bin/bash",
      ...
      "installPhase": "mkdir -p $out/bin\n\ncat <<'EOF' > $out/bin/upper-greet\n#!/usr/bin/env bash\n/nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet/bin/greet \"$@\" | /nix/store/2shqhfsyzz4rnfyysbzgyp5kbfk29750-coreutils-8.32/bin/tr [a-z] [A-Z]\nEOF\n\nchmod +x $out/bin/upper-greet\n",
      "name": "upper-greet",
...

We can see that greet.drv is included as one of inputDrvs. This means that when upper-greet.drv is being built, greet.drv will have to be built first.

The output path of upper-greet.drv is listed in outputs. This shows that the output hash of a derivation is fixed, regardless of the content of the build result.

This is also why the output path of greet.drv is used directly in env.installPhase of upper-greet.drv, even for the case when greet.drv has not been built.

Build Derivation

$ nix-build 04-derivations/02-dependencies/upper-greet.nix
these derivations will be built:
  /nix/store/n61g8616l7g7zv32q52yrzmzr850mjp0-upper-greet.drv
building '/nix/store/n61g8616l7g7zv32q52yrzmzr850mjp0-upper-greet.drv'...
unpacking sources
patching sources
configuring
no configure script, doing nothing
building
building upper-greet...
installing
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet
strip is /nix/store/bnjps68g8ax6abzvys2xpx12imrx8949-binutils-2.31.1/bin/strip
stripping (with command strip and flags -S) in /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet/bin
patching script interpreter paths in /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet
/nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet/bin/upper-greet: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23/bin/bash"
checking for references to /build/ in /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet...
/nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet

As expected, the greet results are turned into upper case.

$ result/bin/upper-greet John
HELLO, JOHN!

The absolute paths to greet and coreutils are extended:

$ cat result/bin/upper-greet
#!/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23/bin/bash
/nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet/bin/greet "$@" | /nix/store/2shqhfsyzz4rnfyysbzgyp5kbfk29750-coreutils-8.32/bin/tr [a-z] [A-Z]

Runtime Dependency

If we query the references of the upper-greet output (not the derivation), we can see that greet is still a runtime dependency of upper-greet.

$ nix-store --query --references /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet
/nix/store/2shqhfsyzz4rnfyysbzgyp5kbfk29750-coreutils-8.32
/nix/store/qdp56fi357fgxxnkjrwx1g67hrk775im-bash-4.4-p23
/nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet

We can use nix why-depends to find out why Nix thinks greet is a runtime dependency to upper-greet:

$ nix why-depends /nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet
/nix/store/dj2vp64gbja0bp65lngrw9q4lrm1a8r3-upper-greet
╚═══bin/upper-greet: …ash-4.4-p23/bin/bash./nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet/bin/greet "$@" | /nix/sto…
    => /nix/store/l6xy4qjr8x3ni16skfilw0fvnda13szq-greet

Scrive Nix Workshop

Dependencies

Upper Greet

Show Derivation

Build Derivation

Runtime Dependency