I’ll keep this post short and sweet. I want to write a little blurb that I’ve been mostly enjoying using nix eval to introspect.

Scenario 1 I wanted to set JAVA_HOME to a particular JDK.

You can use eval to determine the JAVA_HOME. The gotcha here however, is that the JDK may not necessarily be in your _/nix/home.

❯ nix eval nixpkgs.jdk8.home --raw
/nix/store/0jfk27bplglsqh7m945pvayjwh2fy1m0-openjdk-8u272-b10/lib/openjdk

A more complete solution is the following.

❯ export JAVA_HOME=$(nix build nixpkgs.jdk8 && nix eval nixpkgs.jdk8.home --raw)

Scenario 2 I wanted to discover the calculated /nix/store path for a particular derivation.

❯ nix eval nixpkgs.hello.outPath --raw
/nix/store/jmmw0d3nmklwafcwylvrjb9v69wrbcxf-hello-2.10

💡 You can also do something similar to find out the derivation path.

❯ nix eval nixpkgs.hello.drvPath --raw
/nix/store/wqnx5cgcabxkfp771fmjr1nw7mjr9zlm-hello-2.10.drv

Scenario 3 I want a quick one-liner for what I can do with nix repl.

❯ nix eval '(builtins.attrNames (import <nixpkgs>{}).hello)' --json | jq
[
  "__ignoreNulls",
  "all",
  "args",
  "buildInputs",
  "builder",
  "configureFlags",
  "depsBuildBuild",
  "depsBuildBuildPropagated",
  "depsBuildTarget",
  "depsBuildTargetPropagated",
  "depsHostHost",
  "depsHostHostPropagated",
  "depsTargetTarget",
  "depsTargetTargetPropagated",
  "doCheck",
  "doInstallCheck",
  "drvAttrs",
  "drvPath",
  "inputDerivation",
  "meta",
  "name",
  "nativeBuildInputs",
  "out",
  "outPath",
  "outputName",
  "outputUnspecified",
  "outputs",
  "override",
  "overrideAttrs",
  "overrideDerivation",
  "passthru",
  "patches",
  "pname",
  "propagatedBuildInputs",
  "propagatedNativeBuildInputs",
  "src",
  "stdenv",
  "strictDeps",
  "system",
  "tests",
  "type",
  "userHook",
  "version"
]

⚠️ I do find however annoying differences between nix repl and nix eval that I’m not clear on.

For instance the following works on nix repl but fails on nix eval

❯ nix repl
Welcome to Nix version 2.3.12. Type :? for help.

nix-repl> :l <nixpkgs>
Added 14373 variables.

nix-repl> hello
«derivation /nix/store/wqnx5cgcabxkfp771fmjr1nw7mjr9zlm-hello-2.10.drv»

❯ nix eval nixpkgs#hello
error: anonymous function at /nix/store/h7b0frzjk6ylyqq471m667yd9bl9n6fm-source/pkgs/build-support/fetchurl/boot.nix:5:1 called with unexpected argument 'meta'

       at /nix/store/h7b0frzjk6ylyqq471m667yd9bl9n6fm-source/pkgs/build-support/fetchzip/default.nix:18:2:

           17|
           18| (fetchurl (let
             |  ^
           19|   basename = baseNameOf (if url != "" then url else builtins.head urls);
(use '--show-trace' to show detailed location information)

If you happen to know why, answer this message on discourse or tweet me.

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