Create a Derivation

Before a package is built, a derivation must be created. The derivation can be thought of as the unamiguous definition of how to build a package. The process of creating a derivation is called "instantiatiation", or sometimes also refered to as evaluation (although this is more general). Every package in nixpkgs has a corresponding derivation. This means that we can create and inspect the derivation for anything exposed in nixpkgs. An example would be:

$ nix-instantiate '<nixpkgs>' -A hello
/nix/store/byqskk0549v1zz1b2a61lb7llfn4h5bw-hello-2.10.drv

# or using flakes, nix>=2.4

$ nix eval nixpkgs#hello.drvPath
"/nix/store/byqskk0549v1zz1b2a61lb7llfn4h5bw-hello-2.10.drv"

Inspect the contents of a derivation

To inspect the contents of the drv, one can use the nix show-derivation utility.

$ nix show-derivation /nix/store/byqskk0549v1zz1b2a61lb7llfn4h5bw-hello-2.10.drv
{
  "/nix/store/byqskk0549v1zz1b2a61lb7llfn4h5bw-hello-2.10.drv": {
    "outputs": {
      "out": {
        "path": "/nix/store/f4bywv8hjwl0ckv7l077pnap81h6qxw4-hello-2.10"
      }
...

Defining characteristics of a derivation

There's a few important features of a derivation:

• It's a description of how to build the package from source
• The output paths are determined before the build begins
• All dependencies are resolved as part of instantiation, and may have a similar derivation description of their builds
• Any additional flags (makeFlags, configuration flags, cflags, or ld flags) are explicitly stated
• There's no ambiguity. The system, architecture, and other options have been resolved.
• It's immutable. If you want to change a derivation, you need to evaluate a new one.
• It's unique. The hashing scheme ensures that there should only ever be one derivation; if two derivations match, then they are exactly the same in every way.