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# Verifying Build Reproducibility

You can use Nix's `diff-hook` setting to compare build results. Note that this hook is only executed if the results differ; it is not used for determining if the results are the same.

For purposes of demonstration, we'll use the following Nix file, `deterministic.nix` for testing:

    let
      inherit (import <nixpkgs> {}) runCommand;
    in {
      stable = runCommand "stable" {} ''
        touch $out
      '';

      unstable = runCommand "unstable" {} ''
        echo $RANDOM > $out
      '';
    }

Additionally, `nix.conf` contains:

    diff-hook = /etc/nix/my-diff-hook
    run-diff-hook = true

where `/etc/nix/my-diff-hook` is an executable file containing:

    #!/bin/sh
    exec >&2
    echo "For derivation $3:"
    /run/current-system/sw/bin/diff -r "$1" "$2"

The diff hook is executed by the same user and group who ran the build. However, the diff hook does not have write access to the store path just built.

# Spot-Checking Build Determinism

Verify a path which already exists in the Nix store by passing `--check` to the build command.

If the build passes and is deterministic, Nix will exit with a status code of 0:

    $ nix-build ./deterministic.nix --attr stable
    this derivation will be built:
      /nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv
    building '/nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv'...
    /nix/store/yyxlzw3vqaas7wfp04g0b1xg51f2czgq-stable

    $ nix-build ./deterministic.nix --attr stable --check
    checking outputs of '/nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv'...
    /nix/store/yyxlzw3vqaas7wfp04g0b1xg51f2czgq-stable

If the build is not deterministic, Nix will exit with a status code of 1:

    $ nix-build ./deterministic.nix --attr unstable
    this derivation will be built:
      /nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv
    building '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
    /nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable

    $ nix-build ./deterministic.nix --attr unstable --check
    checking outputs of '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
    error: derivation '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv' may
    not be deterministic: output '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable' differs

In the Nix daemon's log, we will now see:

    For derivation /nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv:
    1c1
    < 8108
    ---
    > 30204

Using `--check` with `--keep-failed` will cause Nix to keep the second build's output in a special, `.check` path:

    $ nix-build ./deterministic.nix --attr unstable --check --keep-failed
    checking outputs of '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
    note: keeping build directory '/tmp/nix-build-unstable.drv-0'
    error: derivation '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv' may
    not be deterministic: output '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable' differs
    from '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check'

In particular, notice the `/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check` output. Nix has copied the build results to that directory where you can examine it.

> Note

> Check paths are not protected against garbage collection, and this path will be deleted on the next garbage collection.

> The path is guaranteed to be alive for the duration of the `diff-hook`'s execution, but may be deleted any time after.

> If the comparison is performed as part of automated tooling, please use the diff-hook or author your tooling to handle the case where the build was not deterministic and also a check path does not exist.

`--check` is only usable if the derivation has been built on the system already. If the derivation has not been built Nix will fail with the error:

    error: some outputs of '/nix/store/hzi1h60z2qf0nb85iwnpvrai3j2w7rr6-unstable.drv'
    are not valid, so checking is not possible

Run the build without `--check`, and then try with `--check` again.