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nix-env-diff is a tiny utility which parses and compares the output of
`nix-env -qaP --out-path`, printing all changed and added out paths (or
attributes if desired). This facilitates a simple way to determine
rebuilds or changed reverse dependencies when working on nixpkgs in a
similar way as nixpkgs-review and ofborg do it.
Both the new evaluation and the base evaluation to compare against have
to be created manually using nix-env, which in turn also allows
considering attribute sets that are normally not evaluated do to missing
`lib.recurseIntoAttrs`. As an example, here is an example building all
changed attributes in `ocaml-ng.ocamlPackages_4_12`:
```
nix-env -qaP -A ocaml-ng.ocamlPackages_4_12 --out-path -f . \
| nix-env-diff --attrs ./base-ocamlPackages_4_12 \
| xargs -n 1 -P 4 nix-instantiate --quiet -A \
| xargs nix-store --realise --builders 'ssh://edwin'
```
`./base-ocamlPackages_4_12` contains the result of the `nix-env`
invocation executed on the master branch.
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