At the beginning of each phase of the build (such as unpacking, building or installing), the set of all shell variables is written to the file env-vars at the top-level build directory. This is useful for debugging: it allows you to recreate the environment in which a build was performed. For instance, if a build fails, then assuming you used the -K flag, you can go to the output directory and switch to the environment of the builder: $ nix-build -K ./foo.nix ... fails, keeping build directory `/tmp/nix-1234-0' $ cd /tmp/nix-1234-0 $ source env-vars (edit some files...) $ make (execution continues with the same GCC, make, etc.)