nix-super/doc/manual/expressions/debug-build.xml

<section xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"
      version="5.0"
      xml:id="sec-debug-build">

<title>Debugging Build Failures</title>

<para>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 <filename>env-vars</filename> 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 <option>-K</option> flag, you
can go to the output directory and <quote>switch</quote> to the
environment of the builder:

<screen>
$ nix-build -K ./foo.nix
... fails, keeping build directory `/tmp/nix-1234-0'

$ cd /tmp/nix-1234-0

$ source env-vars

<lineannotation>(edit some files...)</lineannotation>

$ make

<lineannotation>(execution continues with the same GCC, make, etc.)</lineannotation></screen>

</para>

</section>