Thanks. I'm not using make, so I don't really need a make rule, but I can see that gcc -M does something useful:

  $ gcc -M src/value.c
  value.o: src/value.c src/memory.h src/value.h

The result of my grep/sed gives me just the header dependencies alone, which is really all I need:

  $ cat obj/value.i
  src/memory.h src/value.h

So I might consider replacing the grep/sed with gcc -M, and strip off the first two entries, which would give me the equivalent result, but it doesn't strike me as a "must do" right now. Again, I don't use make, just /bin/sh, so I don't need a make rule per se.

Thanks for the advice about the ".i" suffix. I don't think there's a conflict though, because all my intermediate files go into a completely separate "obj" directory, and no pre-processed source will be going there. However, I will consider choosing a different suffix just for the heck of it. I'm sure anything I choose (such as ".dep" or ".inc") will mean something to somebody somewhere, but since I'm in the entirely orthogonal context of the "obj" directory I don't think it matters.