[jlgreco]

That's a good question. I had though it worked because the truncate would not actually happen until tac actually wrote something, but now that I look at it more I don't think that is actually the case. `cat > foo` immediately truncates foo before anything is written. Also `tac foo > foo` doesn't work, which discredits my hypothesis that tac's buffering was causing it.

Furthermore `cat foo | cat > foo` does not work, but `cat foo | cat | cat > foo` does work.

I suspect this behavior is actually a race condition of some sort.

Edit: definetly a race condition, shown by larger files:

  % wc -l foo
  1310720 foo
  % cat foo | cat | cat > foo
  % wc -l foo
  16384 1