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  • "while … > sum never overwrites sum" -- can you elaborate on that? What command, exactly, never overwrites sum? Commented Aug 26, 2018 at 18:49
  • paste out sum | awk '{$1 += $3; $2 += $4; NF = 2; print}' > sum. If you try that and then cat sum, you should see that sum has not changed. Commented Aug 26, 2018 at 18:52