yyy можно заменить на sh + tee

[ghost]

#!/usr/bin/env bash
> /tmp/buf
while IFS= read -r line;do
  echo "$line" >> /tmp/buf
  echo "$line"
done

превратить в

#!/bin/sh
tee /tmp/buf

tee это POSIX (man 1p tee соглы :)).

[rwxrob]

Sometimes you don't have tee and frankly I wanted pure bash instead of shell out inefficiently.