// returns file with program output (STDOUT+STDERR)
static File backtickToConsole_lineBuf(S cmd, fO onLine) {
  O onOutput = new O {
    new LinesBuffer buf;

    void get(S s) {
      buf.append(fixNewLines(s));
      for (S line : buf.getAndClearCompleteLines())
        pcallF(onLine, line);
    }
  };
  
  ret backtickToConsoleX(cmd, onOutput);
}