看到这个简单的echo.py：
import sys
if __name__ == "__main__":
for line in sys.stdin:
sys.stderr.write("DEBUG: got line: " + line)
sys.stdout.write(line)
运行：
ls | python echo.py 2>debug_output.txt | sort
输出：
echo.py
test.py
test.sh
debug_output.txt内容：
DEBUG: got line: echo.py
DEBUG: got line: test.py
DEBUG: got line: test.sh
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