python - plumbum.commands.processes.ProcessExecutionError: null を返すコマンドの場合

Question

実行したいシェル コマンド。何も返されません。

echo helloWorld | grep 'dummy'

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鉛直バージョン:

次の行は機能します：

out=(echo["helloWorld"] | grep["h"])().strip()

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しかし、次の行はそうではありません。理由は何でしょうか?

out=(echo["helloWorld"] | grep["dummy"])().strip()
print(out)

私が持っているエラー:

Traceback (most recent call last):
  File "dd.py", line 6, in <module>
    out=(echo["helloWorld"] | grep["dummy"])().strip()
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/base.py", line 103, in __call__
    return self.run(args, **kwargs)[1]
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/base.py", line 240, in run
    return p.run()
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/base.py", line 201, in runner
    return run_proc(p, retcode, timeout)
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/processes.py", line 232, in run_proc
    return _check_process(proc, retcode, timeout, stdout, stderr)
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/processes.py", line 23, in _check_process
    proc.verify(retcode, timeout, stdout, stderr)
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/commands/base.py", line 412, in verify
    dstproc_verify(retcode, timeout, stdout, stderr)
  File "/home/user/venv/lib/python3.5/site-packages/plumbum/machines/base.py", line 26, in verify
    stderr)
plumbum.commands.processes.ProcessExecutionError: Command line: ['/bin/grep', 'dummy']
Exit code: 1

[Q]このエラーを修正するにはどうすればよいですか?