如果我不在 subprocess.Popen() 中使用 stdout=subprocess.PIPE 有什么区别? [英] What is the difference if I don't use stdout=subprocess.PIPE in subprocess.Popen()?

问题描述

我最近在 Python 中注意到 subprocess.Popen() 有一个参数:

I recently noted in Python the subprocess.Popen() has an argument:

stdout=None(default)

我还看到有人使用 stdout=subprocess.PIPE.

I also saw people using stdout=subprocess.PIPE.

有什么区别?我应该使用哪一种?

What is the difference? Which one should I use?

另一个问题是，为什么有时wait()函数不能等到进程真正完成?我用过:

Another question would be, why the wait() function can't wait until the process is really done sometimes? I used:

a = sp.Popen(....,shell=True)
a.wait()
a2 = sp.Popen(...,shell=True)
a2.wait()

有时 a2 命令会在命令 a 完成之前执行.

sometimes the a2 command is executed before the command a is done.

推荐答案

stdout=None 的意思是，进程中的 stdout 句柄直接从父进程继承，简单来说，它基本上意味着，它被打印到控制台(同样适用于 stderr).

stdout=None means, the stdout-handle from the process is directly inherited from the parent, in easier words it basically means, it gets printed to the console (same applies for stderr).

然后你有选项stderr=STDOUT，这将stderr重定向到stdout，这意味着stdout<的输出/code> 和 stderr 被转发到同一个文件句柄.

Then you have the option stderr=STDOUT, this redirects stderr to the stdout, which means the output of stdout and stderr are forwarded to the same file handle.

如果你设置了stdout=PIPE，Python会将进程中的数据重定向到一个新的文件句柄，可以通过p.stdout(p 是一个 Popen 对象).您可以使用它来捕获进程的输出，或者在 stdin 的情况下将数据(不断地)发送到 stdin.但大多数情况下您想使用 p.communicate，它允许您将数据发送到进程一次(如果需要)并返回完整的 stderr 和 stdout 如果过程完成！

If you set stdout=PIPE, Python will redirect the data from the process to a new file handle, which can be accessed through p.stdout (p beeing a Popen object). You would use this to capture the output of the process, or for the case of stdin to send data (constantly) to stdin. But mostly you want to use p.communicate, which allows you to send data to the process once (if you need to) and returns the complete stderr and stdout if the process is completed!

一个更有趣的事实，你可以将任何 file-object 传递给 stdin/stderr/stdout，例如还有一个用 open 打开的文件(对象必须提供一个 fileno() 方法).

One more interesting fact, you can pass any file-object to stdin/stderr/stdout, e.g. also a file opened with open (the object has to provide a fileno() method).

解决您的wait 问题.这不应该是这样的！作为解决方法，您可以使用 p.poll() 来检查进程是否退出！wait 调用的返回值是什么?

To your wait problem. This should not be the case! As workaround you could use p.poll() to check if the process did exit! What is the return-value of the wait call?

此外，您应该避免 shell=True 特别是如果您将用户输入作为第一个参数传递，恶意用户可能会利用它来利用您的程序！它还启动了一个 shell 进程，这意味着额外的开销.当然，有 1% 的情况您实际上需要 shell=True，我无法用您的简约示例来判断这一点.

Furthermore, you should avoid shell=True especially if you pass user-input as first argument, this could be used by a malicious user to exploit your program! Also it launches a shell process which means additional overhead. Of course there is the 1% of cases where you actually need shell=True, I can't judge this with your minimalistic example.

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