Why does shell=True eat my subprocess.Popen stdout?

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Chapters
00:00 Why Does Shell=True Eat My Subprocess.Popen Stdout?
00:34 Accepted Answer Score 19
01:08 Thank you

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Full question
https://stackoverflow.com/questions/1066...

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Content licensed under CC BY-SA
https://meta.stackexchange.com/help/lice...

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Tags
#python #subprocess #pipe #popen

#avk47

ACCEPTED ANSWER

Score 19

When you pass shell=True, Popen expects a single string argument, not a list. So when you do this:

p1 = Popen(['echo','hello'], stdout=PIPE, shell=True)

What happens is this:

execve("/bin/sh", ["/bin/sh", "-c", "echo", "hello"], ...)

That is, it calls sh -c "echo", and hello is effectively ignored (technically it becomes a positional argument to the shell). So the shell runs echo, which prints \n, which is why you see that in your output.

If you use shell=True, you need to do this:

p1 = Popen('echo hello', stdout=PIPE, shell=True)