Why Does Shell=true Eat My Subprocess.popen Stdout?

It seems that using shell=True in the first process of a chain somehow drops the stdout from downstream tasks: p1 = Popen(['echo','hello'], stdout=PIPE) p2 = Popen('cat', stdin=p1.

Solution 1:

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)