Menu
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.stdout, stdout=PIPE)
p2.communicate()
# outputs correctly ('hello\n', None)
Making the first process use shell=True kills the output somehow...
p1 = Popen(['echo','hello'], stdout=PIPE, shell=True)
p2 = Popen('cat', stdin=p1.stdout, stdout=PIPE)
p2.communicate()
# outputs incorrectly ('\n', None)
shell=True on the second process doesn't seem to matter. Is this expected behavior?
689
From the docs: args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names).
On POSIX with shell=True , the shell defaults to /bin/sh . If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt.
The subprocess module defines one class, Popen and a few wrapper functions that use that class. The constructor for Popen takes arguments to set up the new process so the parent can communicate with it via pipes. It provides all of the functionality of the other modules and functions it replaces, and more.
To capture the output of the subprocess. run method, use an additional argument named “capture_output=True”. You can individually access stdout and stderr values by using “output. stdout” and “output.
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)
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
