Return code of subprocess.run when using shell pipes

Question

I am trying to understand the return code of subprocess.run :

>>> subprocess.run('false | true', shell=True)
CompletedProcess(args='false | true', returncode=0)

>>> subprocess.run(['false', '|', 'true'])
CompletedProcess(args=['false', '|', 'true'], returncode=1)

>>> subprocess.run(['false', '|', 'true'], shell=True)
CompletedProcess(args=['false', '|', 'true'], returncode=1)

It looks like specifying cmd as a list of strings also sets the pipefail shell option:

$ false | true
$ echo $?
0

$ set -o pipefail
$ false | true
$ echo $?
1

I can't find any information about this in the documentation of the subprocess module. I am using python 3.6. Does anyone know how this works?

Answer 1

This doesn't do what you think:

>>> subprocess.run(['false', '|', 'true'], shell=True)

When you pass a list of strings to subprocess.run and specify shell=True , only the first item is interpreted as a command (the remaining items are provided as arguments to the shell). Compare the output of:

>>> subprocess.run('echo hello world', shell=True)
hello world
CompletedProcess(args='echo hello world', returncode=0)

With:

>>> subprocess.run(['echo', 'hello', 'world'], shell=True)

CompletedProcess(args=['echo', 'hello', 'world'], returncode=0)

With:

>>> subprocess.run(['echo $0 $1', 'hello', 'world'], shell=True)
hello world
CompletedProcess(args=['echo $0 $1', 'hello', 'world'], returncode=0)

So what you have there is equivalent to:

>>> subprocess.run('false', shell=True)

And do you know why the return codes of subprocess.run('false | true', shell=True) and subprocess.run(['false', '|', 'true']) are different?

Because the first runs false | true false | true , which is a pipeline. The exit code of a pipeline is the exit code of the last command in the pipeline (in this case true ). The second call, as discussed here, just runs false .