[SOLVED] How do I test that a given shell has been executed in Python?

How do I test that a given shell has been executed in Python?

I want to test a Python function that executes a shell command.

According to testfixtures there are two approaches:

execute a real process and check the result
mock the subprocess module and check the expected interactions

My function is called run_in_shell. Although the subprocess module is the obvious way to implement it, my function doesn't explicitly depend on it, so I'm trying to do the "real" test.

import subprocess

def run_in_shell(command, shell_name=None):
    """Run command in default shell. Use named shell instead if given."""
    subprocess.run(command, shell=True, executable=shell_name)

This test shows that the function can execute a command with the default shell.

import pytest

def test_with_command_string(capfd):
    run_in_shell("echo 'hello'")
    cap = capfd.readouterr()
    assert cap.out.strip() == "hello"
    assert cap.err.strip() == ""

I also want to show that it can execute in the user's chosen shell such as /bin/bash.

The invocation is simple enough. This prints hello to the terminal.

run_in_shell("echo 'hello'", shell_name="/bin/bash")

Without mocking, how do I show that it executed /bin/bash to do so?

I tried ptracer to trace the system calls, but the output disappointed me.

def callback(syscall):
    name = syscall.name
    args = ",".join([str(a.value) for a in syscall.args])
    print(f"{name}({args})")

with ptracer.context(callback):
    run_in_shell("echo 'hello'")

with ptracer.context(callback):
    run_in_shell("echo 'hello'", shell_name="/bin/bash")

I was hoping to see a clone or fork call with the name of the shell, but there is nothing so clear. I don't understand the strings in the read calls, and I don't see any write calls.

hello
pipe2((23, 24),524288)
clone(18874385,0,None,261939,0)
close(24)
read(23,bytearray(b'U\r\r\n'),50000)
close(23)
wait4(262130,0,0,None)

hello
pipe2((24, 25),524288)
clone(18874385,0,None,261939,0)
futex(2,129,1,0,9693984,9694016)
futex(0,129,1,0,None,9694016)
close(25)
read(24,bytearray(b'\x1d'),50000)
close(24)
wait4(262308,0,0,None)

At this point I'm out of my depth. I've surely misunderstood what the system calls are really doing. What am I missing?

Is it possible and practical to test this using Python and PyTest? If not, I'll redefine my function to depend explicitly on one of the subprocess functions, and then I can use a mock to test that it sends the right messages to the function.

Solution

As Teejay Bruno shows, you can use the command to introspect the shell. The shell already stores its name in the $0 variable, so you can just echo that and check the standard output.

def test_executes_in_specified_shell(stack, capfd):
    run_in_shell("echo $0", shell_name="/bin/bash")
    cap = capfd.readouterr()
    assert cap.out.strip() == "/bin/bash"
    assert cap.err.strip() == ""

I think $0 should work in any POSIX-compliant shell, although I can't find clear documentation for that. It certainly works in Bash and dash.