os.system() returns the (encoded) process exit value. 0 means success:
On Unix, the return value is the exit status of the process encoded in the format specified for
wait(). Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent.
The output you see is written to stdout, so your console or terminal, and not returned to the Python caller.
If you wanted to capture stdout, use subprocess.check_output() instead:
x = subprocess.check_output(['whoami'])