I'm with Chris. The information should be stored in the os module. I suggest os.shell var or os.get_shell() function like this:

def get_shell():
    for path in confstr("CS_PATH").split(pathsep):
        sh = sep.join((path, "sh"))
        try:
            mode = stat(sh).st_mode
        except OSError:
            pass
        else:
            if st.S_ISREG(mode):
                return sh
    raise FileNotFound("sh")

According to all examples S_ISREG() is sufficient here.

On Windows the function should use the env var COMSPEC instead of hard coding "cmd.exe".