latex l3sys get exit code of script

I am using l3sys of the latex3 interfaces, i need to get the exit code of a script after running it. I am running sys_get_shell:nnN, i tried giving it echo $? and echo $, but the shell for some reason interprets $? as the literal characters not the exit code of the last command, thus echo $? fails because it is missing one $ for a math environment, and echo $? fails with ! TeX capacity exceeded, sorry [input stack size=5000]. for some reason, that i do not know.

Is there a way to make this work, or is there an interface to the exit code that i missed, or can i make a command fail if the script fails in some other way?