SGID does not work, why?
 

I'm fairly proficient but it's been awhile since I've used sticky bits. So puzzle me this, why isn't this working?

me@Z800 ~ $ ls -l /usr/bin/cu
-rwxr-sr-x 1 root dialout 167168 Oct 23 2015 /usr/bin/cu

me@Z800 ~ $ ls -l /dev/ttyUSB0
crw-rw---- 1 root dialout 188, 0 May 13 17:07 /dev/ttyUSB0

me@Z800 ~ $ cu -l /dev/ttyUSB0 -s 9600
cu: open (/dev/ttyUSB0): Permission denied
cu: /dev/ttyUSB0: Line in use

PLEASE spare me the "just add yourself to dialout in /etc/groups"--I already know that.

Thanks all!

Oh and:

me@Z800 ~ $ uname -a
Linux Z800 4.13.0-36-generic #40~16.04.1-Ubuntu SMP Fri Feb 16 23:25:58 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

that is displayed, that usb port is already in use.