cannot open /dev/ttyS0: Permission denied, but in dialout group

Question

I am trying to use my 9 pin serial port to connect my automotive ECU to my computer (megasquirt controller). I've done this before with Ubuntu 10.04 but now that I've just upgraded to 12.04 it simply will not connect. I didn't have any issue before. Now I simply cannot open this port.

Using chmod I get this:

~$ chmod o+rw /dev/ttyS0
chmod: changing permissions of `/dev/ttyS0': Operation not permitted

Solution

I suggest you should just double-check if you are a member of the dialout group, let us suppose your username is joeuser, do the following:

groups joeuser

..this will list all the groups you belong to. If you don't belong to the dialout group then add yourself to it, for example:

sudo gpasswd --add joeuser dialout

You then need to log out and log back in again for it to be effective. Then see if it fixes your problem.

Comments

[Alex Nolasco]

Emphasis on logout/login and trying again.

[albfan]

I want to remark that log out and log back in is the key to get this to work.

[Nandan Vasudevan]

Thanks Colin! However on Ubuntu 20.04, logging out did not work. I had to reboot my system.