Problem

Tried to use the screen program to connect my lab PC to a Raspberry PI via the USB ports and an RS232 cable.

Ran the command screen /dev/ttyUSB0

Got the following error messages in quick succession.

“Cannot access line ‘/dev/ttyUSB0 for r/w: permission denied”

“Couldn’t find a PTY”

Attempted solutions

I tried to look in the activity monitor to see if there are any processes running, and there was none. So this was not the problem.

Tried to change the /dev/ptmx file permissions

Ran the command ls -l /dev/ptmx

Output crw-rw-rw- 1 root root 5, 2 “datetime” /dev/ptmx

So it looked like the permission were already set. So this was not the problem either.

Tried to mount /dev/pts by running
sudo mount devpts /dev/pts -t devpts -o mode=620

I got the message that the mounting was already done. Move on.

Finally found this idea of adding to the dialout group looking at this post.
https://askubuntu.com/questions/58119/changing-permissions-on-serial-port

Ran the command – sudo usermod -a -G dialout $USER

Restarted the computer and voila, it worked. Was able to use screen program to make the connection.

Spend a few hours to figure this out. Hope this helps someone out. Peace.