screen a serial get could not find a PTY Error

Finally, I find the solution, there are a screen process not close normality.

So, in the Activity Monitor I closed it, then re-start the screen, it works.

In my case simply unplugging the device from the computer (i.e., USB to serial adapter) then plugging back in again cleared the issue.

I had a similar problem and I attempted a number of different solutions. Just wanted to share what made it work for me.

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

1. 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.

2. 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.

3. 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.

4. 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.