If you get the following errors starting screen as a user:
No more PTYs. Sorry, could not find a PTY. [screen is terminating]
You have to change /dev/ptmx file permissions:
# ls -l /dev/ptmx crw-r--r-- 1 root root 5, 2 nov 18 23:28 /dev/ptmx # chmod a+w /dev/ptmx # ls -l /dev/ptmx crw-rw-rw- 1 root root 5, 2 nov 18 23:34 /dev/ptmx
Under Ubuntu, there was no problem. Permissions already set:
$ ls -l /dev/ptmx crw-rw-rw- 1 root tty 5, 2 2009-11-18 23:37 /dev/ptmx
References :
man ptmxubuntu$ ls -l /dev/ptmx
Thanks, this fixed the problem on a Debian Squeeze VM preload image.
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
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.
What works for me: unplug device from computer (i.e., usb serial adapter) then plug back in again