Common causes and fixes when you’re already in the dialout group but still get “permission denied” when accessing /dev/ttyUSB0
If you’re already in the dialout group but still get “permission denied” when accessing /dev/ttyUSB0, here are the common causes and fixes:
1. You need to log out and log back in (or reboot)
Group membership changes only take effect after a new login session. If you haven’t done this yet, do:
- Log out of your desktop session and log back in
- Or reboot the system
Then check with:
groups
Make sure dialout is listed.
2. Check current permissions of /dev/ttyUSB0
Run:
ls -l /dev/ttyUSB0
Example output:
crw-rw---- 1 root dialout 188, 0 Jun 9 14:00 /dev/ttyUSB0
- Owner:
root - Group:
dialout - Permissions:
rwfor owner and group, no permissions for others
If it looks different (e.g., owned by root but group not dialout), something’s off.
3. Check if some process is locking the port
Sometimes another process (like minicom, screen, or an IDE) is using /dev/ttyUSB0.
Check with:
lsof /dev/ttyUSB0
If any process is using it, stop or kill it.
4. Temporary test: use sudo
Try running your program or command with sudo:
sudo screen /dev/ttyUSB0 9600
If this works, the problem is definitely permissions.
5. Check udev rules or device creation
Sometimes the device permissions or group are reset incorrectly by udev.
To see current udev rules affecting USB serial:
udevadm info -a -n /dev/ttyUSB0
You can also reload udev rules and trigger:
sudo udevadm control --reload-rules
sudo udevadm trigger
6. As a workaround: change permissions manually
Not recommended long-term, but you can test by:
sudo chmod 666 /dev/ttyUSB0
This allows anyone to read/write the device. If this fixes it, then it’s definitely a permissions issue.
Summary
- Make sure you’re really in the
dialoutgroup (groupscommand) - Log out/in or reboot to apply group changes
- Check device ownership and permissions (
ls -l /dev/ttyUSB0) - Check for other processes using the port (
lsof) - Try with
sudoto confirm permission issue