Since I updated from 10.04 to 12.04, I am unable to access the device connected in /ttyUSB0 if I don't have root access. To be more specific, I use a usb-to-serial com port converter to access and configure Cisco equipment with a software called Putty. It happens that, since I upgraded to 12.04, Putty only sees the connected device when I run it as gksudo putty. If not, it says "unable to connect to port /ttyUSB0". I have managed to change the permissions via chmod 666, and it works, but if I disconnect the usb and reconnect, it goes back to root only. How do I permanently allow non-root access? I have a clue it might be via udev rules, but I have no idea how to do it.

obs. Manufacturer and drivers "PROLIFIC"

10 Answers 10


It's possible this is related to modemmanager, as detailed here.

I removed that (sudo apt-get remove modemmanager) and it fixed my problem.

You still need to be in the dialout group though.

  • 1
    What if I can't remove modemmanager? I have a friend that has a 3G modem, and it seems he can't remove modemmanager because of that. How can he access /ttyUSB0 on 12.04 without root, but without removing modemmanager? – Leonardo Montenegro Jul 19 '13 at 12:22
  • You can try by stopping it? sudo stop modemmanager It worked for me. – Aleks Aug 12 '14 at 9:19
  • Been a few years, but this is still the case in Ubuntu 17.10. Had to do a sudo apt-get purge modemmanager before sudo usermod -a -G dialout $USER had the desired effect. – Stéphane Nov 6 '17 at 3:29

The device is most likely attached to user group dialout. Just add your user to the dialout group so you have appropriate permissions on the device.

sudo usermod -a -G dialout $USER

(You may need to logout and back in for the new group to take effect.)

No need to mess around with permissions or udev rules.

  • 3
    I sign in to upvote. The cleanest solution. – 0x5f3759df Apr 10 '13 at 10:15
  • 1
    @tom-hennen I guess you should add it first as a comment rather than edit it directly. – MadMike Oct 29 '14 at 13:33
  • 2
    This doesn't work for Debian – Mike May 16 '15 at 16:51
  • 1
    Kudos for including "may need to logout and back in". – Rob Stoecklein Jul 26 '18 at 13:57
  • 1
    In Ubuntu (I'm on 18.04), logout/login doesn't work. we have to reboot after doing usermod. – Sujay Phadke Mar 2 at 6:42
sudo adduser <the user you want to add> dialout
sudo reboot

Mentioned by "Try now" worked for me. Check that You have dialout as group for ttyUSB0:

ls -l /dev/ttyUSB0

in my case the output is:

crw-rw---T 1 root dialout 188, 0 Feb 12 12:01 /dev/ttyUSB0
  • in Linux Mint 18 Cinnamon I get this error when calling ' ls -l /dev/ttyUSB0 ' ls: cannot access '/dev/ttyUSB0': No such file or directory – flyingdrifter Dec 8 '16 at 22:56

This worked for me. Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

sudo adduser <the user you want to add> dialout
sudo reboot
id -Gn

Try now.


As reported by Rinzwind, start by typing:

sudo usermod -a -G dialout $USER

But that is only part of the solution, as then you must reboot the system:

sudo reboot
  • 6
    1) Please don't post comments as answers. 2) No need to reboot; just log out and log back in. – gertvdijk Sep 11 '14 at 12:07
  • I had to reboot to get this to work. A Logoff didn't propagate the change. – user1182988 Jun 3 at 20:19

I tried both

sudo usermod -a -G dialout $USER #(worked perfectly)

And sudo chmod 666 /dev/ttyUSB0 (worked but had to type the command again)

I also removed the mode manager but did not really help. Therefore, the best command that worked for me was

sudo usermod -a -G dialout $USER

On Ubuntu 18.04 I had to add myself to the tty group.

I got hinted by the minicom command output:

minicom  /dev/ttyUSB0
minicom: cannot open /dev/tty8: Permission denied

ls -la /dev/tty8
crw--w---- 1 root tty 4, 8 juil. 31 08:27 /dev/tty

On Ubuntu 18.04:

  1. sudo adduser <user> dialout
  2. Full reboot, for some reason mere logout/login is not sufficient (bug report.).

I had this problem with /dev/ttyS0 in Mint Rosa. Adding the user to the dialout group is required but in my case the device access was restricted and I had to expand it to give r/w access to group level also.

  • 1
    Welcome to Ask Ubuntu! I recommend editing this answer to expand it with specific details about how to do this. (See also How do I write a good answer? for general advice about what sorts of answers are considered most valuable on Ask Ubuntu.) – David Foerster Sep 4 '16 at 18:42

navigate to /etc/ folder and edit the group file add your username like this dialout:x:20:USER

worked perfect for me :)

  • 4
    It is generally considered bad practice to edit these files directly as some simple mistakes can cause loss of access. – flickerfly Apr 10 '15 at 20:49

protected by Community Apr 10 '18 at 17:38

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