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Fixing ‘Permission Denied’ Error When Connecting to Serial Port in Ubuntu

Ubuntu 16

In the world of Ubuntu, one of the most common issues faced by users is the ‘Permission Denied’ error when trying to access the serial port. This error typically arises when the user lacks the necessary permissions to access the /dev/ttyS0 serial port. In this article, we will delve into the details of this issue and provide a comprehensive guide on how to resolve it.

Quick Answer

To fix the ‘Permission Denied’ error when connecting to the serial port in Ubuntu, you need to add yourself to the ‘dialout’ group using the ‘gpasswd’ command with the ‘–add’ option. After adding yourself to the group, log out and log back in or reboot your system for the changes to take effect.

Understanding the ‘Permission Denied’ Error

Before we jump into the solution, it’s important to understand what the ‘Permission Denied’ error signifies. In Ubuntu, certain operations require specific permissions. When you try to perform an operation without the required permissions, the system responds with a ‘Permission Denied’ error.

In the case of accessing the serial port, you need to be a member of the dialout group. If you are not a member of this group, you will encounter the ‘Permission Denied’ error.

Checking Your Group Membership

The first step towards resolving this error is to check if you are a member of the dialout group. This can be done using the groups command followed by your username. Here’s how to do it:

groups joeuser

Replace joeuser with your actual username. This command will return a list of groups that you are a member of. If dialout is not in the list, you need to add yourself to this group.

Adding Yourself to the Dialout Group

To add yourself to the dialout group, you can use the gpasswd command with the --add option. Here’s the command:

sudo gpasswd --add joeuser dialout

Again, replace joeuser with your actual username. The --add option tells gpasswd to add the user to the specified group. The sudo is necessary because changing group membership is a privileged operation.

Logging Out and Logging Back In

After adding yourself to the dialout group, you need to log out and log back in for the changes to take effect. This is because the group membership is only updated at login.

However, some users have reported that simply logging out and logging back in may not work on Ubuntu 20.04, and a system reboot may be required.

Additional Troubleshooting

If you’re still encountering the ‘Permission Denied’ error after following the above steps, you might need to add yourself to the uucp group. This has been reported as a necessary step for some users on Ubuntu 14.04.

To add yourself to the uucp group, you can use the same gpasswd command as before, but replace dialout with uucp:

sudo gpasswd --add joeuser uucp

Conclusion

The ‘Permission Denied’ error when accessing the /dev/ttyS0 serial port in Ubuntu is typically due to a lack of necessary permissions. By checking your group membership and adding yourself to the dialout (and possibly uucp) group, you can resolve this error. Remember to log out and log back in or reboot your system for the changes to take effect.

By understanding the permissions required and how to grant them, you can ensure smooth operation of your Ubuntu system and avoid frustrating ‘Permission Denied’ errors.

How do I check if I am a member of the `dialout` group?

To check if you are a member of the dialout group, open the terminal and enter the command groups <your_username>. Replace <your_username> with your actual username. This command will display a list of groups that you are a member of, including the dialout group if you are a member.

How do I add myself to the `dialout` group?

To add yourself to the dialout group, open the terminal and enter the command sudo gpasswd --add <your_username> dialout. Replace <your_username> with your actual username. You will be prompted to enter your password. After successful execution of the command, you will be added to the dialout group.

Do I need to log out and log back in for the changes to take effect?

Yes, after adding yourself to the dialout group, you need to log out of your Ubuntu session and then log back in for the changes to take effect. This is necessary because group membership is updated at login.

What should I do if logging out and logging back in doesn’t work?

If simply logging out and logging back in doesn’t resolve the ‘Permission Denied’ error, you may need to reboot your system. Some users have reported that a system reboot is required for the changes in group membership to take effect, especially on Ubuntu 20.04.

Are there any additional troubleshooting steps if the error persists?

If you are still encountering the ‘Permission Denied’ error after following the above steps, you can try adding yourself to the uucp group. Use the command sudo gpasswd --add <your_username> uucp to add yourself to the uucp group. Remember to replace <your_username> with your actual username. This step has been reported as necessary for some users on Ubuntu 14.04.

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