Overview

When uploading a sketch you may get the following error

avrdude: ser_open(): can't open device "/dev/ttyACM0": Permission denied

or, when opening the serial Monitor

Error opening serial port '/dev/ttyACM0

Note: The number in ttyACM* may be different.

If this happens, start by ensuring that you are using the correct port:

• Arduino IDE: Tools > Port > [select port]
• Web Editor: Select port using the dropdown.

If the problem persists there may be an issue with permissions for the serial port. See below for step-by-step instructions on how to set the correct permissions.

How to set serial port permissions

Open Terminal and type

ls -l /dev/ttyACM*

Press enter. Terminal will return something like

crw-rw---- 1 root dialout 188, 0 5 apr 23.01 ttyACM0

What we are interested in is the group name, which is probably called dialout.

Note: The number at the end of ttyACM may be different, or multiple entries might be returned.

Now we want to add your user to that group. Input the following command in the terminal, replacing <group> with the group name from above, and <username> with your username, both without brackets.

sudo usermod -a -G <group> <username>

Press enter. Input password if prompted.

Lastly, for the new permissions to take effect, you need to log out and in to your user account.

You can verify the changes by typing

groups

into the Terminal and pressing Enter. The group you added should appear in the list.