How to fix Raspberry Pi GPIO "No access to /dev/mem. Try running as root!"

Problem:

You want to toggle a Raspberry Pi GPIO pin using RPi.GPIO but you see an error message like

Traceback (most recent call last):
  File "gpio.py", line 8, in <module>
    GPIO.setup(pin1, GPIO.OUT)
RuntimeError: No access to /dev/mem.  Try running as root!

Solution

Add your user to the gpio group:

sudo usermod -a -G gpio $USER

then log out and log back in(or reboot, if that doesn’t help).

The reason for this error is that /dev/gpiomem is only accessible by users belonging to the gpio group. By default, only the pi user belongs to the gpio group - hence, if you run a script as any users other than pi or root, you will see that error message.