root@celestia:~# echo 19 > /sys/class/gpio/export 
root@celestia:~# ls /sys/class/gpio
export	gpio19	gpiochip0  unexport
root@celestia:~# echo 19 > /sys/class/gpio/unexport 
root@celestia:~# ls /sys/class/gpio
export	gpiochip0  unexport