Example
In this example we will use GPIO29 and use it as a switch.

With latest linux kernels you may need to first get the gpio base

cat /sys/class/gpio/gpiochip*/base | head -n1
200

and sum the base to your GPIO:
200 + 29 = 229

Now first step is making GPIO available in Linux:

echo "229" > /sys/class/gpio/export

then you need to decide if it will be input or output, as we will use it as a switch so we need output

echo "out" > /sys/class/gpio/gpio229/direction

and last line turns GPIO on or off with 1 or 0:

echo "1" > /sys/class/gpio/gpio229/value