GPIO

We use BCM pad 18 for testing with a LED + serial resistor. Connect the LED cathode (shorer wire) to GND (J8 pin 14) and LED anode (longer wire) via a serial resistor for current limiting to J8 pin 12 which is routed to BCM pin 18.

Now, boot, login and from the command prompt on your running Pi:

Export (==make visible) GPIO pin 18:

echo 18 > /sys/class/gpio/export

Make pin 18 an output:

echo out > /sys/class/gpio/gpio18/direction 

Set its value to 1 (high), the LED should switch on:

echo 1 > /sys/class/gpio/gpio18/value

Set its value to 0 (low), the LED should switch off:

echo 0 > /sys/class/gpio/gpio18/value

If you like bash hacking, let the LED blink from the console:

while [  true ]
do  
  echo 1 > /sys/class/gpio/gpio18/value
  sleep 1
  echo 0 > /sys/class/gpio/gpio18/value
  sleep 1
done

Export must be done only once in the beginning when @/sys/class/gpio/gpio18/@ does not yet exists. Its good style to unexport the pin if no longer needed (which also switches the LED off):

echo 18 > /sys/class/gpio/unexport

This code can be one-to-one translated into C code using POSIX style file access functions. For a complete example see: http://www.netzmafia.de/skripten/hardware/RasPi/RasPi_GPIO_C.html