For mcp23008:

p=232; z=/sys/class/gpio; echo $p > ${z}/export; echo out > "${z}/gpio${p}/direction"; echo 1 > "${z}/gpio${p}/value"; sleep 1; echo 0 > "${z}/gpio${p}/value"; echo $p > "${z}/unexport"

For mcp23017:

# find out which i2c address the device listens to:
i2cdetect -y 0
 
# you should find 0x20
 
# Set all bits from port a to output:
i2cset -y 0 0x20 0x00 0x00
 
# Write value to register GPIOA (0x12) oder OLATA (0x14)
# For example: for bit 0, write:
i2cset -y 0 0x20 0x14 0x01