String handling in Computercraft test

function mv(x)
if x == nil then
x = "f"
end
print("Zonk")

print(x)

s = string.lower(x)
l = string.len(s)  -- length of string

print(s)
print(l)
print("Zonk")

for i=1,l,1 do
	e = string.sub(s,i,i)
	if e=="f" then
		turtle.forward()
	elseif e == "b" then
		turtle.back()
	elseif e == "d" then
		turtle.down()
	elseif e == "u" then
		turtle.up()
	elseif e == "r" then
		turtle.turnRight()
	elseif e == "l" then
		turtle.turnLeft()
	elseif e == "t" then
		turtle.turnRight()
		turtle.turnRight()
	else
		print ("Wrong Move *mv* input character")
		return(0)

end
end
end

mv("fffrffflbbuutffdd")