Bollinger Bands

study(shorttitle="BB", title="Bollinger Bands", overlay=true)
source = close
length = input(20, minval=1), mult = input(2.0, minval=0.001, maxval=50)
basis = sma(source, length)
dev = mult * stdev(source, length)
upper = basis + dev
lower = basis - dev
plot(basis, color=red)
p1 = plot(upper, color=blue)
p2 = plot(lower, color=blue)
fill(p1, p2)