gistfile1.rb

# Conditional decrement.
#
# Decrement the value of a key only if the current value is greater than
# a specified value.

require 'rubygems'
require 'redis'

r = Redis.new
cond_decr = <<LUA
    local value = tonumber(redis('get',KEYS[1]))
    if value == nil then return {err="Value at key is not integer"} end
    if value > tonumber(ARGV[1])
    then
        value = value - 1
        redis('set',KEYS[1],value)
    end
    return value
LUA

r.set(:x,4)
5.times {
    puts(r.eval(cond_decr,1,:x,0))
}

@doub People could do something like https://gist.github.com/952677 if they need it. It might be better to leave such things to third party lua libs and keep the redis code simple.