Ruby method to set a Redis key if it's not already defined using a Lua script.

redis_set_if_not_defined.rb

require 'redis'

# Lua script to set a Redis key if it doesn't exist yet.
# Returns result of SET if key is not set, otherwise
# returns the previous value of the key.
SetScript = <<EOF
local val = redis.call('GET', KEYS[1])
if val == false then
  return redis.call('SET', KEYS[1], ARGV[1])
end
return redis.call('GET', KEYS[1])
EOF

def set_if_not_def(key, value)
  Redis.new.eval SetScript, [key], [value]
end

Of course, this is a silly example given that Redis provides SETNX and HSETNX.