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Lua Fundamentals

Lua is Portuguese for moon. It is also the name of the scripting language we use in KumoMTA. Because it is a name, Lua is always capitalized.

Understanding Lua is not required to deploy and use KumoMTA, but it will help you leverage the full power of this incredibly flexible system. Lua is easy to learn, easy to read, and easy to implement.

You can find many resources at the [official Lua site including online documentation and physical reference books.

Here is a simplified primer to help you read the KumoMTA script/configs:

-- A single line comment in Lua is 2 dashes (--)

--[[ A multi line comment in Lua
     is framed in 2 dashes and 2 square brackets
local myvar
local myvar = 32

Global variables are implied by excluding the word "local".

myvar = 32 -- this is a GLOBAL variable


In KumoMTA, variables should always be declared as "local" unless you intend for the value to be constant for the life of the program.

The concurrency model used by KumoMTA means that global variables can be shared in unpredictable ways. If you need to share values that change across the life of the program, you should use a data store such as sqlite or redis.

Lua supports the following relational operators:

Symbol Meaning
== equality
~= inequality
< less than
> greater than
<= less than or equal to
>= greater than or equal to

You can concatenate strings with two dots surrounded by spaces.

print('This' .. ' is ' .. 'true.')

Functions, conditionals, and loops always end with "end"

if x == 2 then
  y = 6

function dostuff(things)

Last update: 2023-11-06
Created: 2023-03-01