Table of contents

Getting Started with Lua Scripts

Lua is case-sensitive.

Prerequisites

Install Lua locally. You can search Google for installation instructions.

Basics

Comments

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Reserved Keywords

Reserved keyword documentation

Variables

Types of Undeclared or Uninitialized Variables

The default value of both undeclared variables and variables declared without initialization is nil.

Local Variables

Declare a local variable with the local keyword, for example:

-- This is a local variable.
local a = 10

function max()
    -- This is also a local variable.
    local a = 10
end

Global Variables

Variables without the local keyword are global, for example:

-- This is a global variable.
a = 10

Note: even inside a function, a variable is global if it is not declared with local.

function max()
    a = 10
end

Where Are Global Variables Stored?

In a table. See this explanation of _G.

What Happens If Global Variables Are Not Deleted?

So far, I have not found an explanation in the official documentation or books of the effects of leaving global variables undeleted. The book also says that users need not worry about Lua’s memory, because memory management is automatic.

How Do You Delete a Global Variable?

Assign nil to it.

a = 10
-- Delete the global variable.
a = nil

Initializing Multiple Variables

Documentation

The basic syntax places variables in order on the left of the equals sign and their corresponding values in order on the right.

local a, b = 10, 20

If there are fewer values on the right than variables on the left, variables without a corresponding value receive Lua’s default value, nil.

a, b = 10

If there are more values on the right than variables on the left, the extra values are automatically ignored.

a, b = 10, 20, 30

Data Types

Official documentation

Use type to determine a variable’s type:

type(123)

• nil: no value.

  When comparing the type to nil, put nil in quotes.

  type(X) == 'nil'

• boolean

• number: a double-precision floating-point number.

• string: single or double quotes.

  Use # to calculate the string length.

  local string str = 'message'
  print("String length:", #str)

  Another common way to concatenate strings:

  local msg1 = 'msg1'
  print(msg1 .. 'append' .. 'builder')

  Note: with this syntax, the string must be a local variable.

  Common string operations, including searching, case conversion, substrings, and concatenation.

• function

  Lua functions can return multiple values, separated by commas.

  function max()
      a, b = 10, 20
      return a, b
  end
  -- Call the function.
  max()

  Call the function after declaring it.

• table

There are a few other types I have not listed because I do not currently need them at work.

Conditional Expressions

if

local a, b = 10, 10
if (a == b) then
    print("if expression")
end

if else

local a = 10
if (a == c) then

else
    print("if-else expression")
end

Nested if/elseif

local a, b = 10, 10
if (a == c) then

elseif (a == d) then

elseif(a ==b ) then
    print("if-elseif expression")
end

Arrays

Declaring an Array

array = {'index1', 'index3'}

Loops

while Loops

a = 10
while (a < 20) do
    a = a+1
    if (a > 18) then
        break
    end
    print("value of a:", a)
end

for Loops

for i = 10, 1, -1 do
print("value of i:", i)
end

Use the built-in ipairs function to iterate over an array:

local array = {'jack', 'tom', 'bob'}
for k, v in ipairs(array) do
    print("Value in this iteration:", v)
    print('Key in this iteration:', k)
end

repeat…until Loops

repeat is similar to do-while: the loop executes at least once, regardless of the condition.

b = 10
repeat
    b = b+1
    print("value of b:", b)
until(b > 15)

Error Handling

I have not actually worked with this part yet.

Calling Redis from Lua Scripts

Use redis.call. See the documentation for related redis. commands.

local rc, rl, ll, key, value = redis.call, redis.log, redis.LOG_NOTICE, KEYS[1], ARGV[1]
local result = rc("SETEX", key, 60, value)
print("result value is:", result)

Function Libraries

Quickly Looking Up Lua Built-in Functions

You can quickly look up unfamiliar functions in the library list at the bottom of the official documentation and in the Lua standard library section of the official manual.

Resources

Example Code

Example code for this article.

Documentation

• Lua documentation
• Lua tutorial on TutorialsPoint
• The Lua programming book on the official website

IntelliJ IDEA Plugin

• EmmyLua: I recommend installing this plugin for convenient Lua syntax completion and debugging.