Lua Reference

-- print() adds a newline by default
print("Hello, World!")

# Run a Lua script directly
lua script.lua

-- Single-line comment

--[[
    Multi-line
    comment block
--]]

local math = require("math") -- Standard math library
local socket = require("socket") -- Common external library

-- os.exit(code, [close])
-- 0 for success, non-zero for error
os.exit(1)

-- pcall (protected call) catches errors
local status, err = pcall(function()
    error("Something went wrong")
end)

if not status then
    print("Caught error: " .. err)
end

-- assert throws error if condition is false
assert(1 == 1, "This will not throw")

local x = 10      -- Local variable
y = 20            -- Global variable
local name = "Lua" -- String

local int_val = 100
local float_val = 3.14
print(type(int_val))

local n = tonumber("123") -- String to number (123)
local s = tostring(456)   -- Number to string ("456")
local f = tonumber("invalid") -- nil

if 0 then
    print("0 is true")
end
if "" then
    print("Empty string is true")
end

local path = os.getenv("PATH")
print(path ~= nil)

local seconds = os.time() -- Current time in seconds
local date_str = os.date("%Y-%m-%d %H:%M:%S") -- Formatted date
local cpu_time = os.clock() -- CPU time for performance measurement

print(date_str)

local add = 10 + 5
local floor_div = 10 // 3 -- Lua 5.3+ floor division
local power = 2 ^ 10      -- 1024
local neq = 10 ~= 5       -- Not equal
local logic = true and false or true -- and/or/not

local text = "The price is $100"
-- string.match returns captures
local price = string.match(text, "%$(%d+)")
print(price)

-- arg[0] is the script name, arg[1] is the first argument
for i = 1, #arg do
    print("Arg " .. i .. ": " .. arg[i])
end

local function add(a, b)
    return a + b
end

-- Multiple returns
local function swap(a, b)
    return b, a
end

local x, y = swap(10, 20)

local t1 = {val = 1}
local t2 = t1 -- t2 points to same table as t1
t2.val = 2
print(t1.val)

-- Metatable example
local mt = { __add = function(a, b) return a.x + b.x end }
local obj = {x = 10}
setmetatable(obj, mt)

-- While loop
local i = 1
while i <= 3 do
    print("while: " .. i)
    i = i + 1
end

-- Numeric for loop (start, stop, step)
for j = 1, 3 do
    print("for: " .. j)
end

for i = 1, 10 do
    if i > 2 then break end
    print(i)
end

local list = {"a", "b", "c"}
-- ipairs for array-like tables
for i, v in ipairs(list) do
    print(i, v)
end

local map = {key = "val"}
-- pairs for all keys
for k, v in pairs(map) do
    print(k, v)
end

local s = "hello"
local upper = s:upper()         -- "HELLO"
local sub = s:sub(1, 2)         -- "he" (1-indexed)
local concat = "a" .. "b"       -- "ab"
local len = #s                  -- 5

local function join(path, file)
    local sep = package.config:sub(1, 1)
    return path .. sep .. file
end
print(join("/usr", "bin"))

local f = io.open("test.txt", "w")
if f then
    f:write("line 1\n")
    f:write("line 2\n")
    f:close()
end

local f = io.open("test.txt", "r")
if f then
    local content = f:read("*a") -- "*a" reads the entire file
    f:close()
    print(content)
end

-- pack(format, values...)
-- ">i4" is Big-endian 4-byte integer
local binary_data = string.pack(">i4d", 42, 3.14)
local val, pi = string.unpack(">i4d", binary_data)

local and_res = 0x0F & 0xF0 -- 0
local or_res  = 0x0F | 0xF0 -- 255
local lshift  = 1 << 3      -- 8
local not_res = ~0          -- -1 (signed)

local hex_str = string.format("%02x", 255) -- "ff"
local num = tonumber("ff", 16)             -- 255

local list = {10, 20}
table.insert(list, 30)

local map = {a = 1, b = 2}
map["c"] = 3

local set = {["item1"] = true, ["item2"] = true}
print(set["item1"])

local cjson = require("cjson")

local json_text = '{"id": 42, "name": "lua"}'
local data = cjson.decode(json_text)
print(data.name)

local encoded = cjson.encode({status = "ok"})

local socket = require("socket")
local client = socket.connect("example.com", 80)
client:send("GET / HTTP/1.0\r\n\r\n")
local line, err = client:receive()
client:close()

local socket = require("socket")
local udp = socket.udp()
udp:setpeername("127.0.0.1", 12345)
udp:send("hello")
udp:close()

local co = coroutine.create(function()
    print("Coroutine started")
    coroutine.yield("yielded value")
    print("Coroutine resumed")
end)

local status, res = coroutine.resume(co)
print(res)
coroutine.resume(co)

local sqlite3 = require("lsqlite3")
local db = sqlite3.open_memory()

db:exec[[
  CREATE TABLE test (id INTEGER, content TEXT);
  INSERT INTO test VALUES (1, 'Hello World');
]]

for row in db:nrows("SELECT * FROM test") do
  print(row.id, row.content)
end
db:close()