Lua:
Reading command line arguments

How to:

Here’s the skinny on grabbing those arguments in Lua:

-- Save this as 'greet.lua'
local name = arg[1] -- arg[1] is the first command line argument
print("Hello, " .. (name or "stranger") .. "!")

Fire up the terminal and run it:

$ lua greet.lua LuaLearner
Hello, LuaLearner!

No name? No problem:

$ lua greet.lua
Hello, stranger!

Deep Dive

Lua keeps it straightforward with the global arg table. Historically, folks have been reading command line arguments in programming since the dawn of time (well, since the birth of UNIX, at least). It’s a staple of customization.

In Lua, arg is an array with all the command-line goodies. arg[0] is the script name, and arg[1] onwards are the actual arguments. You can scoop them all up with a loop if you’re feeling fancy:

for i = 1, #arg do
  print("Argument " .. i .. ": " .. arg[i])
end

Alternatives? Sure, there are libraries out there for sophisticated argument parsing (like Penlight), but for many cases, arg does the trick without fuss.

As for implementation details, remember Lua’s arrays are 1-based (they start counting at 1), not 0 like many other languages. That’s why arg[1] is the first argument and not arg[0].

See Also

For those hungry for more, here’s some extra chow:

• Lua 5.4 Reference Manual on the arg table: https://www.lua.org/manual/5.4/manual.html#6.1
• “Programming in Lua” (4th edition) for a solid grasp on Lua basics: https://www.lua.org/pil/contents.html
• Penlight, a Lua utility library with enhanced argument parsing: https://github.com/lunarmodules/Penlight