Lua for Programmers Part 4: Tips and Tricks

If you haven’t read the previous parts (one, two and three), then I’d highly recommend doing so. For reference, here’s a list of the parts in this series:

• Part 1: Language Essentials, covers fundamental syntax and concepts such as operators, loops, and functions.
• Part 2: Data and Standard Libraries, covers Lua’s built-in data types and some of the standard libraries.
• Part 3: More Advanced Concepts, deals with things like variable scope, advanced functions, and file loading.
• Part 4: Tips and Tricks, the current part; a collection of small things that you may find useful.

Command Line Arguments

Command line arguments are stored in the arg table. For example, if we had the script foo.lua:

print(arg[-1], arg[0])
for i, v in ipairs(arg) do print(v) end

And we ran it via lua foo.lua arg1 arg2 arg3, the output would be:

lua foo.lua
arg1
arg2
arg3

arg[-1] is the name of the interpreter/program, generally lua. arg[0] is the name of the file run. All entries after this are the command line arguments; this allows you to iterate over them as shown in the example.

... in Files

Since files are loaded as functions, it makes sense that you can use ... inside of them. If we have a file called bar.lua:

print(...) -- prints all arguments given to this file's function

And then we load it like this:

loadfile("bar.lua")(1, 2, 3, 4)

The output will be 1 2 3 4.

As for the other functions, dofile sends no arguments, and require sends a single argument with the path it was given:

require("bar") -- "bar" is the output
require("folder.subfolder.bar") -- "folder.subfolder.bar" is the output

If you use ... in the entry-point file (this would be foo.lua if you executed lua foo.lua, for example) you’ll get a list of command line arguments. So if we were to run bar.lua via lua bar.lua arg1 arg2 arg3, the output would be arg1 arg2 arg3.

_G

_G is a global table which holds all global variables. Here’s an example:

a = 3
print(_G.a) -- 3
_G.b = 4
print(b) -- 4
print(_G._G == _G) -- true

Conclusion

Well, that’s it for now. If you’ve got any suggestions for things that could be added, I’d love hear them.

Thanks for reading the series, I hope it’s been of use to you.