Turning a string into a variable call

I want to set a variable from a string. For example, if string = 'variable' and later on I wanna set variable, the variable inside string, to a new value. How would I do that?

I've heard about something like _G before a string value in some way, and somehow that should convert that into a variable call. I'm probably saying a nonsense but don't get me wrong, I've been a lot of time trying to get this to work and writing a lot of tests in vain.

Don't use string as a variable name; you're overwriting (if string is global) or shadowing (if string is local) the global string table. I'll be using varname.

Simply use varname as a key to index _G; _G is just a regular table after all:

local varname = "variable"
print(_G[varname]) -- print the value of the global variable named "variable"
_G[varname] = 42 -- change the value

similarly, to list all global variables, you can just loop over _G using pairs:

for varname in pairs(_G) do print(varname) end

If you want to call the value, use _G[varname]():

function foo() print"foo!" end
function bar() print"bar!" end
local varname = "foo"
_G[varname]() -- foo!

Depending on your specifics, you might instead want to look up the variable in _ENV / getfenv(1) (which need not necessarily be _G) or search the local variables and upvalues using debug.getlocal and debug.getupvalue respectively.

This seems like an X-Y-Problem though: Why do you need to dynamically index the global table? If you're not the consumer of a poorly designed API, you should be able to re-engineer your API to use its own table rather than polluting the global table. For example, I'd rewrite the above example as:

local t = {}
function t.foo() print"foo!" end
function t.bar() print"bar!" end
local varname = "foo"
t[varname]() -- foo!