“main” function in Lua?

Question

In python, one would usually define a main function, in order to allow the script to be used as module (if needed):

def main():
    print(\"Hello world\")
           


        

Answer 1

What's wrong with this:

$ cat aa.lua
#!/usr/bin/lua

if (arg ~= nil and arg[-1] ~= nil) then
    print "main"
else
    print "library"
end
$ ./aa.lua
main
$ ./aa.lua arg1 arg2
main
$ cat bb.lua
#!/usr/bin/lua

print("in bb")
$ lua -laa bb.lua
library
in bb
$ lua
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> require "aa"
library
> 

Answer 2

There's no "proper" way to do this, since Lua doesn't really distinguish code by where it came from, they are all just functions. That said, this at least seems to work in Lua 5.1:

matthew@silver:~$ cat hybrid.lua 
if pcall(getfenv, 4) then
    print("Library")
else
    print("Main file")
end
matthew@silver:~$ lua hybrid.lua 
Main file
matthew@silver:~$ lua -lhybrid
Library
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> ^C
matthew@silver:~$ lua
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> require "hybrid"
Library
> ^C
matthew@silver:~$

It works by checking whether the stack depth is greater than 3 (the normal depth for a file in the stock Lua interpreter). This test may break between Lua versions though, and even in any embedded/custom Lua builds.

I'll also include this (slightly more portable) alternative, although it's taking an even greater leap in heuristics, and has a failure case (see below):

matthew@silver:~$ cat hybrid2.lua 
function is_main(_arg, ...)
    local n_arg = _arg and #_arg or 0;
    if n_arg == select("#", ...) then
        for i=1,n_arg do
            if _arg[i] ~= select(i, ...) then
                print(_arg[i], "does not match", (select(i, ...)))
                return false;
            end
        end
        return true;
    end
    return false;
end

if is_main(arg, ...) then
    print("Main file");
else
    print("Library");
end
matthew@silver:~$ lua hybrid2.lua 
Main file
matthew@silver:~$ lua -lhybrid2
Library
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> ^C
matthew@silver:~$ lua 
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> require "hybrid2"
Library
>

This one works by comparing the contents of _G.arg with the contents of '...'. In the main chunk they will always be the same. In a module _G.arg will still contain the command-line arguments, but '...' will contain the module name passed to require(). I suspect this is closer to the better solution for you, given that you know your module name. The bug in this code lies when the user executes the main script with 1 argument, and this is the exact name of your module:

matthew@silver:~$ lua -i hybrid2.lua hybrid2
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
Main file
> require "hybrid2"
Main file
> 

Given the above I hope at least you know where you stand, even if it isn't exactly what you had in mind :)

Update: For a version of hybrid.lua that works in Lua 5.1 and 5.2, you can replace getfenv with debug.getlocal:

if pcall(debug.getlocal, 4, 1) then
    print("Library")
else
    print("Main file")
end

Answer 3

you could try to check if the module has been required.

from documentation:

package.loaded A table used by require to control which modules are already loaded. When you require a module modname and package.loaded[modname] is not false, require simply returns the value stored there.

With this you could write:

if not package.loaded['modulename'] then
    main()
end

Answer 4

Maybe you can just deal with the debug library, with the debug.getinfo() function

if debug.getinfo(1).what == "main" then
    -- Main execution
end

See the reference manual for more information.

Answer 5

When Lua requires a module, it passes it the name it's been required with as varargs (...).

So, if your script doesn't intend to take any arguments (from the command line or otherwise), you can use something like

if ... then
  return this_mod --module case
else
  main() --main case
end

Note, however, that this isn't foolproof in the (entirely) possible case that you take arguments. However, at this point, you can combine this with Lukasz's answer to get:

if not package.loaded[...] then
  --main case
else --module case
end

Still not perfect (for instance, if the script is called with a first argument of string or the name of some other already-loaded module), but likely good enough. In other situations, I defer to MattJ's answer.

Answer 6

I'm using lua 5.3 and had issues with most of the suggestions here. This is what worked for my use case:

local my_module = {}
...
if os.getenv('CLI') then
  main()
else
  return my_module
end

When running from the command line I simple defined the environment variable such as:

CLI=1 lua my_script.lua

Works for me™