For me this does not seem to work. Even though the script exists, the condition always evaluates to false. By adding some debugging statements, I found that rc is a boolean (or nil?), and if rc then works for me.

https://www.lua.org/pil/22.2.html is not really clear, but my AI friend told me:

In Lua, os.execute() returns different values depending on the Lua version.

✅ Lua 5.2 and newer (5.2, 5.3, 5.4)

os.execute(command) returns three values:
success, exit_type, code = os.execute("your_command")

Meaning:
success → true if the command terminated normally, nil otherwise
exit_type → string:
"exit" → program exited normally
"signal" → program was killed by a signal (Unix)
code → numeric:
exit status (if "exit")
signal number (if "signal")

Lua 5.1 apparently does only return a number...

💡 Portable pattern

If you want code that works across versions:

local r1, r2, r3 = os.execute(cmd)

if type(r1) == "number" then
    -- Lua 5.1
    local exit_code = r1 / 256
else
    -- Lua 5.2+
    local success = r1
    local exit_code = r3
end

I'm confused, so I can catch 3 returns, even if the command only returns a single one (i.e. r1)? And this does not lead to errors?

Also: does this change than solve your original issue of this thing always evaluating to false? I guess I then just check exit_code == 0 and that would work accross both, right?

Yeah, apparently that works in Lua:

function test()
    return 42
end

> test()
42
> r1, r2, r3 = test()
> r1
42
> r2
nil
> r3
nil

Checking the exit code should work in that case, indeed.

Done, can you check that the current version also works for you? (I tested it and it still works for me - I'm pretty sure I'm on Lua 5.1)

This file will have to be added to the following list to ensure that it gets deployed to cvmfs:
https://github.com/EESSI/software-layer-scripts/blob/main/install_scripts.sh#L210

Done