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

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