sys.cmd seems broken. What am I missing?

pythonprogrammer

Just learning via luaRT, and I have a dead simple window. After the window is shown, I call sys.cmd() to launch another process. Let's say 'notepad.exe', per examples I've seen here, but that doesn't really matter (I've tried a bunch of stuff).

A basic call to sys.cmd() will NOT launch the specified cmd. It never happens. The host windows is there, but the cmd never executes. Even so, sys.cmd() returns true! I can confirm the specified cmd doesn't launch by watching the proc monitor (in case it was launching and dying quickly or something).

Now, if I add the extra arguments to sys.cmd(), specifically standalone=true, THEN the cmd does launch. Ok .. good. BUT, if I close the host window then the launched process also quits, which seems like the opposite of what standalone=true is supposed to do.

So, to reiterate:
result = sys.cmd('start "" notepad.exe')
... nothing happens. but result == true.

result = sys.cmd('start "" notepad.exe', true, true)
... notepad instance is launched, but if I close the host window, then the notepad instance is killed.

Sooo... what's up? What am I doing wrong, or what is wrong with sys.cmd?

Thanks in advance.

Samir

Try to start the program directly without calling start.exe.
Other users have reporter a buggy behaviour. I'm still investigating it

pythonprogrammer

Switching the cmd from ('start "" notepad.exe') to just ('notepad.exe') does cause the cmd to execute. There are definitely odd discrepancies when using start vs. not using it. Also, I observe different behaviors (working vs. not) if I use ('start "" .') vs ('start "" notepad.exe'). Even moving the file around a bit seemed to affect the outcome, if I was trying ('start "" .')!! As in, it would sometimes work and sometimes not.

esferatec

I have tried to reproduce the error with my system. All examples works except for number 5, where no editor is started. But it seems that the function sys.cmd always returns true.

`local ui = require "ui"
local sys = require "sys"

local win = ui.Window("Simple Window")

function win:onShow()
local r1 = sys.cmd('start "" notepad.exe') -- works
print(r1)
local r2 = sys.cmd('start notepad.exe') -- works
print(r2)
local r3 = sys.cmd('start "" notepad.exe', true, true) --works
print(r3)
local r4 = sys.cmd("notepad.exe") --works
print(r4)
local r5 = sys.cmd("notepadf.exe") --works not
print(r5)
end

ui.run(win):wait()`

pythonprogrammer

I've got luaRT source locally now and am investigating. The func responsible for sys.cmd() has a hardcoded return 1, so it's always going to return true in in its host lua.

I've not yet been able to pin down why the call sometimes actually executes the cmd and sometimes doesn't. It's fundamentally unreliable.

pythonprogrammer

oops. no ... there's an lua_pushboolean() affecting the lua return value. So, presumably would reflect a hard fail of the underlying CreateProcessW() call. Though I've yet to witness sys.cmd() return false.

Samir

Using sys.cmd() with start.exe as argument will always return true because the process start.exe started as expected.
If start.exe fails to find the executable sys.cmd will return true as start.exe was executed successfully

pythonprogrammer

sorry ... I hopped onto an imgui py binding and got my tool built and out the door this weekend.
I wanted to use luaRT for this little thing I needed, to try it out, but ... literally got stuck on this first sys.cmd() anomaly which was a showstopper for the task. I spent over a day trying to determine why it would run the actual cmd sometimes and not others, with either no change whatsoever to the cmd or with multiple flavors of syntax that were endorsed as legitimate. Nothing exposed any explanation for the behavior.

Wish I had a more encouraging postmortem on this. 🙁

Samir

I don't know why it may fail at times. Have you tried without start ?

By the way I'm working on a new Process object to have better control over sub processes

pythonprogrammer

I tried every conceivable crafting of the actual cmd that I could think of. It's the inconsistency of the resulting behavior that dissuaded me from continuing.

... more specifically: I would manually run cmd syntax in a shell and observe it working fine. Plunk it identically into sys.cmd() and run that (even using the same directory) and it would silently do nothing in luaRT.

Samir

Ok you should use a sys.Pipe object with the same command Line and check the Pipe:readerror() result in case an error message has been written to stderr.