check whether a script exists by script name

[user4592357]

@JonB
okay i will use full path, that not a problem.

i have one more question, so that script actually outputs something, i'm calling it with QProcess::startDetached(progName, args) where args is the args for the script.
but since it has output, i wanna suppress it, so i added /dev/null:

const auto args = QStringList() <<
		... <<
		"> /dev/null 2>&1";

but it is passed to the script and the script says it's an unknown/unexpected argument. what should i do?

[JonB]

@user4592357
Once you start using symbols like >, 2>&1 you now cannot try to execute some_util directly. Those are interpreted by the (bash) shell. You will have to achieve a:

bash -c 'some_util ... > /dev/null 2>&1'

so the executable is bash with two arguments, -c and some_util ... > /dev/null 2>&1 as a single-argument-string.

This is getting hairy, and probably not the best way to do it. Better use QProcess stdout/err redirection if that's what you need to achieve.

[user4592357]

@JonB
yes, i need to not have output from that script i'm calling. this didn't work, i still get the output:

QProcess process;
process.setStandardOutputFile(QProcess::nullDevice());
if (!process.startDetached(progName, args))
...

[Christian Ehrlicher]

@user4592357 said in check whether a script exists by script name:

!process.startDetache

QProcess::startDetached() is a static function - if you need something from the process, you have to use QProcess::start() as explained in the documentation

[SGaist]

You're using this static overload which is not the same as this member function.

[JonB]

@user4592357
In addition to my learned colleague @Christian-Ehrlicher's excellent catch :)
You also need/should call setStandardErrorFile() (or use MergedChannels), in addition to your Output one.

[user4592357]

thanks for the replies, now i have this and it works. is this code okay?

	QProcess process;
	process.closeReadChannel(QProcess::StandardOutput);
	process.start(progPath, args);
	while (!process.atEnd());
	process.close();

also, why suppress errors? i'd like to see if the called program has any errors.

[JonB]

@user4592357 said in check whether a script exists by script name:

also, why suppress errors? i'd like to see if the called program has any errors.

You wrote

but since it has output, i wanna suppress it

and the code you chose to show had

"> /dev/null 2>&1"

I was just translating what you wrote, where you are sending stderr to nowhere. It's up to you.

[user4592357]

@JonB
okay. and above code is okay? specifically, the while (!process.atEnd()); part.

[JonB]

@user4592357

while (!process.atEnd());

Lord, no, I didn't look! That would occupy all CPU time for parent process!

If you want to do it synchronously that way you must at least use waitForFinished(), see the example & discussion at https://doc.qt.io/qt-5/qprocess.html#synchronous-process-api. It's not the best compared to asynchronous with signal/slot, but I haven't got time, again see the doc if you want to understand.

[user4592357]

@JonB
i see. thanks a lot