check whether a script exists by script name

[user4592357]

i'm on linux, and i need to check whether a script exists.
at first i used this code:

auto sUtilName = "some_util";
QFileInfo fileInfo(sUtilName);
if (fileInfo.exists())
	return sUtilName;

actually, i can execute the script by running some_util on terminal, but qt doesn't find it.
so i had to use full path of the script. but i'd first to check without full path, and if it isn't reachable, only then try full path.

[SGaist]

Hi,

Where is that script supposed to be ?
Where is your application located ?

[JonB]

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

actually, i can execute the script by running some_util on terminal, but qt doesn't find it.

Is it that some_util is being found on your PATH (from your description this sounds quite likely)? Or (depending on your Linux flavor), if some_util is in the current directory when you type it in a terminal, what is the current directory in the terminal, and what is it when you run your Qt program?

Please answer @SGaist's questions above.

[user4592357]

@SGaist
i have some env var set, e.g. "export PROG_DIR=/some/path/" which isn't in $PATH, and the script is available at $PROG_DIR/some_util.
application is located some directory, why does that matter?
e.g. current dir=/path/, running aaplication: ./sub-dir/app

[JonB]

@user4592357
If the current directory is /path/, and some_util is in $PROG_DIR (where $PROG_DIR is not /path/), going QFileInfo fileInfo("some_util"); is not going to find it. That will only look in whatever the current directory happens to be. And (unless you set it yourself) you don't know what it might be for an end user anyway; searching for a relative pathname is not good, unless you actually intend it only to look in the current directory.

It sounds like you need to expand (the environment variable in) the $PROG_DIR/some_util to a full path if you expect to pass it to QFileInfo.

[user4592357]

@JonB
ok, then why can i start the script with QProcess::startDetached(progName, args)?

[JonB]

@user4592357
Because it's on your PATH?

What is the "script" anyway? Is it something interpreted by something which looks at that PROG_DIR environment variable? I don't recognise what is using a PROG_DIR environment variable?

[Christian Ehrlicher]

This post is deleted!

[user4592357]

@JonB
no the script isn't in $PATH.

ok, so at first script isn't available: bash: some_util: command not found
i set the env var: export PROG_DIR=/some/path/, now script is available: some_util
when i do which some_util, i get the value of $PROG_DIR, i.e. /some/path/

[Christian Ehrlicher]

PROG_DIR is no env var which evaluates something to be available in PATH later on so it must be something special in your setup.

[JonB]

@user4592357
I will look around for PROG_DIR now, because I'm interested. EDIT Couldn't find it anywhere!

Meanwhile, for your question, if some_util is not going to be in the current directory, then QFileInfo("some_util") is not going to find it. If it is found in $PROG_DIR then you will need to expand that environment variable in your code before passing to QFileInfo.

OOI: can you show us what your PATH environment is before you do anything in a new terminal/shell? From what you described: Does bash allow $PROG_DIR to be "embedded" as an environment variable inside PATH??

[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