About double quotes in QProcess and display cmd.exe when debug

jsulm

@Ehehe Did you try to check whether there was an error?
See http://doc.qt.io/qt-5/qprocess.html#error and http://doc.qt.io/qt-5/qprocess.html#readyReadStandardError

sierdzio

To see the output of the terminal you need to set the channel mode to QProcess::ForwardedChannels.

This will not show you the command that is run, only the output. I don't think there is a way to see it, but you can try nativeArguments() or program().

JonB

@Ehehe

you can see the second parameter, it is a string and has a pair of double quotes, so in Qt code I use ", but likely they do not be passed correctly,

When you put your own pair of double-quotes into the second argument passed as an argument list to QProcess::start(), Qt code decided you wanted to pass those quotes as part of the argument and so it will have put further quotes (and backslashes) around yours. That's why it would then be wrong.

When you choose the QProcess::start() overload which accepts a list of arguments, Qt will quote each one as necessary: your job is to pass in those arguments without quotes, and let Qt do it for you. When you choose the QProcess::start() overload which accepts a single string of argument(s), it requires you to have done your own quoting, and it does not further-quote it.

SGaist

Hi,

You should use QDir::toNativeSeparators for your file paths. Since you're on Windows, ffmpeg might not understand the forward slash notation.

JonB

In his particular case, OP started by showing his command worked with forward slashes outside of Qt. It would be better to use QDir::toNativeSeparators, but just so he understands why his existing code will work.

SGaist

@JonB good point, I misread that line.

Ehehe

@SGaist Thanks for your reminder. In fact, I get the path through QFileDialog::getOpenFileName and getSaveFileName , so nothing about slash or backslash. :D

Ehehe

@JonB Thanks very much! It's useful!! I don't know if the argument is string (QString), it will send double-quotes automatically. Live and learn.

Ehehe

@JonB I have read the documentation fo QProcess carefully, and also have the same question: Why I do not need to add double-quotes when I choose the QProcess::start() overload which accepts a list of arguments?
I try this command in Windows cmd.exe(without double-quotes in second argument), it is useless :
> ffmpeg.exe -i concat:file1.mp3|file2.mp3 -acodec copy out.mp3
and try these code(have \") in Qt code, it is useful:

QProcess *cmd = new QProcess(this);
QString cmdArgv = QString("ffmpeg.exe -i \"concat:%1\" -acodec copy %2").arg(fileList.join("|")).arg(savePath);
cmd->start(cmdArgv);

Above all, the second argument has a pair of double-quotes as part of itself. However these code is useful too!!!

QProcess *cmd = new QProcess(this);
QString program = "ffmpeg.exe";
QStringList argv;
argv.append(QString("-i"));
argv.append(QString("concat:%1").arg(compaxList.join("|")));
argv.append(QString("-acodec"));
argv.append(QString("copy"));
argv.append(savePath);
cmd->start(program,argv);

This method makes all argument have double-quotes or have no double-quotes, but is useful.
Although my Qt program works successfully, I still can not figure out how QProcess::start() works. I hope your can explain for me, or give me any document link I will learn it by myself.
Thanks.

SGaist

Because you have the | char which is a pipe and is handled by the command line interpreter. If you don't escape that string properly like you have to do on a command line you end up with something that cannot be interpreted.

JonB

@Ehehe
As @SGaist says.

And also, one more time: there are 2 different overloads for QProcess::start():

Originally you used void QProcess::start(const QString &program, const QStringList &arguments, OpenMode mode = ReadWrite). Here QStringList &arguments is a list of separate arguments. So Qt code knows each one is an argument, and quotes each one for you. So you must not quote the arguments yourself.
Then you use void QProcess::start(const QString &command, OpenMode mode = ReadWrite). Here const QString &command is a single string, the executable followed by any arguments. So Qt code does not know where each argument is, and so does no quoting for you. So you must quote the arguments yourself as you construct the string.

Ehehe

@JonB Run this command in cmd.exe, and know little more.
> ffmpeg.exe "-i" "concat:file1.mp3|file2.mp3" "-acodec" "copy" "output.mp3"

void QProcess::start(const QString &program, const QStringList &arguments, OpenMode mode = ReadWrite) quotes each argument, and this style of command line is effective. In the original command line, only second argument has double-quotes, it confused me.

Thanks for your patient explanation!

And also thank you @SGaist .

JonB

@Ehehe

Because you are using the "separate arguments list", Qt quotes (or may quote) each & every argument, just in case. It may look odd, but does no harm, and works.

If you do not want to allow Qt to produce too many unnecessarily-quoted args, you must use the "single string all-arguments". Then you are responsible for pre-quoting, and in your example the only one which requires quoting is the "concat:file1.mp3|file2.mp3", because it contains a | character.