However, p->startCommand("dir C:\\Users") gives me no error and no output.

Well that is interesting. As I said I only just discovered looking at the docs. It's new in Qt 6.0. I do not use Qt 6, nor Windows, so I cannot comment. If you look at he examples at https://doc.qt.io/qt-6/qprocess.html#startCommand you can see them claiming to issue dir commands that way, that's all I can say.

I want to ask that in p->start("cmd", { "/c", "dir C:\\Users" });, what does the /c mean in the second argument?

The Windows/DOS cmd.exe takes various arguments, as you can see from running cmd /?. Including:

/C      Carries out the command specified by string and then terminates

Basically without the /c it runs an interactive Command Prompt, the command-line console. With the /c it interprets and executes the command line string following it, i.e. your dir .... It does not open a Command Prompt window and it returns to your code immediately after executing the command. dir is built into cmd.exe --- along with many other "simple" commands like echo or mkdir --- there is no external dir.exe file to execute, so we need cmd /c dir to perform it.

However, p->startCommand("dir C:\\Users") gives me no error and no output.

I want to ask that in p->start("cmd", { "/c", "dir C:\\Users" });, what does the /c mean in the second argument?

• QProcess::start(const QString &program, const QStringList &arguments = {}, QIODeviceBase::OpenMode mode = ReadWrite) does not accept a complete command line, only a single executable as its first argument. Change over to the correct new overload.

• QProcess::readAllStandardOutput() is not going to read an error message. You must call QProcess::readAllStandardError() too. You also don't attach to signal QProcess::errorOccurred(). I think what you have will be generating one of these, possibly the latter.

• dir is not an executable program under Windows. It is built into cmd.exe, which you must run. (Unless Qt handles this for you, I don't have Qt under Windows.)

• dir C:/Users is wrong/lucky if it works under Windows. Use proper native \s for paths in QProcess commands, it does not know an argument is a path and will not translate /s to \s for you, unlike e.g. QFile.

p->start("cmd", { "/c", "dir C:\\Users" });
p->waitForFinished();
QString output = p->readAllStandardOutput();
QString error = p->readAllStandardError();

UPDATE
At Qt 6.0 it seems they introduced void QProcess::startCommand(const QString &command, QIODeviceBase::OpenMode mode = ReadWrite). So this would probably also work:

p->startCommand("dir C:\\Users")