QProcess::execute capture output

Jan-Willem

I made a simple example.

// in constructor
connect(&process, SIGNAL(readyReadStandardOutput()), this, SLOT(readOutput()));

// slot connected to a QAction
void Widget::runProcess()
{
    QString sh = qgetenv("SHELL");
    process.setProgram(sh);
    process.setArguments(QStringList() << "-i");
    process.start();

    process.write("ls"); // first test
    // process.execute("ls"); // second test

    process.closeWriteChannel();
}

// slot connected to QProcess::finished
void Widget::readOutput()
{
    plainTextEdit->appendPlainText("readOutput");
    plainTextEdit->appendPlainText(process.readAllStandardError());
    plainTextEdit->appendPlainText(process.readAllStandardOutput());
}

In the first test QProcess::readAllStandardOutput() gives me the string "readOutput" and the listing of the current working directory in the plainTextEdit.

For the second test I changed the following in Widget::runProcess():

    // process.write("ls"); // first test
    process.execute("ls"); // second test

In the second test readyReadStandardOutput() is not emitted and the listing of the current working directory is show in the Application Output pane in QtCreator.

I expected the output forwarded to the defined QProcess process, but it seems to be forwarded to the main application. Any ideas how I can get it in the calling QProcess process?

jsulm

@Jan-Willem execute() is a static method! It is not related to your QProcess instance which you connected to your slot.
You should replace

process.setProgram(sh);

with

process.setProgram("ls");

Jan-Willem

@jsulm thanks for the clarification. The I have interpreted the use of execute() wrong.
I know

process.setProgram("ls");

works. But I was trying to create a bash-session because of the variables defined in .bashrc.
The output of the QProcess seems to be only available after the QProcess is finished. Consider this:

process.setProgram("/bin/bash");
process.start();
process.write("pwd\n");
process.write("cd ..\n");
process.write("pwd");

This gives the desired result, but the output is received only when the QProcess is finished. And then the bash-session is closed, so the result of cd is not there anymore.
You see my problem? I can't seem to find a way around it.

SGaist

What exactly do you want to do with your serie of commands ?

jsulm

@Jan-Willem Maybe you should do

process.write("cd ..\n");
process.write("pwd");

after you got the output of the first pwd? Because currently you're flooding the shell with commands without waiting for them to finish.

Jan-Willem

@SGaist I'm experimenting with building a simple terminal window. The series of commands are only for testing. It could be any command for that matter.
@jsulm Thanks, I suspected that. But the output never comes, unless I close the writingchannel or what for the process to finish.

If I write:

process.write("pwd\n");
process.write("ls\n");
process.write("pwd\n");

it works as expected. The output after each step is shown. cmd does not generate output, but the result is also not shown on the last call of pwd.

When I create a custom object through which the process is started and then connect it to a Q(Plain)TextEdit, then everything seems to work. I'm trying to understand the reason why.

SGaist

Do you have something like Konsole in mind ?

Jan-Willem

Yes, but much more simple.
I have looked into that code and from a derived project QTermWidget.

It seems I now have a working solution, but I try to understand why this works.

Jan-Willem

I have started from scratch to see what my problem actually was, but I wasn't able to reproduce it.
I probably got lost somewhere in my code.

Well, below is the working code for the curious.

@SGaist @jsulm Thanks for your help!

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    QPlainTextEdit *plainTextEdit = new QPlainTextEdit(this);
    plainTextEdit->setReadOnly(true);

    QLineEdit *lineEdit = new QLineEdit(this);
    lineEdit->setClearButtonEnabled(true);

    QVBoxLayout *layout = new QVBoxLayout;
    layout->addWidget(plainTextEdit);
    layout->addWidget(lineEdit);
    setLayout(layout);

    connect(&process, &QProcess::readyReadStandardOutput, [=](){ plainTextEdit->appendPlainText(process.readAllStandardOutput()); });
    connect(&process, &QProcess::readyReadStandardError, [=](){ plainTextEdit->appendPlainText(process.readAllStandardError()); });
    connect(&process, static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished), this, &Widget::close);
    connect(lineEdit, &QLineEdit::returnPressed, [=](){ process.write(lineEdit->text().toLatin1() + "\n"); lineEdit->clear(); });

    process.start("sh");
}

Widget::~Widget()
{
    process.close();
}

SGaist

If you have a recent enough version of Qt, you can use qOverload in place of static_cast. It will do the same but will be more readable.

Jan-Willem

@SGaist Good one! Though I had to use the C++11 version since C++14 is still in Debian Sid.

// C++11
connect(process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), this, &MainWindow::close);

For those who want to know, this is the C++14-version:

// C++14
connect(process, qOverload<int, QProcess::ExitStatus>(&QProcess::finished), this, &MainWindow::close);

Since the actual problem of this thread is solved, I will mark this thread as solved tomorrow. In the mean time comments on the above code can be made.