[Solved]I can't make QProcess work

[Nirvana]

I am trying to get a Linux console output. Here is my code

p.start("gsettings get org.gnome.desktop.wm.preferences theme");
    QString p_stdout = p.readAllStandardOutput();
    p.waitForFinished();
    qDebug()<<"theme ="<<p_stdout;

p.start("gsettings get org.gnome.desktop.wm.preferences theme");
    QString p_stdout = p.readAllStandardOutput();
    p.waitForFinished();
    qDebug()<<"theme ="<<p_stdout;

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in terminal, I get Ambiance output which is my current theme. But QT terminal gives theme ="" output.
What am I doing wrong?
Thanks