QProcess not firing readyRead() or readyReadStandardOutput() signals

Anthony Abboud

Hi Qt Community,

I am currently working on an app that will be monitoring the journalctl of the linux system, and display a popup when a specific line from another tool will be logged in it.

I am trying to reproduce a command via QProcess which works when directly in terminal. I want to perform a read of the journal every few seconds.

journalctl -q -u aktualizr* | grep "Update available"

If data with this filter exists, then I want to display a popup on my application to indicate to the user that said process has been initiated.

See below the code I'm working on:

#include <QDebug>
#include <QProcess>

// Check journalctl every 3 seconds
#define UPDATES_CHECK_CYCLE 3000

UpdatesWorker::UpdatesWorker(QObject *parent) : QObject(parent)
{
    // All constructor code has been moved the the init function (slot) so that it can be called after this object is assigned a thread in order to make sure everything has the correct thread scope.
}

void UpdatesWorker::initUpdatesChecker()
{
    m_processUpdatesChecker = new QProcess();
    connect(m_processUpdatesChecker, SIGNAL(readyRead()), this, SLOT(updateInfoReceived()));

    // Set up timer to check journalctl file at a fixed interval
    updatesCheckTimer = new QTimer();
    updatesCheckTimer->setSingleShot(true);
    updatesCheckTimer->setInterval(UPDATES_CHECK_CYCLE);
    connect(updatesCheckTimer, SIGNAL(timeout()), SLOT(checkUpdates()));
    updatesCheckTimer->start();    
}

// Every 3 seconds, this function will be called and execute the journalctl read and filtering
void UpdatesWorker::checkUpdates(){
    if(m_processUpdatesChecker->state() == QProcess::Running || m_processUpdatesChecker->state() == QProcess::Starting){
        m_processUpdatesChecker->close();
    }
    else{
        qDebug() << "Checking journal" << Qt::endl;
        m_processUpdatesChecker->start("/bin/sh", QStringList() << "-c" << "'journalctl -q -u aktualizr* | grep \"Update available\"'");
    }
    updatesCheckTimer->start();  
}

// If the search filter returns something, this function should be called and send a signal to the UI to display a popup
void UpdatesWorker::updateInfoReceived()
{
    qDebug() << "Update initiated" << Qt::endl;
    updatesCheckTimer->stop(); 
    SystemManager::Instance()->signalUpdateInitiated();
}

As of right now, the check every 3 seconds is performed correctly, but I am unable to make the app fire the readyRead() signal in order to trigger the function updateInfoReceived(). I've looked up doc and tried to use the signal readyReadStandardOutput() instead, but still nothing. Manually executing the line in terminal shows the filtered output I'm looking for.

I've tried to follow similar posts where the same concept was solved, but I can't seem to identify what I'm missing.

Thanks in advance!

SGaist

Hi,

Usually, the culprit is the pipe. The solution is to chain two QProcess (the output of the first sent to the input of the second) however, in your case, you could parse the output of journalctl directly in your application.

Anthony Abboud

Hi @SGaist ,

I've attempted to chain two QProcess instead of using pipe as per the example here: QProcess - setStandardOutputProcess

However the signal still didn't fire.

To debug further, I simplified the command to remove the grep part, so the process call is now only journalctl -q -u aktualizr*, which outputs a bunch of results via terminal, as expected.

Then, implementing into the code like so:

else{
        qDebug() << "Checking journal" << Qt::endl;
        m_processUpdatesChecker->start("journalctl", QStringList() << "-q" << "-u aktualizr*");
}

But I still get no readyRead signals.

Performing a readAllStandardOutput() and readAllStandardError() outputs nothing either.

SGaist

Did you connect the error signals as well ?

By the way, you did not completely split the arguments, this might also be part of the issue.

JonB

@Anthony-Abboud
Both the issues @SGaist has mentioned. And when do you call readAllStandardError(), do you wait for the process to finish first?

m_processUpdatesChecker->start("/bin/sh", QStringList() << "-c" << "'journalctl -q -u aktualizr* | grep \"Update available\"'");

Enclosing the argument to -c inside its own 's (single-quotes) is wrong. Remove those to make this work.

m_processUpdatesChecker->start("journalctl", QStringList() << "-q" << "-u aktualizr*");

-u aktualizr* should not be a single argument.