The Logs are not showing in Application Window.

Aviral 0

Hi, i am making a Tail Log application. It is executing perfectly but the output window is not showing any results.
Below is the code:
LogTail.cpp

#include "logtail.h"
#include <QApplication>
#include <QTextEdit>
#include<QDebug>
//start id=constructor
LogTail::LogTail(QString fn) {
    //if (fn == QString()) {
    //    fn = "C:/Users/arpit.k/OneDrive - Accord Software & Systems Pvt. Ltd/Documents/QT/LogTail3/sample.log";
   // }
    connect (this, SIGNAL(readyReadStandardOutput()),
        this, SLOT(logOutput()));       /* When there is input ready, call this slot.*/
    QStringList argv;

    argv << "-f" << fn;                 /* tail -f filename */
    start("tail", argv);                /* Returns immediately, and now there is a child process running, "attached"
    to this process. When this process exits, the child tail process will also terminate. */
}
LogTail::~LogTail() {
    terminate();                        /* Attempts to terminate this process. */
}
//end

//start id=logOutput
// tail sends its output to stdout.
void LogTail::logOutput() {             /* Slot called whenever there is input to read. */
    QByteArray bytes = readAllStandardOutput();
    QStringList lines = QString(bytes).split("\n");
    foreach (QString line, lines) {
        //qDebug() << line;
        emit logString(line);
    }
}
//end

main.cpp

#include "logtail.h"
#include <QTextEdit>
#include <QApplication>

int main (int argc, char* argv[]) {
    QApplication app(argc, argv);
    QStringList al = app.arguments();
    QTextEdit textEdit;
    textEdit.setWindowTitle("Debug");
    textEdit.setWindowTitle("logtail demo");
    QString filename = "sample.log";
    if (al.size() > 1) filename = al[1];
    LogTail tail(filename);             /* Create object, starts process too. */
    tail.connect (&tail, SIGNAL(logString(const QString&)),
        &textEdit, SLOT(append(const QString&)));
    textEdit.show();
    return app.exec();
}

Screenshot 2023-02-09 101847.png

Note: sample.log file contains log data

jsulm

@Aviral-0 How should this work at all? Who is emitting readyReadStandardOutput() signal?
Why don't you use QProcess to run the command?

Aviral 0

@jsulm If I am using this, then its not able to detect the file and doesn't display anything:

#include <QApplication>
#include <QFile>
#include <QPlainTextEdit>
#include <QProcess>
#include <QTextStream>
#include <QTimer>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QPlainTextEdit *textEdit = new QPlainTextEdit;
    textEdit->setReadOnly(true);
    textEdit->show();

    QProcess *process = new QProcess;
    process->start("tail", QStringList() << "-n" << "10" << "log.txt");
    process->waitForStarted();
    textEdit->setPlainText(QString(process->readAll()));

    QTimer *timer = new QTimer;
    QObject::connect(timer, &QTimer::timeout, [textEdit, process] {
        process->start("tail", QStringList() << "-n" << "10" << "log.txt");
        process->waitForFinished();
        textEdit->setPlainText(QString(process->readAll()));
    });
    timer->start(1000);

    return a.exec();
}

What I am getting wrong?

jsulm

@Aviral-0 said in The Logs are not showing in Application Window.:

What I am getting wrong?

You're not using https://doc.qt.io/qt-6/qprocess.html#readyReadStandardOutput from QProcess to execute a slot everytime there is something to read from the process. So connect a slot to that signal and read the output of the process using https://doc.qt.io/qt-6/qprocess.html#readAllStandardOutput

Also, you have zero error handling! Please add it (see https://doc.qt.io/qt-6/qprocess.html#errorOccurred and https://doc.qt.io/qt-6/qprocess.html#error).

Another thing to consider: you're using relative path to the log file - if the file is not inside the working directory tail will not find it...

Aviral 0

@jsulm Please can you write me the code of readyReadStandardOutput
As I am very new to QT. I would really appreciate your efforts.
Also with file path: I have given absolute file path and also put the file in the code folder. still its not detecting

jsulm

@Aviral-0 You should know how to connect a slot to a signal, so please at least try to write the code by yourself...

Aviral 0

@jsulm Hi, I have tried this:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QPlainTextEdit *textEdit = new QPlainTextEdit;
    textEdit->setReadOnly(true);
    textEdit->show();

    QProcess *process = new QProcess;
    process->start("tail", QStringList() << "-n" << "10" << "../logs/sample.log");
    process->waitForStarted();
    QObject::connect(process, &QProcess::readyReadStandardOutput, [textEdit, process] {
            textEdit->appendPlainText(QString(process->readAllStandardOutput()));
        });
    textEdit->setPlainText(QString(process->readAll()));

    QTimer *timer = new QTimer;
    QObject::connect(timer, &QTimer::timeout, [textEdit, process] {
        process->start("tail", QStringList() << "-n" << "10" << "../logs/sample.log");
        process->waitForFinished();
        textEdit->setPlainText(QString(process->readAll()));
    });
    timer->start(1000);

    return a.exec();
}

Still not getting results. :(

jsulm

@Aviral-0 I repeat myself: add error handling and make sure the path to log file is correct and can be found by tail at runtime...
I would also do the connect before calling waitForStarted (which is actually not needed if using signals/slots).
There is also no need for a timer to detect termination of a process, there is https://doc.qt.io/qt-6/qprocess.html#finished for that.

Aviral 0

@jsulm Hi, This is the updated one:
And still not able to read file:

#include <QApplication>
#include <QDebug>
#include <QFile>
#include <QPlainTextEdit>
#include <QProcess>
#include <QTextStream>
#include <QTimer>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QPlainTextEdit *textEdit = new QPlainTextEdit;
    textEdit->setReadOnly(true);
    textEdit->show();

    QProcess *process = new QProcess;
    process->start("tail", QStringList() << "-n" << "10" << "sample.log");

    QObject::connect(process, &QProcess::readyReadStandardOutput, [textEdit, process] {
        textEdit->setPlainText(QString(process->readAllStandardOutput()));
    });
    QObject::connect(process, &QProcess::errorOccurred, [](QProcess::ProcessError error) {
        qWarning() << "Error occurred:" << error;
    });
    QObject::connect(process, &QProcess::finished, [process](int exitCode, QProcess::ExitStatus exitStatus) {
        if (exitStatus == QProcess::CrashExit) {
            qWarning() << "Process crashed with code" << exitCode;
        } else {
            qDebug() << "Process finished with code" << exitCode;
        }

        process->start("tail", QStringList() << "-n" << "10" << "sample.log");
    });

    return a.exec();
}

I have put the copy of file in built folder, source folder and everywhere but still its not able to read.
Also after putting absolute path of the file, still not worked!

jsulm

@Aviral-0 said in The Logs are not showing in Application Window.:

I have put the copy of file in built folder, source folder and everywhere but still its not able to read

Why don't you simply use an absolute path for testing?
Was errorOccurred signal emitted?

Aviral 0

@jsulm I have used absolute path and it is not working and also no error was generated

jsulm

@Aviral-0 Could be that tail prints out an error on stderr. To check that connect a slot to https://doc.qt.io/qt-6/qprocess.html#readyReadStandardError and print out what https://doc.qt.io/qt-6/qprocess.html#readAllStandardError returns there.

JonB

@Aviral-0
We have said this before, but one more time. You are not doing yourself any favors by doing this via QProcess & tail command. @jsulm wrote in https://forum.qt.io/topic/142476/how-to-display-recent-logs-and-can-be-filter-according-to-the-checkboxes/12

@Aviral-0 You can open the file, seek to file end and then connect a slot to https://doc.qt.io/qt-6/qfilesystemwatcher.html#fileChanged signal. In the slot you simply read to the end of the file and add the data you read to your log widget.

Or just use a QTimer instead of QFileSystemWatcher, which may well be what tail -f does. Up to you, just saying.

Actually, I now see that for this question you are only using tail -n not tail -f. Which makes it even easier to do this by reading the file in your process.

Aviral 0

@JonB I was using QTimer, but @jsulm suggested my not to https://forum.qt.io/post/746836
Also I replaced tail -n with tail -f and its not working. I am really confused what is not working.

jsulm

@Aviral-0 said in The Logs are not showing in Application Window.:

I was using QTimer, but @jsulm suggested my not to https://forum.qt.io/post/746836

Sorry, but this is just wrong!
I suggested not to use QTimer to detect when the process finishes.
@JonB is talking about something completelly different (not using QProcess). Please read more carefully!

JonB

@Aviral-0
No he did not. He said you don't need a QTimer for

There is also no need for a timer to detect termination of a process, there is https://doc.qt.io/qt-6/qprocess.html#finished for that.

That is detecting a program finishes, not for picking up the tail of some output.

Also I replaced tail -n with tail -f and its not working. I am really confused what is not working

Why are you attempting to run an external process to print out the last lines in some log file when you could do it yourself in your app by reading the file? I have said before: you can do it via QProcess/tail if you really want to, but since you are having problems with that maybe it would be simpler not to do it that way.

Aviral 0

@JonB @jsulm I know it would be frustrating for you both. But Can anyone please write in simple technical terms of what functions I have to use. In a process execution manner?

jsulm

@Aviral-0 said in The Logs are not showing in Application Window.:

But Can anyone please write in simple technical terms of what functions

Please first clarify what exactly you are asking.
Do you want code to read a file?

JonB

@Aviral-0
First please answer @jsulm.
Then could you make clear: tail -n and tail -f do somewhat different things. Which of the two's behaviour are you actually looking for?

Aviral 0

@jsulm Yes, I want a application which displays Recent 10 Lines of Logs from a Log File. And it should be updating dynamically, any change in log file appears in real time in application window.

Secondly, User be able to pause the Log updates and Be able to select or copy any log lines.

Third, a checkbox filter to filter the application with log types: like If I click on INFO then it should display only INFO logs, similarly for DEBUG and WARN.

The Base version code is this which runs and displays last 10 Logs but is not dynamically updating new logs from file:

#include <QApplication>
#include <QProcess>
#include <QTextEdit>
#include <QVBoxLayout>
#include <QWidget>

int main(int argc, char *argv[]) {
  QApplication app(argc, argv);
  QWidget window;
  QVBoxLayout layout(&window);
  QTextEdit textEdit;
  layout.addWidget(&textEdit);
  window.show();

  QProcess process;
  process.start("tail", QStringList() << "-n" << "10" << "/home/arpitk/QT Projects/logtail1/sample.log");
  process.waitForFinished();
  QByteArray output = process.readAllStandardOutput();
  textEdit.setPlainText(output);

  return app.exec();
}