Qt - get output from running process

Question

I would like to get output from running process on Linux in Qt.

My code looks like this:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <qprocess.h>
#include <qthread.h>

QProcess process;

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    process.start("htop");

    connect(&process, SIGNAL(readyReadStandardOutput()), this, SLOT(getData()));
}

void getData(){

    QByteArray out;
    out = process.readAllStandardOutput();
}

MainWindow::~MainWindow()
{
    delete ui;
}

But I want to get realtime (changing) output for example from htop and save it to string.

Reading the docs turned up [this](http://doc.qt.io/qt-5/qprocess.html#readAllStandardOutput). So doing it in a loop will probably get you there... — StoryTeller - Unslander Monica, Dec 12 '16 at 13:34
I tested it in Qt debugger but i get no output. Array out is empty. — tomsk, Dec 12 '16 at 13:38
Did you call it more than once? The running process must generate output to read before you can actually read it. Which means it must be scheduled to be run first — StoryTeller - Unslander Monica, Dec 12 '16 at 13:39
You need to read Qt documentation: http://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput — Dmitry Sazonov, Dec 12 '16 at 13:48
Thank you and how to connect that signal with function to get data from stream? — tomsk, Dec 12 '16 at 14:03
You connect the signal the same way you connect any other signal in `Qt`. Although process can't be a local variable. — drescherjm, Dec 12 '16 at 14:23
change `connect(NULL, SIGNAL(process.readyReadStandardOutput(stdout)), NULL, SLOT(getData));` to `connect(process, SIGNAL(readyReadStandardOutput()), this, SLOT(getData()));` — drescherjm, Dec 12 '16 at 14:43
thank you, do i have to add/change something in mainwindow.h? Because im getting this: no matching function for call to ‘MainWindow::connect(QProcess&, const char*, MainWindow*, const char*)’ — tomsk, Dec 12 '16 at 14:49
I am sorry it should be `connect(&process, SIGNAL(readyReadStandardOutput()), this, SLOT(getData()));` since process is not a pointer. — drescherjm, Dec 12 '16 at 15:24
I changed my code, but if i set breakpoints inside getData(); my program doesnt stop, it looks like signal doesnt call getData(); — tomsk, Dec 12 '16 at 18:29
I believe you got a lot of help. We're not a code writing service. — sehe, Dec 14 '16 at 19:59

score 5 · Accepted Answer · answered Dec 14 '16 at 20:36

Because the sample "htop" interests me, here's a hint.

htop is an "interactive" terminal application (using curses to "draw" a animated terminal image), as opposed to a run-of-the-mill UNIX-style filter (that takes input from a file-like source, and provides a sequential output stream to any file-like destination).

So it's not quite as easy to "capture" it live. In fact the only class of application that supports this is called a terminal emulator. Let's use tmux as a terminal emulator that is capable of writing "screenshots" to a file.

$ SESS_ID=$(uuidgen)
$ COLUMNS=80 LINES=25 tmux new-session -s "$SESS_ID" -d htop

This starts a new session, running htop in the background. We generated a unique ID so we can control it without interfering with other tmux sessions. You can list it to check what the name is:

$ tmux list-sessions
a9946cbf-9863-4ac1-a063-02724e580f88: 1 windows (created Wed Dec 14 21:10:42 2016) [170x42]

Now you can use capture-pane to get the contents of that window:

$ tmux capture-pane -t "$SESS_ID" -p

In fact, running it repeatedly gives you a (monochrome) live mirror of the htop (every 2 seconds, by default):

$ watch tmux capture-pane -t "$SESS_ID" -p

Now. You want color, of course. Use ansifilter:

$ tmux capture-pane -t "$SESS_ID" -p -e | ansifilter -H > shot.html

Voila. I'm sure Qt has a nice Widget to display HTML content. I tested it running this

$ while sleep 1; do tmux capture-pane -t "$SESS_ID" -p -e | ansifilter -H > shot.html; done

And opening shot.html in my browser. Every time I reload, I get an up-to-date screenshot:

Oh, PS when you're done clean up that session using

$ tmux kill-session -t "$SESS_ID"

No. The only way is to use terminal emulation and get the screen contents from there. I happen to have used tmux. You don't need to use HTML. I just chose that because it was a simple solution to get the ANSI escapes rendered to something non-terminal. You're free to choose other tools, this is just a proof of concept. — sehe, Dec 14 '16 at 20:59

Qt - get output from running process

1 Answers1