reading and writing to QProcess in Qt Console Application

Question

Noted: this appears to be a specific issue question but hopefully it can be edited for all to related to

I need to interact with a QProcess object.

The Problem:

I am not getting any output from QProcess after calling QProcess:write(input)

More Info:

Going through the doc pages led me to create an example below:

I have a script requesting user input, and finally displaying and appropriate message based on the user input.

Testing:

After adding a "log" feature to my script for testing, the following occurs:

script executes
script requests user input (confirmed by the 'first' qDebug() << p->readAll())
script accepts input from QProcess (confirmed by script 'log output')

After this, no output is received. The following 2 debug statements both fire (i.e. wait 30s each)

if (!p->waitForReadyRead()) {
    qDebug() << "waitForReadyRead() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
}
if (!p->waitForFinished()) {
    qDebug() << "waitForFinished() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
}

Followed by:

QString s = QString(p->readAll() + p->readAllStandardOutput());

where s is an empty string.

The issue is s should contain either "success" or "failed"

Calling Code:

QString cmd = QString("sh -c \"/path/to/bashscript.sh\"");
QString input = QString("Name");
QString result = runCommand(cmd, input)

Process Code:

//takes 2 parameters, 
//    cmd which is the code to be executed by the shell
//    input which acts as the user input

QString runCommand(QString cmd, QString input){
    QProcess *p = new QProcess(new QObject());
    p->setProcessChannelMode(QProcess::MergedChannels);   //no actual reason to do this
    p->start(cmd);
    if (p->waitForStarted()) {
        if (!p->waitForReadyRead()) {
            qDebug() << "waitForReadyRead() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
        }
        if (!p->waitForFinished()) {

            //reads current stdout, this will show the input request from the bash script
            //e.g. please enter your name:
            qDebug() << p->readAll();  

            //here I write the input (the name) to the process, which is received by the script
            p->write(ps.toLatin1());

            //the script should then display a message i.e. ("success" o "failed")
            if (!p->waitForReadyRead()) {
                qDebug() << "waitForReadyRead() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
            }
            if (!p->waitForFinished()) {
                qDebug() << "waitForFinished() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
            }
        }
        QString s = QString(p->readAll() + p->readAllStandardOutput());
        return s;
    }
    else{
        qDebug() << "waitForStarted() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
    }
    p->waitForFinished();
    p->kill();
    return QString();
}

script.sh (-rwxrwxr-x)

#!/bin/bash
#returns 
#    "success" on non empty $n value
#    "failed: on empty $n value
#
echo "enter your name:"
read n
if [[ ! -z $n ]];
then
        echo "success"
        exit 0;
else
        echo "failed"
        exit 1;
fi

UPDATE

@KevinKrammer I modified the run command as you said, also using the QStringList with the args.

Still does not get output, infact the waitForReadyRead() and waitForFinished() returns false instantly.

Called with:

QString r = runCommand(QString("text"));

Process Code:

QString runCommand(QString input){      

    QProcess *p = new QProcess(new QObject());    
    p->setProcessChannelMode(QProcess::MergedChannels);

    //script is the same script refered to earlier, and the `cd /home/dev` IS required
    p->start("sh", QStringList() << "-c" << "cd /home/dev" << "./script");
    ;
    if (p->waitForStarted()) {
        if (!p->waitForReadyRead(5000)) {
            qDebug() << "waitForReadyRead() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
        }
        qDebug() << p->readAll();
        p->write(input.toLatin1());
        if(!p->waitForFinished(5000)){
            qDebug() << "waitForFinished() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
        }
        QString s = QString(p->readAll() + p->readAllStandardOutput());
        return s;
    }
    else{
        qDebug() << "waitForStarted() [false] : CODE: " << QVariant(p->error()).toString() << " | ERROR STRING: " << p->errorString();
    }
    p->waitForFinished();
    p->kill();
    return QString();
}

Terminal Output of the Process:

started
readChannelFinished
exit code =  "0"
waitForReadyRead() [false] : CODE:  "5"  | ERROR STRING:  "Unknown error"
""
waitForFinished() [false] : CODE:  "5"  | ERROR STRING:  "Unknown error"
Press <RETURN> to close this window...

Thoughts on this?

UPDATE 2

@Tarod Thank you for taking the time to make a solution.

It works, however not completely is expected.

I copied over your code, exactly.

Made a few changes in the mReadyReadStandardOutput()

See additional info below.

The problem:

After running the application (and script), I get a result -> AWESOME

Everytime it is the incorrect result i.e. "failed". -> NOT AWESOME

Terminal Output:

void MyProcess::myReadyRead()
void MyProcess::myReadyReadStandardOutput()
"enter your name:\n"
""
void MyProcess::myReadyRead()
void MyProcess::myReadyReadStandardOutput()
"failed\n"
Press <RETURN> to close this window...

script contents:

#!/bin/bash
echo "enter your name:"
read n
echo $n > "/tmp/log_test.txt"
if [[ ! -z "$n" ]];
then
        echo "success"
        exit 0;
else
        echo "failed"
        exit 1;
fi

/tmp/log_test.txt output

myname

running this manually from console:

dev@dev-W55xEU:~$ ls -la script 
-rwxrwxr-x 1 dev dev 155 Jan 25 14:53 script*

dev@dev-W55xEU:~$ ./script 
enter your name:
TEST_NAME
success

dev@dev-W55xEU:~$ cat /tmp/log_test.txt 
TEST_NAME

Full code:

#include <QCoreApplication>
#include <QProcess>
#include <QDebug>

class MyProcess : public QProcess
{
    Q_OBJECT

public:
    MyProcess(QObject *parent = 0);
    ~MyProcess() {}

public slots:
    void myReadyRead();
    void myReadyReadStandardOutput();
};

MyProcess::MyProcess(QObject *parent)
{
    connect(this,SIGNAL(readyRead()),
            this,SLOT(myReadyRead()));
    connect(this,SIGNAL(readyReadStandardOutput()),
            this,SLOT(myReadyReadStandardOutput()));
}

void MyProcess::myReadyRead() {
    qDebug() << Q_FUNC_INFO;
}

void MyProcess::myReadyReadStandardOutput() {
    qDebug() << Q_FUNC_INFO;
    // Note we need to add \n (it's like pressing enter key)
    QString s = this->readAllStandardOutput();
    qDebug() << s;
    if (s.contains("enter your name")) {
        this->write(QString("myname" + QString("\n")).toLatin1());
        qDebug() << this->readAllStandardOutput();
    }
}

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

    MyProcess *myProcess = new MyProcess();

    QString program = "/home/dev/script";

    myProcess->start("/bin/sh", QStringList() << program);

    a.exec();
}

#include "main.moc"

script issue? QProcess issue?

why are you calling `waitForFinished` before writing? Given your program the order would be to wait for start, then loop on wait for read until you've received the expected string, the write, then wait for finished. For the command I would also recommend to pass command and arguments separately, i.e. the command as the first argument to `start()` and all its parameters as the second, as this will also handle espacing correctly and automatically. — Kevin Krammer, Jan 25 '17 at 10:26
@KevinKrammer Thank you for suggestion, I will apply it and return with a result — CybeX, Jan 25 '17 at 10:40
What happens if you only execute `sh` instead of your script? What about `/bin/sh`? — Georg Schölly, Jan 25 '17 at 11:44
`waitForFinished` returns `false` if the process is already finished. The unknown error is just the default value for `error()`. I think this is a bad API decision, but explains at least why you get `false` there. — Georg Schölly, Jan 25 '17 at 11:59
@KGCybeX if your command is a script, why not just execute the script instead of executing a shell that executes the script? — Kevin Krammer, Jan 26 '17 at 12:36

Tarod · Accepted Answer · 2017-01-25T14:37:20.277

4

Unfortunately I don't have all your code, so I made an example. I hope it helps you.

If I compare my code to yours, I think the problem could be you are not calling readAllStandardOutput() after writing or maybe you are not calling exec() in your main.cpp.

#include <QCoreApplication>
#include <QProcess>
#include <QDebug>

class MyProcess : public QProcess
{
    Q_OBJECT

public:
    MyProcess(QObject *parent = 0);
    ~MyProcess() {}

public slots:
    void myReadyRead();
    void myReadyReadStandardOutput();
};

MyProcess::MyProcess(QObject *parent)
{
    connect(this,SIGNAL(readyRead()),
            this,SLOT(myReadyRead()));
    connect(this,SIGNAL(readyReadStandardOutput()),
            this,SLOT(myReadyReadStandardOutput()));
}

void MyProcess::myReadyRead() {
    qDebug() << Q_FUNC_INFO;
}

void MyProcess::myReadyReadStandardOutput() {
    qDebug() << Q_FUNC_INFO;
    // Note we need to add \n (it's like pressing enter key)
    this->write(QString("myname" + QString("\n")).toLatin1());
    // Next line no required
    // qDebug() << this->readAll();
    qDebug() << this->readAllStandardOutput();

}

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

    MyProcess *myProcess = new MyProcess();

    QString program = "/home/fran/code/myscript.sh";

    myProcess->start("/bin/sh", QStringList() << program);

    a.exec();
}

#include "main.moc"

Script to test the application:

echo "enter your name:"
read n
if [ ! -z "$n" ];
then
    echo "success"
    exit 0;
else
    echo "failed"
    exit 1;
fi

edited Jan 25 '17 at 14:37

answered Jan 25 '17 at 11:40

Tarod

6,732
5
44
50

1

According to the docs `QProcess` should also work without the event loop, so no `app.exec()` is required. – Georg Schölly Jan 25 '17 at 11:43
OK. Anyway, in my example is required due to the signal/slot methods. Thank you, @georgschölly – Tarod Jan 25 '17 at 11:47
@Tarod I was looking for the signal slot method, on the forum.qt.io it is suggested to use the Eventloop, although I feel more comfortable using the signal slot method for now. I will test this, I really hope it works :p – CybeX Jan 25 '17 at 11:49
@Torad please see update 2, I think I/we are narrowing in on the issue... – CybeX Jan 25 '17 at 13:38
@Torad if you don't mind, could you apply my exact example. This would give clarity to the root cause of my troubles :p – CybeX Jan 25 '17 at 13:41
1

@KGCybeX Unfortunately I don't have the code right know. Maybe it's the `if` sentence in the script. Try `if [ ! -z "$n" ];` instead. – Tarod Jan 25 '17 at 14:05
@Tarod Now I really feel stupid. `if [ ! -z "$n" ];` does solve my problem. I get the correct output now. Last question *promise*, why does your suggestion work and not `if [[ ! -z $n ]];` any thoughts on that? – CybeX Jan 25 '17 at 14:15
@Torad thanks for ALL the help, I cannot tell you how much this means to me. Please add the script with its correction to your answer, I am sure someone else struggling, like me, will be grateful to come across this post! – CybeX Jan 25 '17 at 14:18
@KGCybeX You're welcome, it's been a pleasure to help you. About the `if` sentence in the script, you have a lot of information [here](http://stackoverflow.com/questions/2188199/how-to-use-double-or-single-bracket-parentheses-curly-bracesRead) and [here](http://stackoverflow.com/questions/669452/is-preferable-over-in-bash-scripts). Please read also the comments in the answers. – Tarod Jan 25 '17 at 15:25

reading and writing to QProcess in Qt Console Application

1 Answers1

Linked