how to echo directly from dynamic input

[Mijaz]

This code worked fine:

system("cd /sys/bus/iio/devices/iio:device1/; echo 1111 > out_altvoltage1_TX_LO_frequency");
QProcess process1;
process1.start("cat /sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
process1.waitForFinished(-1); // will wait forever until finished
QString stdout1 = process1.readAllStandardOutput();
qDebug() << stdout1;
ui->lineEdit_3->setText(stdout1);

// output display in lineEdit_3 : 1111 (this is correct)

This code not working correctly:
QString input = ui->lineEdit_1->text();
// entered value 2222 to the lineEdit_1 in GUI
ui->lineEdit_2->setText(input);
// diplay lineEdit_2 : 2222 (this is correct)
// I want to directly echo the input as given below but this not updating the (out_altvoltage1_TX_LO_frequency) register value.

system("cd /sys/bus/iio/devices/iio:device1/; echo input > out_altvoltage1_TX_LO_frequency");   
QProcess process1;
process1.start("cat /sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
process1.waitForFinished(-1); // will wait forever until finished
QString stdout1 = process1.readAllStandardOutput();
qDebug() << stdout1;
ui->lineEdit_3->setText(stdout1);

// output display in lineEdit_3 : 1111 (this is not correct)

[jsulm]

@Mijaz OK, that means you want to write to /sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency file, because

cd /sys/bus/iio/devices/iio:device1/
echo 111111 > out_altvoltage1_TX_LO_frequency

does exactly that.
Then simply use QFile to write:

QFile f("/sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
if (f.open(QIODevice::WriteOnly | QIODevice::Text)) {
    f.write(ui->lineEdit_2->text().toLatin1());
    f.close();
}

[jsulm]

@Mijaz This looks really overcomplicated: why don't you simply use QFile to read from /sys/bus... instead of spawning a process for such a simple task? There is really no need to use cat in a process to read a file...

[Mijaz]

Hi @jsulm yes, you are correct I can read from QFile also. But here main problem is
echo input > out_altvoltage1_TX_LO_frequency;
How can do this in qt;

[jsulm]

@Mijaz said in how to echo directly from dynamic input:

How can do this in qt;

This writes into a file, you can do it also with QFile.

[Mijaz]

@jsulm
Do you mean like this?

QFile f("echo /sys/bus/iio/devices/iio:device1/input > out_altvoltage1_TX_LO_frequency");
if (f.open(QIODevice::ReadOnly | QIODevice::Text))
{
QByteArray ba = f.readAll();
ui->lineEdit_3->setText(QString(ba));
f.close();
}

[jsulm]

@Mijaz said in how to echo directly from dynamic input:

you mean like this?

No, I don't.
Just open the file using QFile and read/write from/to it:

QFile f("/sys/bus/iio/devices/iio:device1/input");
if (f.open(QIODevice::ReadOnly | QIODevice::Text)) {
    QByteArray ba = f.readAll();
    ui->lineEdit_3->setText(QString(ba));
    f.close();
}

Be careful with QIODevice::Text: don't use it if file contains binary data.

[jsulm]

@Mijaz The line where you get that error does not make any sense.
I don't know what you are trying to do there...

[Mijaz]

@jsulm
QFile f("/sys/bus/iio/devices/iio:device1/input");
if (f.open(QIODevice::ReadOnly | QIODevice::Text)) {
QByteArray ba = f.readAll();
ui->lineEdit_3->setText(QString(ba));
f.close();
}

I executed above your code, no any output display.

[jsulm]

@Mijaz said in how to echo directly from dynamic input:

/sys/bus/iio/devices/iio:device1/input

Is this a valid path?
Did you check return value of open()?
Did you check what https://doc.qt.io/qt-5/qiodevice.html#errorString returns?
Does /sys/bus/iio/devices/iio:device1/input contain text or binary data?

[Mijaz]

@jsulm
I want to update register (out_altvoltage1_TX_LO_frequency) with the new value;
In real time when I will enter new value to lineEdit_1 then it should be display in lineEdit_2 as well as update the register (out_altvoltage1_TX_LO_frequency) which will recheck in lineEdit_3.

QString input = ui->lineEdit_1->text();
ui->lineEdit_2->setText(input);
QFile f("/sys/bus/iio/devices/iio:device1/input");
if (f.open(QIODevice::ReadOnly | QIODevice::Text))
{
QByteArray ba = f.readAll();
ui->lineEdit_3->setText(QString(ba));
f.close();
}

[jsulm]

@Mijaz You did not answer my questions from my previous post.
Do you mean you want to update out_altvoltage1_TX_LO_frequency inside /sys/bus/iio/devices/iio:device1/input ?
Because this:

echo /sys/bus/iio/devices/iio:device1/input > out_altvoltage1_TX_LO_frequency

would write the content of /sys/bus/iio/devices/iio:device1/input into file out_altvoltage1_TX_LO_frequency in current working directory.

[Mijaz]

@jsulm
No input is not a directory it is the string in which I have stored value in about command like this:

QString input = ui->lineEdit_1->text();
ui->lineEdit_2->setText(input);
Now I want to use this "input" in which I already stored value to update my out_altvoltage1_TX_LO_frequency.
//
Detail explenation of my work:
Step 1:
I firstly executed following commands on my board where I am deploying my project like this:

cd /sys/bus/iio/devices/iio:device1/
echo 111111 > out_altvoltage1_TX_LO_frequency
This worked fine;
Step 2:
I create project on my PC using following commands and GUI.

system("cd /sys/bus/iio/devices/iio:device1/; echo 222222 > out_altvoltage1_TX_LO_frequency");
QProcess process1;
process1.start("cat /sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
process1.waitForFinished(-1); // will wait forever until finished
QString stdout1 = process1.readAllStandardOutput();
qDebug() << stdout1;
ui->lineEdit_3->setText(stdout1);
This also work correctly.
Step 3:
Now, because it is my task requirement that I need update register (out_altvoltage1_TX_LO_frequency) value at real-time, depends on the situation not remained fixed like (222222), therefore I update code like this:

QString input = ui->lineEdit_1->text();
ui->lineEdit_2->setText(input);
QFile f("/sys/bus/iio/devices/iio:device1/input");
if (f.open(QIODevice::ReadOnly | QIODevice::Text))
{
QByteArray ba = f.readAll();
ui->lineEdit_3->setText(QString(ba));
f.close();
}

whenever I will enter a new value to lineEdit_1 that should update register (out_altvoltage1_TX_LO_frequency) immediately,

this code not working as my requirements.

[jsulm]

@Mijaz OK, that means you want to write to /sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency file, because

cd /sys/bus/iio/devices/iio:device1/
echo 111111 > out_altvoltage1_TX_LO_frequency

does exactly that.
Then simply use QFile to write:

QFile f("/sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
if (f.open(QIODevice::WriteOnly | QIODevice::Text)) {
    f.write(ui->lineEdit_2->text().toLatin1());
    f.close();
}

[Mijaz]

@jsulm

QFile f("/sys/bus/iio/devices/iio:device1/out_altvoltage1_TX_LO_frequency");
if (f.open(QIODevice::WriteOnly | QIODevice::Text))
{
f.write(ui->lineEdit_2->text().toLatin1());
f.close();
}

this still not updating register ( out_altvoltage1_TX_LO_frequency) value.

[jsulm]

@Mijaz You still did not answer any of my previous questions?
Did you check what happens? Please use debugger and see whether your if block was executed at all.
Did you check return value of open()?
Did you check what https://doc.qt.io/qt-5/qiodevice.html#errorString returns?

[JonB]

@jsulm
Do you realise this thread is just a rehash of @Mijaz's own https://forum.qt.io/topic/107974/system-vs-process-command/ ? He asks exactly the same question there. You will see the same questions about the cat command and the same answers about using QFile, the same issue about the current directory and paths, .... I tried to help, but he chose not to supply the necessary information, and is just going through the same stuff again. It gets so frustrating here when people choose not to follow through the advice they are given when they do not like it, and instead create a new thread to get the information all over again...

[jsulm]

@JonB Yes, I saw that thread and I agree that it is very frustrating if people do not answer questions and do not follow advices - what's the point to ask then in a forum? But it is how it is...

[Mijaz]

@jsulm
Thank you so much for your kind help to find a proper solution.

[jsulm]

@Mijaz Does it work now?

[Mijaz]

@jsulm
yes, it is working correctly.