Adjust screen backlight

[Jakob Clausen]

Hi.
I am trying to adjust the screen backlight.
I can do it from the terminal using

echo 26 > /sys/class/backlight/lvds0_backlight/brightness

Where 26 is maximum brightness.
But if I try it from the code it does not work.

qDebug() << startCommand(value);
QProcess::execute(startCommand(value));

QString Brightness::startCommand(uint8_t value)
{
    return "echo echo " + QString::number(value) + " > /sys/class/backlight/lvds0_backlight/brightness";
}

which gives the output:

"echo echo 26 > /sys/class/backlight/lvds0_backlight/brightness"
echo 26 > /sys/class/backlight/lvds0_backlight/brightness

[Christian Ehrlicher]

@Jakob-Clausen said in Adjust screen backlight:

But if I try it from the code it does not work.

"echo echo 26 > /sys/class/backlight/lvds0_backlight/brightness"

What do oyu expect when you double 'echo' ?

[Jakob Clausen]

@Christian-Ehrlicher
If do it with a single echo, then I get the output

"echo 12 > /sys/class/backlight/lvds0_backlight/brightness"
12 > /sys/class/backlight/lvds0_backlight/brightness

and still no effect.

[JonB]

@Jakob-Clausen
Whether you are Linux or Windows in both cases it is only a shell which interprets a redirection symbol like >. You cannot execute this line as the program argument to QProcess::execute(). If you are Linux try

QProcess::execute("/bin/sh", QStringList() << "-c" << "echo " + QString::number(value) + " > /sys/class/backlight/lvds0_backlight/brightness");

Having said that, this is not a good way to implement it. There is no point firing a command to call "echo" to send "26" to a path. Just open that device path for write and send it the number, no QProcess needed.

[Jakob Clausen]

It works.
The commandline suggestion worked.
And the better idea also worked.