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c++qtshellqprocesschain

Piping (or command chaining) with QProcess

I'm using Qt and bash over it, need to execute something like:

bash: cat file | grep string

in Qt:

QString cmd = "cat file | grep string";
QProcess *process = new QProcess;
process->start(cmd);
process->waitForBytesWritten();
process->waitForFinished();
qDebug() << process->readAll();

The problem is in pipe ("|"), and process returs nothing. If there is no ("|"), like 

"cat file"

everything is ok. I tried smth. like 

"cat file \\| grep string", 
"cat file \| grep string"

but result is the same. If I copy the command and run it in bash everything is ok. 

QString::toAscii().data()

and other transforms also have bad result.

Solution

  • The problem is you cannot run a system command with QProcess, but only a single process. So the workaround will be to pass your command as an argument to bash:

    process.start("bash", QStringList() << "-c" << "cat file | grep string");