Piping (or command chaining) with QProcess

Question

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.

@LaszloPapp No argument there... Since there is a dedicated API your answer is better. On the other hand, invoking a shell may allow for more complex commands, e.g. using process substitution, shell globbing etc, so it may have some advantages under some conditions. — user000001, Jan 03 '14 at 11:12

score 29 · Accepted Answer · answered Jan 03 '14 at 10:56

29

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");

answered Jan 03 '14 at 10:56

Dmitry Markin

1,105
8
8

Why not use QProcess for piped commands with the high-level API as per my answer? :-) – lpapp Jan 03 '14 at 10:57
Your answer is perfectly OK, this is just a workaround. – Dmitry Markin Jan 03 '14 at 10:58
What do you mean by "system command"? You probably meant something more specific, like "shell command". – lrineau Jan 03 '14 at 11:04
I meant `system` call like `os.system()` in Python or `system` from ``. – Dmitry Markin Jan 03 '14 at 11:07
1

@DmitryMarkin: OT, but do not use os.system() in python. People should use the subprocess module as per documentation. – lpapp Jan 03 '14 at 11:49

score 28 · Answer 2 · edited Jun 20 '20 at 09:12

The quick and dirty hack would be this:

QString cmd = "/bin/sh -c \"cat file | grep string\"";

You could also avoid the escaping in there with C++11's R"", but the point is that do not use bash in there because that will make it only work with bash. It will not work on embedded with busybox without bash, just ash, or any other common desktop shell.

/bin/sh is usually a symlink to the shell interpreter used, so that will eventually work.

BUT!

I think you are thinking a bit too low-level when using a high-level C++/OOP framework such as Qt. I would not recommend to invoke the commands in the low-level way when you run it from bash. There is some dedicated high-level convenience API for this use case.

Based on the official documentation, QProcess is supposed to work for pipe'd commands:

void QProcess::setStandardOutputProcess(QProcess * destination)

Pipes the standard output stream of this process to the destination process' standard input.

In other words, the command1 | command2 shell command command can be achieved in the following way:

QProcess process1;
QProcess process2;

process1.setStandardOutputProcess(&process2);

process1.start("cat file");
process2.start("grep string");
process2.setProcessChannelMode(QProcess::ForwardedChannels);

// Wait for it to start
if(!process1.waitForStarted())
    return 0;

bool retval = false;
QByteArray buffer;
while ((retval = process2.waitForFinished()));
    buffer.append(process2.readAll());

if (!retval) {
    qDebug() << "Process 2 error:" << process2.errorString();
    return 1;
}

qDebug() << "Buffer data" << buffer;

This is not the main point, but a useful suggestion: do not use QString::toAscii(). That API has been deprecated in Qt 5.

I wish if I can use this method, but the problem for me is that it did not work the way the accepted answer did. I'm not piping commands, but simply trying to perform two consecutive commands: gcc *.c; ./a.out argv[1] ... when I use your code with setStandardOutputProcess, I get weird results. — 7kemZmani, Jul 28 '16 at 18:50
Any thoughts on long-running processes? What if process1 is a telnet session and I want process2 to rewrite the output of that process1 telnet session. I would still send commands to process1, but need it to pipe output to process2? Code welcome. — RoundSparrow hilltx, Aug 30 '17 at 02:17

score 4 · Answer 3 · answered Jan 03 '14 at 10:55

4

The problem is that when you call process->start(cmd), the commands following the the call to cat are all interpreted as arguments to cat, so the pipe is not doing what you're expecting. If you start with a call to bash with a parameter of a string, you should get what you want: -

QString cmd = "bash -c \"cat file | grep string\"";

Alternatively, you could just call "cat file" and do the search on the returned QString when you read the output from the QProcess

answered Jan 03 '14 at 10:55

TheDarkKnight

25,660
4
50
79

the command runs fine, however the output i get is bit garbaged...for this command "netstat -i | grep enss33 | awk '{print $3}', it get result as 0x804d2045085 , it should be just 5085... – krisdigitx Jun 20 '14 at 13:29
@krisdigitx, I suspect that may be due to how the awk command is being passed with the ' characters. If you remove the awk and just pipe to grep, does that work as expected? If that works, add the awk, but escape the quotes. – TheDarkKnight Jun 20 '14 at 13:33
I have escaped the single quotes...still the same output... http://fpaste.org/111471/32714861/ – krisdigitx Jun 20 '14 at 13:38
@krisdigitx Using stdout as a variable name is probably a bad idea. This works for me: http://pastebin.com/1jZWDL0C – TheDarkKnight Jun 20 '14 at 13:50
Qdebug works for me also, using "output" variable made no difference with the output using cout also.....so I think the problem is with cout handling the output.... – krisdigitx Jun 20 '14 at 13:58
With Qt, I don't see a reason to use cout. You could have also used fprintf and see if that makes a difference. Glad it's working now ;O) – TheDarkKnight Jun 20 '14 at 14:06
the cout output is actually correct, there something else in the program which is causing this...http://fpaste.org/111490/03274758/ – krisdigitx Jun 20 '14 at 14:33
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/55990/discussion-between-merlin069-and-krisdigitx). – TheDarkKnight Jun 20 '14 at 14:37

score 0 · Answer 4 · answered Aug 09 '15 at 07:08

0

how about this :

QString program = "program";
QStringList arguments;

download = new QProcess(this);
download->start(program, arguments);

answered Aug 09 '15 at 07:08

bulldog68

197
1
10

2

Consider to add more details to make it be better answer than current one. – Victor Polevoy Aug 09 '15 at 10:18

score 0 · Answer 5 · answered Mar 15 '21 at 17:05

If Google brought you here and you are using PyQt5 or PySide2

        process1 = QProcess()
        process2 = QProcess()
        process1.setStandardOutputProcess(process2)
        process1.start(cat, [file])
        process2.start(grep, [string])

Piping (or command chaining) with QProcess

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