Run a command with different result between in QProcess and in terminal

Mihan

Hi
I find some special commands run in QProcess will get different result (different from in terminal ).
e.g. I use mplayer -af volume=-20 xxx.wav > /dev/null .
In terminal it just play the audio with no debug.
But in QProcess it will run like mplayer -af volume=-20 xxx.wav , mplayer -af volume=-20 > ,mplayer -af volume=-20 /dev/null.

e.g. sed -i \'s/iface eth0 inet static/iface eth0 inet dhcp/g\' /etc/network/interfaces
in terminal it works well.
in QProcess, it returns:
sed: -e expression #1, char 1: unknown command: `''

What should I pay attention to?

Regards
Mihan

jsulm

@Mihan said in Run a command with different result between in QProcess and in terminal:

But in QProcess it will run like mplayer -af volume=-20 xxx.wav , mplayer -af volume=-20 > ,mplayer -af volume=-20 /dev/null

Please show how you're doing it in code.

"What should I pay attention to?" - back slash (\) needs to be escaped as \\

Mihan

@jsulm
Sure, Here is the code

void PlayInMplayer(QString file, int volume)
{
    QString order = QString("mplayer -af volume=%1 %2 > /dev/null")
                           .arg(QString::number(volume))
                           .arg(file);
    QProcess::execute(order);
}

JonB

@Mihan
You cannot have something like a redirection symbol, >, in a command you try to execute directly. Only the shell interprets that. So to do the command as shown you must go e.g.:

QProcess::execute("/bin/bash", QStringList() << "-c" << order);

In the case of this particular command, you might be better doing the output redirection via QProcess::setStandardOutputFile() instead of passing > /dev/null.

Similarly for your other question where quoting is involved. ' ... ' is dealt with by the shell. Either use bash -c on the command line, or pass your items as separate arguments (without quoting) in the QStringList arguments to execute().

Mihan

@JonB said in Run a command with different result between in QProcess and in terminal:

You cannot have something like a redirection symbol, >

Thank you so much , That's what I need!

JonB

@Mihan
The list of symbols interpreted by the Linux shell, which you therefore cannot pass directly to execute without going through bash -c or thinking through, includes but is not limited to:

< > | & ; ' " $ * [ ( ~ # \ <space>

So kind of most symbols!

Mihan

@JonB
By the way, does system() interpret symbols by the linux shell? I used system() to process commands before.

JonB

@Mihan
It's not that system() does any interpretation, it's that system(command) precisely does /bin/sh -c command, technically

execl("/bin/sh", "sh", "-c", command, (char *) 0);

Therefore to do the equivalent from QProcess you would go:

QProcess::execute("/bin/sh", QStringList() << "-c" << command)

Mihan

@JonB said in Run a command with different result between in QProcess and in terminal:

execl("/bin/sh", "sh", "-c", command, (char *) 0);

Yep, I also find the same in man system.

But I can't find the explaination about this in wiki, so I don't pay attention to it so that I make a mistake.
I think it should be added in wiki.

JonB

@Mihan
In the Qt wiki? I'm afraid that's not its job. There are millions of OS-specific issues, it just covers Qt stuff and does not get involved in those.

Mihan

@JonB Ok, so maybe I make a great topic for linux Qt's user :).

Pablo J. Rogina

@Mihan said in Run a command with different result between in QProcess and in terminal:

I think it should be added in wiki.

You're more than welcomed to do that!