qt run shell commands via qprocess

Question

I am developing a small QT application to interact with the terminal, to send commands to the terminal and to read back information printed out.

Example: get output of all processes using ps -aux

PROBLEM

I am able to write information out to the terminal but I dont think it is in the system scope, actual example:

Command passed to shell interpreter : "echo "pre"; ps -aux; echo "post"

edit from comment:

I need to send specific complete commands, I am not looking for shortened or alternative commands, I require a method of sending a command like this : ps -aux | grep chrome | tr -s " " | cut -d " " -f 2 and reading its output. This example is getting all pid's of all running chrome processes

Interpreters attempted :

sh
/bin/bash

Code:

QProcess *proc_ovpn = new QProcess(this);
proc_ovpn->waitForFinished();
proc_ovpn->start("sh",QStringList() << "-c" << "echo \"pre\";ps -aux; echo \"post\"");
proc_ovpn->setProcessChannelMode(QProcess::MergedChannels);
QString str(proc_ovpn->readAllStandardOutput());
return str;                             <<< ======= //breakpoint here

Debug information:

When breakpoint is reached, debug information as follows:

Locals      
    str ""  QString
    this    @0x555555ad7be0 Interface
Inspector       
Expressions     
Return Value        
Tooltip     
    10000000    10000000    int

It was suggested to run shell code using this method above from a post on SO, could not find it again.

I am at a loss, I do not understand why running these commands to not interact directly with the system (and its information),

Any advice?

Do you need a shell for ps -aux? Why not just use ps as the command and -aux as the argument? — Kevin Krammer
– Kevin Krammer, Commented Nov 3, 2016 at 17:09
@KevinKrammer I need to send complete shell commands and read output, as the one described above, another example is ps -aux | grep chrome | tr -s " " | cut -d " " -f 2, this will retrieve all the pid's of all chrome processes. I need to send commands like the one I mentioned and retrieve its output. — CybeX
– CybeX, Commented Nov 3, 2016 at 17:27
Wouldn't it be easier to just call the ps command and then read and process its output? — Kevin Krammer
– Kevin Krammer, Commented Nov 3, 2016 at 17:30
@KevinKrammer you seem to be persistent on the idea of an alternative for a specific command. I need to send ANY command to the shell and read the response. How will I process this then ifconfig | grep tun0 > /dev/null;r=$?;echo $r. It is exactly the same as the previous. Do you understand now? — CybeX
– CybeX, Commented Nov 3, 2016 at 17:43
"any command" as in the user types those? Why would the user not just use a terminal then? — Kevin Krammer
– Kevin Krammer, Commented Nov 4, 2016 at 14:00

HazemGomaa · Accepted Answer · 2016-11-03 17:47:52Z

3

you need to use waitForFinished() after start, not before

proc_ovpn->start("sh",QStringList() << "-c" << "echo \"pre\";ps -aux; echo \"post\"");
proc_ovpn->waitForFinished();

Note that waitForFinished() blocks until the process (that has been invoked by start) has finished ...

Also, you may check if the process is started successfully and/or if waitForFinished timed out

proc_ovpn->start("sh",QStringList() << "-c" << "echo \"pre\";ps -aux; echo \"post\"");

if(!proc_ovpn->waitForStarted()) //default wait time 30 sec
    qWarning() << " cannot start process ";

int waitTime = 60000 ; //60 sec
if (!proc_ovpn->waitForFinished(waitTime))
         qWarning() << "timeout .. ";

edited Nov 3, 2016 at 17:47

answered Nov 3, 2016 at 17:31

HazemGomaa

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2 Comments

CybeX Over a year ago

Quickly read up on the waitForFinished(), it makes sense to have it before, why does it need to be after?

HazemGomaa Over a year ago

@KGCybeX you welcome ! .. actually it makes sense to 1-start 2-wait to start 3-wait to finish ..

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