system() library function is not writing output in a text file

Himanshu Rohilla

Hi All,

I am writing an application that will be used to set the selected WIFI network. The user will enter SSID and its password and on a button click, I am calling a function that will take these 2 parameters. The code snippet for this function is as follows:

QString strCmdWifi = "nmcli dev wifi connect '"+ssid+"' password '"+pwd+"' &> out.txt";

qDebug() << "strCmdWifi" << strCmdWifi; 		// Prints correct command


        int nCmdResponse = system(strCmdWifi.toStdString().c_str());

        if(nCmdResponse == 0)
        {
            QFile file("out.txt");

            if(file.open(QIODevice::ReadOnly | QIODevice::Text))
            {
                QString strVal(file.readAll());
				
				qDebug() << "strVal" << strVal;  // Prints blank

                file.close();

                if(strVal.startsWith("Error",Qt::CaseInsensitive) || strVal.contains("Error",Qt::CaseInsensitive))
                {
                    // Handling error here.
                }
				else
				{
					// Showing successful msg
				}
            }
        }

"ssid" and "pwd" are parameters. I am not getting any error in the out.txt file if I pass the wrong "pwd" value. The application is running with sudo permission. If I run the command ( strCmdWifi variable) on a terminal, it writes an error in a file but when I am trying from the Qt application, it is not writing an error in the file.

JonB

@Himanshu-Rohilla
This seems to be a question about system() and maybe nmcli, not about Qt.

Verify that the redirection works in a much simpler command, e.g. ls -l &> out.txt. Verify some other command which produces an error message (ls -abcdefghijklm &> out.txt) sends it to the file too.

ChrisW67

You are using a relative path the the output file. Verify that the current working directory is where you think it is if you are looking for the file manually.

The error message when you provide an invalid password is almost certainly written to stderr. You are using a bash redirection &> to attempt capturing both stdout and stderr. system() runs the command with sh which may not support that syntax.

You do not need the temporary file at all if you use QProcess.

Your method is fragile. Think what occurs if the password provided by the user contains a single quote (or something more deliberately structured to be malicious).

JonB

@ChrisW67 said in system() library function is not writing output in a text file:

system() runs the command with sh which may not support that syntax

On Ubuntu at least /bin/sh is a link to /bin/bash. But we shall see if the OP tries e.g. the ls -l &> out.txt.

ChrisW67

Curious. On my Ubuntu 22.04 system:

$ ls -l /bin/sh /bin/dash
-rwxr-xr-x 1 root root 125688 Mar 23  2022 /bin/dash
lrwxrwxrwx 1 root root      4 Mar 23  2022 /bin/sh -> dash

dash is minimal, POSIX-compliant sh replacement.

JonB

@ChrisW67
You're doubtless correct, sorry. I didn't fire my Ubuntu up, I took it from man sh in Google -> https://linux.die.net/man/1/sh. Still waiting to hear back from the OP as to their findings :)

Kent-Dorfman

system() is the naive OS interface mecahnism. It will stall the calling program while executing. an async QProcess is a better mechanism if you must call a subshell...The sophisitcated Linux way is to use the dbus system to talk directly to NetworkManager.