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Qt, implementing an SFTP client

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  • SPlattenS SPlatten

    @JonB , the sftp I'm using in present in the linux distribution on the device.

    JonBJ Online
    JonBJ Online
    JonB
    wrote on last edited by JonB
    #11

    @splatten
    OK, well I don't know about that! What about man sftp (host, port, user at least on command line)? On, say, Ubuntu I see:

     The final usage format allows for automated sessions using the -b option.
    In such cases, it is necessary to configure non-interactive authentica‐
    tion to obviate the need to enter a password at connection time (see
    sshd(8) and ssh-keygen(1) for details).
    

    Anyway, you use QProcess methods like https://doc.qt.io/qt-5/qprocess.html#readAllStandardOutput or (better) https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput signal to read from/write to the subprocess. You'll want to deal with all of stdin, stdout, & stderr, plus error signals, for proper control of your sub-process.

    1 Reply Last reply
    2
    • SPlattenS Offline
      SPlattenS Offline
      SPlatten
      wrote on last edited by SPlatten
      #12

      Here is my sample code:

      QProcess* pProc = new QProcess(this);
      QString strHost = clsMainWnd::strGetHostName()
             ,strPort = clsMainWnd::strGetPort()
             ,strUsername = clsMainWnd::strGetUsername();
      QStringList slstCmdArgs;
      slstCmdArgs << (strUsername + QString("@") + strHost)
                  << (QString("-P") + strPort);
      pProc->start("sftp", slstCmdArgs);
      while( pProc->waitForStarted() != true ) {
      }
      QByteArray bytaryIn = pProc->readAllStandardOutput();
      qDebug() << bytaryIn;
      

      I've grabbed the content of slstCmdArgs and tried it in a terminal and it works, but in code I never see anything in "bytaryIn".

      An example of what the code above does:

          sftp User@HostName -P22
      

      What I am expecting to see is:

          User@HostName's password:
      

      [Edit] I've now re-written this code to:

      QString strHost = clsMainWnd::strGetHostName()
             ,strPort = clsMainWnd::strGetPort()
             ,strUsername = clsMainWnd::strGetUsername();
      QStringList slstCmdArgs;
      slstCmdArgs << (strUsername + QString("@") + strHost)
                  << (QString("-P") + strPort);
      mpSFTP = new QProcess(this);
      connect(mpSFTP, SIGNAL(readyReadStandardOutput())
                     ,this, SLOT(readSFTPoutput()));
      mpSFTP->start("sftp", slstCmdArgs);
      

      Kind Regards,
      Sy

      jsulmJ JonBJ 2 Replies Last reply
      0
      • SPlattenS SPlatten

        Here is my sample code:

        QProcess* pProc = new QProcess(this);
        QString strHost = clsMainWnd::strGetHostName()
               ,strPort = clsMainWnd::strGetPort()
               ,strUsername = clsMainWnd::strGetUsername();
        QStringList slstCmdArgs;
        slstCmdArgs << (strUsername + QString("@") + strHost)
                    << (QString("-P") + strPort);
        pProc->start("sftp", slstCmdArgs);
        while( pProc->waitForStarted() != true ) {
        }
        QByteArray bytaryIn = pProc->readAllStandardOutput();
        qDebug() << bytaryIn;
        

        I've grabbed the content of slstCmdArgs and tried it in a terminal and it works, but in code I never see anything in "bytaryIn".

        An example of what the code above does:

            sftp User@HostName -P22
        

        What I am expecting to see is:

            User@HostName's password:
        

        [Edit] I've now re-written this code to:

        QString strHost = clsMainWnd::strGetHostName()
               ,strPort = clsMainWnd::strGetPort()
               ,strUsername = clsMainWnd::strGetUsername();
        QStringList slstCmdArgs;
        slstCmdArgs << (strUsername + QString("@") + strHost)
                    << (QString("-P") + strPort);
        mpSFTP = new QProcess(this);
        connect(mpSFTP, SIGNAL(readyReadStandardOutput())
                       ,this, SLOT(readSFTPoutput()));
        mpSFTP->start("sftp", slstCmdArgs);
        
        jsulmJ Offline
        jsulmJ Offline
        jsulm
        Lifetime Qt Champion
        wrote on last edited by
        #13

        @splatten said in Qt, implementing an SFTP client:

        QByteArray bytaryIn = pProc->readAllStandardOutput();

        Don't do it like this - you don't know whether there is already anything to read. Use https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput signal instead.

        https://forum.qt.io/topic/113070/qt-code-of-conduct

        1 Reply Last reply
        4
        • SPlattenS Offline
          SPlattenS Offline
          SPlatten
          wrote on last edited by
          #14

          @jsulm, having re-written the code, my slot never gets called, meaning the signal can't be emitted.

          Kind Regards,
          Sy

          1 Reply Last reply
          0
          • SPlattenS SPlatten

            Here is my sample code:

            QProcess* pProc = new QProcess(this);
            QString strHost = clsMainWnd::strGetHostName()
                   ,strPort = clsMainWnd::strGetPort()
                   ,strUsername = clsMainWnd::strGetUsername();
            QStringList slstCmdArgs;
            slstCmdArgs << (strUsername + QString("@") + strHost)
                        << (QString("-P") + strPort);
            pProc->start("sftp", slstCmdArgs);
            while( pProc->waitForStarted() != true ) {
            }
            QByteArray bytaryIn = pProc->readAllStandardOutput();
            qDebug() << bytaryIn;
            

            I've grabbed the content of slstCmdArgs and tried it in a terminal and it works, but in code I never see anything in "bytaryIn".

            An example of what the code above does:

                sftp User@HostName -P22
            

            What I am expecting to see is:

                User@HostName's password:
            

            [Edit] I've now re-written this code to:

            QString strHost = clsMainWnd::strGetHostName()
                   ,strPort = clsMainWnd::strGetPort()
                   ,strUsername = clsMainWnd::strGetUsername();
            QStringList slstCmdArgs;
            slstCmdArgs << (strUsername + QString("@") + strHost)
                        << (QString("-P") + strPort);
            mpSFTP = new QProcess(this);
            connect(mpSFTP, SIGNAL(readyReadStandardOutput())
                           ,this, SLOT(readSFTPoutput()));
            mpSFTP->start("sftp", slstCmdArgs);
            
            JonBJ Online
            JonBJ Online
            JonB
            wrote on last edited by JonB
            #15

            @splatten
            Which is why I said to use https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput for better/proper behaviour....

            There is a wrinkle you may have to face. It is possible that your command-line sftp executable will not prompt for password if it detects that the user is not in an interactive terminal (stdin). E.g. its code may quite possibly have

            if (isatty(0))
                prompt_for_password();
            else
               // do not prompt for password
            

            That will be problematic if it is the case. You could try in a terminal sftp < /dev/null or sftp < file: if that does not still prompt for password you have a (potential) problem....

            1 Reply Last reply
            1
            • SPlattenS Offline
              SPlattenS Offline
              SPlatten
              wrote on last edited by
              #16

              @JonB , would this help:

                  mpSFTP->setProcessChannelMode(QProcess::MergedChannels);
              

              Kind Regards,
              Sy

              JonBJ 1 Reply Last reply
              0
              • SPlattenS SPlatten

                @JonB , would this help:

                    mpSFTP->setProcessChannelMode(QProcess::MergedChannels);
                
                JonBJ Online
                JonBJ Online
                JonB
                wrote on last edited by JonB
                #17

                @splatten
                Help what, in what way? If the sftp requires a terminal stdin for password prompt, then no, if, say, you have a purely interactive Qt application. You can try it. But the first thing to test is whether your sftp behaves differently if its stdin is not attached to a terminal?

                1 Reply Last reply
                1
                • SPlattenS Offline
                  SPlattenS Offline
                  SPlatten
                  wrote on last edited by
                  #18

                  Now the slot gets called and in my slot I have:

                      qDebug() << "readAllStandardOutput:";
                      qDebug() << mpSFTP->readAllStandardOutput();
                      qDebug() << "readChannel:";
                      qDebug() << mpSFTP->readChannel();
                  

                  In the output I see:

                      readAllStandardOutput:
                      "ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory
                      "
                      readChannel:
                      0
                  

                  Kind Regards,
                  Sy

                  jsulmJ JonBJ 2 Replies Last reply
                  0
                  • SPlattenS SPlatten

                    Now the slot gets called and in my slot I have:

                        qDebug() << "readAllStandardOutput:";
                        qDebug() << mpSFTP->readAllStandardOutput();
                        qDebug() << "readChannel:";
                        qDebug() << mpSFTP->readChannel();
                    

                    In the output I see:

                        readAllStandardOutput:
                        "ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory
                        "
                        readChannel:
                        0
                    
                    jsulmJ Offline
                    jsulmJ Offline
                    jsulm
                    Lifetime Qt Champion
                    wrote on last edited by jsulm
                    #19

                    @splatten What happens if you execute exact same command manually in a terminal?
                    Also connect a slot to the error signal from QProcess and check whether you get any errors.

                    https://forum.qt.io/topic/113070/qt-code-of-conduct

                    1 Reply Last reply
                    0
                    • SPlattenS Offline
                      SPlattenS Offline
                      SPlatten
                      wrote on last edited by SPlatten
                      #20

                      @jsulm , When I execute "sftp" with the command line arguments, it works without any issues.

                      Kind Regards,
                      Sy

                      1 Reply Last reply
                      0
                      • SPlattenS SPlatten

                        Now the slot gets called and in my slot I have:

                            qDebug() << "readAllStandardOutput:";
                            qDebug() << mpSFTP->readAllStandardOutput();
                            qDebug() << "readChannel:";
                            qDebug() << mpSFTP->readChannel();
                        

                        In the output I see:

                            readAllStandardOutput:
                            "ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory
                            "
                            readChannel:
                            0
                        
                        JonBJ Online
                        JonBJ Online
                        JonB
                        wrote on last edited by JonB
                        #21

                        @splatten
                        As I have said before, so last time now:

                        ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory

                        This is because your sftp is not willing/able to access stdin to get a password. Try what I said about seeing how your sftp behaves with input/output redirected when run from a terminal. I will not comment again, and leave it to you.

                        1 Reply Last reply
                        0
                        • SPlattenS Offline
                          SPlattenS Offline
                          SPlatten
                          wrote on last edited by
                          #22

                          ssh_askpass is not available on this linux distribution.

                          Kind Regards,
                          Sy

                          JonBJ 1 Reply Last reply
                          0
                          • SPlattenS SPlatten

                            ssh_askpass is not available on this linux distribution.

                            JonBJ Online
                            JonBJ Online
                            JonB
                            wrote on last edited by
                            #23

                            @splatten
                            Yes, so you can't use it.

                            1 Reply Last reply
                            0
                            • SPlattenS Offline
                              SPlattenS Offline
                              SPlatten
                              wrote on last edited by
                              #24

                              @JonB, I've tried everything you have suggested so far and not been able to make any progress due to either incorrect syntax or missing commands.

                              Kind Regards,
                              Sy

                              JonBJ 1 Reply Last reply
                              0
                              • SPlattenS SPlatten

                                @JonB, I've tried everything you have suggested so far and not been able to make any progress due to either incorrect syntax or missing commands.

                                JonBJ Online
                                JonBJ Online
                                JonB
                                wrote on last edited by JonB
                                #25

                                @splatten
                                I am suggesting your sftp client will not work to prompt for a password if you do not run it from a terminal. It will not work when you try redirection from a parent process like Qt.

                                The man page for my sftp under Ubuntu at least is pretty explicit about requiring either an "interactive" session for authentication, or must use ssh facilities for non-password authentication. Does your system have man sftp for you to read through?

                                1 Reply Last reply
                                1

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