Monday, June 1, 2020

Configure an Ubuntu VirtualBox guest instance for ssh access from the host OS

I use VirtualBox to run guest OS such as Ubuntu 18.04 virtual machines for trying out stuff and experiments to prevent messing up my host operating system, e.g. Ubuntu. For convenience, the guest Ubuntu OS running in the VirtualBox can be accessed remotely from the host OS by creating a network adapter that bridges between the host and the guest. This post describes the configuration needed to allow the host OS to connect to the guest OS remotely through a secured shell (ssh).

The following steps assume an Ubuntu virtual machine has been created in VirtualBox and shutdown.

Create a bridge network adapter
  1. Run VirtualBox.

    The VirtualBox Manager is displayed.

  2. Select the Ubuntu virtual machine e.g. vm1. Click Settings.

    The Settings appear.

  3. Select Network.


  4. Select the Adapter 2 tab. Toggle on the Enable Network Adapter field.


  5. Choose Bridged Adapter in the Attached to field.
  6. In the Name field, choose the network you want the virtual machine to join, e.g. wlp3s0. Click OK.

    The bridged network adapter is created in the virtual machine.
 Identify the virtual machine's IP address on the bridged network
  1. In VirtualBox, run the guest virtual machine, e.g. vm1.
  2. Log in to the guest OS. Open up a Terminal.
  3. Type in the ifconfig command at the prompt:

    A list of network adapters and related information is displayed.


  4. Note down the virtual machine's ip address, e.g. 192.168.0.189.

Install SSH server
If the SSH server is not installed in the guest Ubuntu OS, then do the following steps.
  1. Open a Terminal.
  2. At the prompt, type in the command:

    $ sudo apt-get install openssh-server
  3. At the prompt, type in the command to enable the ssh service.

    $ sudo systemctl enable ssh
  4. Start the ssh service by entering the following command.

    $ sudo systemctl start ssh
Remote access to the guest virtual machine
Now the virtual machine can be accessed from the host operating system. With the virtual machine running, the following commands can be used to access the guest OS.
  1. Open up a Terminal. Type in the following command:

    $ ssh remote_user@192.168.0.189

    Note: where remote_user is the login name for the guest OS and 192.168.0.189 is the IP address of the virtual machine on the host's network.

Monday, May 4, 2020

Gazebo: make an animated box using a separate model file(s)

The Gazebo tutorial at http://gazebosim.org/tutorials?tut=actor&cat=build_robot shows how to make a box actor rotate around the vertical axis using only a world file. I wanted to use create a separate actor model and include it in the world file so I don't have to repeat the code. After fiddling around, I managed to figure out the procedure to do it. This post summarizes the steps.

Create an actor model
  1. In the user's home directory ~/.gazebo/models/, create a model folder, e.g. animated_box

    $ cd ~/.gazebo/models/
    $ mkdir -p animated_box/materials/scripts
    $ mkdir -p animated_box/materials/textures


  2. Change directory into the animated_box folder.

    $ cd animated_box


  3. Using a text editor, create a model.config file. Type in the following:
    <?xml version="1.0" encoding="utf-8"?>
    <model>
     <name>Animated Box</name>
     <version>1.0</version>
     <sdf version="1.4">model.sdf</sdf>
     <description>My animated box</description>
    </model>
    


  4. Using a text editor, create a model.sdf file. Type in the following:

    Note: instead of the model tag, use the actor tag.
    <?xml version="1.0" encoding="utf-8"?>
    <sdf version="1.4">
     <actor name="animated_box">
      <static>false</static>
       <link name="link">
        <visual name="visual">
         <geometry>
          <box>
           <size>0.2 0.2 0.2</size>
          </box>
         </geometry>
        </visual>
       </link>
       <script>
        <loop>true</loop>
        <delay_start>0.0</delay_start>
        <auto_start>true</auto_start>
        <trajectory id="0" type="square">
         <waypoint>
          <time>0.0</time>
          <pose>-1 -1 1 0 0 0</pose>
         </waypoint>
         <waypoint>
          <time>1.0</time>
          <pose>-1 1 1 0 0 0</pose>
         </waypoint>
         <waypoint>
          <time>2.0</time>
          <pose>1 1 1 0 0 0</pose>
         </waypoint>
         <waypoint>
          <time>3.0</time>
          <pose>1 -1 1 0 0 0</pose>
         </waypoint>
         <waypoint>
          <time>4.0</time>
          <pose>-1 -1 1 0 0 0</pose>
         </waypoint>
        </trajectory>
       </script>
    
     </actor>
    </sdf>
    


Create and run the world file
  1.  Using a text editor, create a world file, e.g. my_animated_box.world. Type in the following commands:
    <?xml version="1.0" ?>
    <sdf version="1.3">
     <world name="default">
      <include>
      <uri>model://sun</uri>
      </include>
      <include>
       <uri>model://ground_plane</uri>
      </include>
      <include>
       <uri>model://animated_box</uri>
      </include>
     </world>
    </sdf>
    

    Note: the previously created animated_box model is included as a model.

  2. Run the world file.

    $ gazebo my_animated_box.world

    Gazebo opens and an animated box is shown circling the z axis.

    Monday, April 13, 2020

    Setup to launch ROS nodes on a remote computer on a network

    ROS nodes can be setup to run on a remote computer from a local computer on the same network. However, there are some setup to be done. After reading the tutorials and trying out on my own, the following steps summarized what worked for me.

    Create known hosts
    On the local Linux computer e.g. a Raspberry Pi (local1) do the following:
    1. Open a Terminal.
       
    2. Type in the command.

      $  ssh -oHostKeyAlgorithms='ssh-rsa' remote_user1@remote1

      The prompt appears: Are you sure you want to continue connecting (yes/no)?
    3. Type in yes. Press RETURN.
    4. When prompted, type in the password for the remote1 computer's remote_user1.

      The Terminal is now connected to the computer remote1 and remote_user1 is logged in.

      The remote1 computer name is encrypted with the RSA encryption and stored in the Raspberry Pi's /home/local_user1/.ssh/known_hosts file.

    5. Type in exit.

      The connection to remote1 is closed.
    6. If necessary, repeat the previous steps 2 to 4 for the IP address of the computer remote1.

      $ ssh -oHostKeyAlgorithms='ssh-rsa' user1@192.168.8.101

      Note: where 192.168.8.101 is the IP address for the computer remote1.
    Create SSH public and private keys for authentication
    1. On the local1 computer, open a Terminal.
    2. Type in the command:

      $ ssh-keygen -t rsa

      Enter file in which to save the key (/home/local_user1/.ssh/id_rsa):
    3. Press RETURN.

      Enter passphrase (empty for no passphrase):
    4. Press RETURN.

      Enter same passphrase again:
    5. Press RETURN.

      The private key is generated in /home/local_user1/.ssh/id_rsa.
      The public key is generated in /home/local_user1/.ssh/id_rsa.pub
      .
    Install the public key(s) to the remote computer
    1. On the computer local1, open a Terminal.
    2. Type in the command:

      $ ssh-copy-id remote_user1@remote1
    3. When prompted, type in the password for remote_user1.

      The public keys are installed on computer remote1.
    Create a remote ROS environment shell script file
    The following steps should be executed on the remote computer remote1.
    1. Using a text editor, create a shell script file e.g. /opt/ros/melodic/env_remote1.sh with the following content.

      #!/bin/bash
      
      export ROS_MASTER_URI=http://remote1:11311
      
      source /opt/ros/melodic/setup.bash
      source /home/remote_user1/catkin_ws/devel/setup.bash
      
      exec "$@"
      

    2. Open a Terminal. Make the shell script executable.

      $ sudo chmod a+x env_remote1.sh

    Create and run local launch file
    The following should be done on the local computer local1.
    1. Using a text editor, create a launch file e.g. run_remote.launch.
      <launch>
              <machine
                      name="remote1"
                      address="remote1"
                      env-loader="/opt/ros/melodic/env_remote1.sh"
                      default="true"
                      user="remote_user1"
              />
              <node machine="remote1" pkg="beginner_tutorials" name="hello_doubles" type="hello_doubles" />
      </launch>
      

      Note: this launch file will run the hello_doubles node from the beginner_tutorials package on the remote1 computer.

    2. Open a Terminal. Type in the following command assuming the launch file is in the current directory:

      $ roslaunch remote.launch
      The following messages may appear. Ws06 in this example screenshot is the remote computer.

      Monday, March 16, 2020

      How to run a shell script from a ROS launch file

      I wanted to execute a bash shell script from the ROS launch file but the ROS Wiki were not very clear. After some trial and error, I figured out how to do it. The following steps illustrate the procedure I used:

      Create a shell script
      1. In the ROS workspace package, e.g. /path/to/workspace/package/script/ folder, create a shell script e.g. run_script.sh.
      2. Type in the script commands, e.g. see the code listing below.

        Note 1: ensure the shebang statement is at the top i.e. #!/bin/bash and a exit status code (0 for success or other values) is returned from the script.

        Note 2: Use the chmod command to make the script executable, e.g. $ chmod a+x run_script.sh

      #!/bin/bash
      
      # just print this out
      echo "Hello ROS world"
      
      # exit gracefully by returning a status 
      exit 0
      


      Create a launch file
      1. In the ROS workspace package launch folder, create a launch file e.g. /path/to/workspace/package/launch/hello_script.launch.
      2. Using a text editor, type in the following:

        Note: fill in the package name, e.g. beginner_tutorials, and the type, which should be the shell script name; name is any label you want to associate with the script node.


      <launch>
              <node pkg="beginner_tutorials"
                      type="run_script.sh" name="run_script"
                      output="screen"
              />
      </launch>
      

      Run the launch file
      1. In a terminal, type in the ros launch command:

        $ roslaunch beginner_tutorials hello_script.launch

        Note: change beginner_tutorials to your package name and hello_script.launch to the launch file created previously.

        The script is executed as shown in the print out of "Hello ROS world" below.

      Monday, March 9, 2020

      Using Saga GIS' Terrain Analysis Swath Profile (interactive) function

      Saga GIS has a couple of interactive terrain profiling functions, a single profile and a swath profile. This post shows how to use the interactive swath terrain profile command.

      Load a grid file such as a USGS SRTM file
      1. Run Saga GIS. Select Geoprocessing | File | Grid | Import | Import USGS SRTM Grid.

        The Import USGS SRTM Grid dialog box appears.
      2. Click the Browse button in the Files field. Choose an SRTM file e.g. N21E093.hgt.

      3. Click Open.

      4. Click Okay.

        The SRTM file is loaded and shown in the Data tab.
      Start the swath profile command
      1.  Under the Data tab, mouse right click on the loaded grid file e.g. N21E093.
      2. In the pop up context menu, choose Add to Map.

        The grid file is displayed in a map window.
      3. Select Geoprocessing | Terrain Analysis | Profiles | Swath Profle [Interactive].

        The Swath Profile dialog box appears.
      4. In the Grid system field, choose the loaded grid file's system e.g. 0.000833; 1201x 1201y; 93x 21y option.
      5. In the DEM field, choose the loaded grid file, e.g. N21E093.
      6. Optional. Change the Swath Width if necessary.

        The message Interactive tool execution has been started is displayed in the Messages pane.
      Digitize the swath profile
      1. In the Toolbar, click the Action icon (that looks like a black NW arrow).
      2. In the map window, click a few points to draw the swath profile.


      3. To complete the drawing, press the mouse right button.
      4. To exit the interactive command, select Geoprocessing | Swath Profle [Interactive].

        The Tool Execution prompt appears.
      5. Click Yes.

        The message: "Interactive tool execution has been stopped" is shown in the Messages pane.

        Note: This may take a while as the command will sample the terrain to calculate the points.
      Display the swath profile graphically
      1.  In the Data pane, mouse right click on the newly created profile points e.g. Profile [N21E093].

      2. In the pop up menu, choose Attributes | Diagram.

        The Properties dialog box appear.
      3. In the X Axis Values field, choose D (for Distance).
      4. In the X Axis Label field, choose D.
      5. In the Attributes field, toggle on Z, Z [min], Z[max].
      6. Set other options if necessary.
      7. Click Okay.

        The profile line(s) are displayed.

      Monday, March 2, 2020

      git authentication failed after changing password or installing a new version of git for Windows

      After installing a new version of git for Windows or changing the git repository password, the following git error messages may appear when trying to access the git repository: "...access denied...fatal authentication failed for ..."
       
      This problem can be resolved by resetting the git password in the Windows Credential Manager.

      1. Click the Windows Start button.
      2. Start typing in "Credential Manager". When the Credential Manager icon is displayed, click the icon.

        The Credential Manager dialog box appears.

      3. Click the Windows Credentials icon.
      4. Scroll down the credential list and look for a git generic credential, e.g. git:https://gitlab.com. Click on the credential.

        The git credential properties appear.

      5. Click Remove.



        The git credential is removed.
      6. Close the Credential Manager.

      Now, running any git command to access the repository will cause Windows to pop up a dialog box to enter the login and pasword, as shown below.
      Entering the correct login and password will allow the git command to be executed successfully.


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