Unable to insert the virtual optical disk in an Ubuntu virtual machine

When you try to install a new version of the VirtualBox Guest Addition on an Ubuntu virtual machine you may get the error message

Unable to insert the virtual optical disk C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso into the machine Ubuntu 64.

Could not mount the media/drive ‘C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso’ (VERR_PDM_MEDIA_LOCKED).

Result Code: E_FAIL (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}
Callee: IMachine {85cd948e-a71f-4289-281e-0ca7ad48cd89}

To mount the new CD, first eject the currently mounted virtual CD

  1. Click the CD icon on the left side of the screen
  2. Click the eject icon next of the currently mounted Guest Addition CD

Raw-mode is unavailable courtesy of Hyper-V error in VirtualBox

For VirtualBox to be able to start virtual machines, Hyper-V has to be turned off in Windows 10.

If you get the error message when you try to launch a virtual machine in VirtualBox

Raw-mode is unavailable courtesy of Hyper-V

Check the Hyper-V setting

  1. Open a command prompt as an administrator,
  2. Execute the command:

    The default value of the hypervisorlaunchtype is “Auto”

    For VirtualBox to be able to launch virtual machines, we have to turn off hypervisorlaunchtype

Disable Hyper-V

To disable Hyper-V

  1. Open a command prompt as an administrator
  2. Execute the command
    bcdedit /set hypervisorlaunchtype off
  3. Restart the computer. Do not shut down and start the computer, that does not work.

Enable Hyper-V

Docker for Windows needs the hardware virtualization that Hyper-V provides. To enbale Hyper-V again

  1. Open a command prompt as an administrator
  2. Execute the command
    bcdedit /set hypervisorlaunchtype auto
  3. Restart the computer for the change to take effect.

Set up a user to connect to a Linux server with a private-public key pair

To secure a Linux server, disable password authentication on it. This way only those users can connect to it, who have access to an authorized private key.

To enable users to connect to a Linux server with a private-public key pair

Generate an RSA key pair

  1. In a Bash terminal on your workstation execute
  2. Follow the prompts to specify the name of the key file pair. In most of the cases, you don’t need to protect the key with a password.
    1. If you don’t specify the file name, the key will be saved as ~/.ssh/id_rsa
    2. If you specify a file name, the key files will be saved in the current directory
  3. The public key file will get the “.pub” extension, the private file has no extension

Upload the public key to the Linux server

  1. Log into the server with the “ssh” command using a username and password
  2. Add the public part of the key to the user configuration
    1. Switch to sudo mode, this command will ask for the password again
      sudo -i
    2. Navigate to the user home directory
      cd /home/USER_NAME/
    3. Add the public key to the user’s authorized_keys file. Open the file with a text editor and copy the public key into a new line.
      vi authorized_keys
    4. To test the configuration, on your workstation navigate to the directory where the new key is located, and log into the server with

Turn off password authentication

  1. Make sure you can log in with the new key !!!
  2. Execute the command
    sed -i 's/#PasswordAuthentication yes/PasswordAuthentication no/' /etc/ssh/sshd_config

Error loading table definition of a PostgreSQL table

When a PostgreSQL table name contains uppercase letters there is no known way to get the table definition SQL script.

When we right-click a table name in SQLPro for Postgres the has an uppercase letter and select Script as, CREATE to, New query window

we get the error message

* Error loading table definition.
* ERROR: relation “public.users” does not exist

As the error message tells us, PostgreSQL is looking for the table name with all lowercase letters.

The pg_dump utility acts the same way.


The error message is

pg_dump: no matching tables were found

The same process works if the table name only contains lowercase letters.

Convert an existing PostgreSQL database to all lowercase table and column names

To avoid similar errors caused by upper case letters in table and column names, convert the existing database definition to all lowercase

    1. Backup your database from the command prompt
    2. Open the backup file in a text editor
    3. Add underscores to the table and column names to match the .NET entity framework user identity processes.
      1. The PostgreSQL driver converts the first letter of all lowercase table and column names to upper-case and the letter that follows an underscore. For example, asp_net_users will be converted to AspNetUsers.
        1. Replace the table names with TableName -> table_name ( leave __EFMigrationsHistory as is)
        2. Replace the column names with “ColumnName” -> “column_name”
        3. Replace the primary key names with PK_ -> pk_
        4. Replace the index names with IX_ -> ix_
        5. Replace the foreign key names with FK_ -> fk_
    4. Drop the database. In SQL Pro for Postgres
      1. Right-click the database and select Drop database
    5. Re-create the database. In SQL Pro for Postgres
      1. Rith-click the server name and select Create database
    6. Import the modified backup file from the command prompt
    7. Reverse engineer the database back to the .NET data layer project
      1. Delete the class files that correspond to the database tables
      2. Delete the MY_DATABSEContext.cs file
      3. Import execute the command in the data layer project directory
        dotnet ef dbcontext scaffold --force "Host=MY_DATABASE_ADDRESS;Database=MY_DATABASE_NAME;Username=MY_DATABSE_USER;Password=MY_PASSWORD" Npgsql.EntityFrameworkCore.PostgreSQL

Copy files between Linux machines

The rsync command allows you to copy files using SSH connection, between your workstation and another Linux machine. You have to be logged into one of the machines, this command cannot copy files between two remote machines.

To copy a file from a remote server to your local workstation, execute


To copy a file to a remote server, swap the source and target definitions after the SSH_KEY_NAME option.

Using tmux terminal multiplexer

The tmux terminal multiplexer allows us to open multiple terminal windows in the same SSH session and continue the command execution even when we log out of the SSH session. This way we can execute long-running copy commands overnight without keeping the SSH session open.

Install tmux

On CentOS family Linux

yum install tmux

To start tmux and attach to the last session

tmux a

To keep the current session active

To continue the session execution even when you log out of the server, detach from the session before closing the connection. See the commands below.


To switch to binding mode


Commands in binding mode

d detach
% vertical split
horizontal split
arrows move between panes
c new window
p previous window
n next window
l last used pane
q display pane numbers
[ enable scroll with arrow keys or pgup, pgdown
q quit scroll mode
x close the pane (will ask you to press “y” to confirm)


cmd-shift + zoom in
cmd-shift – zoom out
cmd-shift 0 (zero) 100% zoom

Copy files between Windows and Linux computers

There are many tools to copy files between Windows and Linux computers, I have found this method the simplest.

It does not require any software installation on the Windows machine, and only one package installation on the Linux machine.

Share a folder on the Windows machine

Share a folder on the Windows machine and allow access to it for a user. If the Windows computer is in the Windows domain, the domain user does not have to be a member of any security group on the Windows machine.

If you copy files from Windows to Linux, make the folder read-only for the user. If you copy files to the Windows machine, allow write access to the folder for the user.

Set up the Linux machine

  1. Install the cifs-utils on the Linux machine
    1. On Red Hat, CentOS, and Amazon Linux
      sudo yum install cifs-utils
    1. On Ubuntu
      sudo apt-get install cifs-utils

Mount the shared Windows folder on the Linux machine

  1. On the Linux machine create a directory to mount the Windows folder to
    mkdir /tmp/windows
  2. Mount the Windows share
    sudo mount.cifs '\\WINDOWS_SERVER_IP\attachments' /tmp/windows -o domain=MY_DOMAIN,username=MY_USERNAME,password=MY_PASSWORD,vers=1.0

    First, you will be asked for the root password on the Linux machine.
    If you do not specify your password in the line above, you will be also asked to enter your password on the Windows machine.

Access the Windows share

  1. On the Linux machine navigate to the mount directory
    cd /tmp/windows
  2. List the files of the Windows share
    ls -al


If you get the error message when you issue the mount command

mount error(16): Device or resource busy

try to unmount (umount !) the share first and try the mount again

umount /tmp/windows


delimited by end-of-file (wanted `EOF’)

When you create a file in a script and use an end of file delimiter,

if ...
  cat <<EOF> $chef_dir/attrib.json

and you indent the word EOF to look nicer, you will get the error message

warning: here-document at line … delimited by end-of-file (wanted `EOF’)
STDERR> …: line …: syntax error: unexpected end of file

To correct the script, move the EOF to the beginning of the line

if ...
  cat <<EOF> $chef_dir/attrib.json

org.apache.jasper.JasperException: java.lang.IllegalStateException: No output folder

This error can happen on Linux and Windows, and the cause could be the same.

On Windows Server 2012R2 I opened a command prompt. To be able to delete a directory from the Tomcat webapps folder, I have issued the shutdown command.

When I started the server with the startup command, and navigated to my local website, my Java Spring MVC web application displayed the error message

org.apache.jasper.JasperException: java.lang.IllegalStateException: No output folder

The problem was, that the Tomcat service runs under root on Linux, and SYSTEM on Windows.

When you issue the startup command, Tomcat will run under the user account you logged in with.


Stop and start the TomCat Linux or Windows service instead of issuing the shutdown and startup commands from the terminal or command prompt.