"Remote desktop session has ended" error when connecting to a guest on VirtualBox

I had a problem connecting from Windows using build-in Remote Desktop Connection program to a a guest on a headless VirtualBox server. I found that the reason wast that I did not checked "Allow me to save credentials" option in a Remote Desktop Connection.

With this option, the client will ask for username/password before connecting to the VirtualBox. Thus it seems, that VirtualBox requires user credentials at the very beginning of the session. Without this option, connections will fail.

I was using VirtualBox 3.2.4 in a headless mode running on a CentOS 5.5 x86_64.

Example of SSH tunneling

Example:
ssh -N -L 5904:127.0.0.1:5901 username@remoteserverThe above command forwards everything on localhost port 5904 to the port 5901 on remoteserver. This is what I use to connect to a VNC service on port 5901 on a remoteserver at my job from my home computer. Thus, at home, after establishing the above tunnel I can use a vncviewer as follows:vncviewer :4

VirtualBox: start and stop a guest in a headless mode

Lets assume that we have a guest VirtualBox machine called "xp". To start it in a headless mode we can use VBoxHeadless --startvm "xp"To stop it we can use VBoxManage controlvm "xp" savestateSince it is in a headless state we have to access it using a RDP viewer, e.g. rdesktop command in Linux:desktop -u username -p password -a 16 serverIPOf course, before we can use it in this way, we have to enable remote destkop for the "xp" machine and define authentication method. In my case, I used External method, since it uses the linux user account under which my VirtualBox is running.

bash: send a command to a screen session

"Screen is a full-screen window manager that multiplexes a physical terminal between several processes, typically interactive shells." It is a very useful tool, that I often use, as it allows me to run "multiple terminals in one" even when I log out from the system.

So how to send command to screen session from other terminal. In other words how to remote-control a screen session from within a script. This post is useful for this.

In particular, I wanted to execute matlab scripts in screen windows. To do this I used the following:
1. Create screen sessionscreen -S matlab2. Create few windows in the screen session using Ctr-a c
3. Send matlab command to each as followsfor ((i=0;i<$NO_WINDOWS;i+=1)); do
echo $i
sleep 2
screen -S matlab -p $i -X exec matlab -nodesktop -r "bashCalculations('part',$i+1)"
donewhere bashCalculations is my matlab script bashCalculations.m that takes part argument. So,, each screen window executes different part of my script.

Ubuntu 10.04: How to remove chat and mail icons (indicators) from system tray

The 10.04 LTS release of Ubuntu seems to be very, very good. However, for some strange reason, creators of this release made some strange decisions about Ubuntu's look-and-feel. One of the things that annoyed me was the assumption that every Ubuntu user, uses Twitter, Facebook or other social network. Thus, they incorporated the tools for using social networking sites into the distibution. They did it by adding chat and status icons/notifiers into Gnome system try. However, what they did not do, was the ability to easily remove them. For example, when you remove massage notifier (mail icon) using "Remove From Panel" option, the volume control icon also disappears.

Thus you need to remove these icons some other way, which in this case, is to remove two packages: indicator-me and indicator-messages:sudo apt-get remove indicator-me indicator-messagesAfter that just log-out and log-into the gnome session and the icons should be gone:

EDIT
It also should work for Ubuntu 11.04 (Natty Narwhal).

Matlab: Convert numerical array to cell with string representation of numbers

Short Matlab script that takes an array (i.e. vector) of numbers and returns cell with fileds that are string representation of the numbers in the array.For example: >> a=[1 2 3.24 -43 4.56];
>> numarray2cellstring(a)

ans =

'1' '2' '3.24' '-43' '4.56'

imagemagick: crop all images

Example:convert rose: -crop 40x30+10+10 +repage repage.gif

Change font and font size in Fluxbox

In Fluxbox 1.1 a font and a font size can be changed by editing a overlay file, usually located in ~/.fluxbox/. So just add the following lines to this file and restart fluxboxmenu.title.font: sans-12:bold
menu.frame.font: sans-12:bold
toolbar.clock.font: sans-10:bold
toolbar.workspace.font: sans-12:bold
*font: sans-12Of course you have to select the font you like or the size.

bash: split string to array

There are few possible ways of splitting a string with delimiter-separated values to an array in Bash. Here I present five of them.

1. Using tr command

Output:> [123]
> [456]
> [567]
> [5]
> [343]This method will produce incorrect array due to space in "567 5", but it works fine if there are no spaces in the $STR variable.

2. Using IFS (Internal Field Separator) variable

Output:> [123]
> [456]
> [567 5]
> [343]This method works fine even if there are spaces.

3. Using read command

Output:> [123]
> [456]
> [567 5]
> [343] This method also works fine even though there are spaces in the $STR variable.

4. Using sed command

Output:> [123]
> [456]
> [567]
> [5]
> [343]This method will also produce incorrect array due to space in "567 5", but it works fine if there are no spaces in the $STR variable.

5. Using set command

Output:> [123]
> [456]
> [567 5]
> [343] This method also works fine.

References

This post was mainly inspired by this StackOverflow question and my need to perform string 2 array conversions in bash.

bash: read a specific line or/and a column from a file

If we have column-row wise txt files, we can use awk and sed programs to read the rows and columns of such files.

Read specific line

To read a specific line from a txt file, the following commands can be usedawk 'NR==12' file.txt sed -n '12p' file.txt In both cases, line 12 will be returned.

Read specific column

awk '{print $1, $2}' file The code will return first and second columns from file.txt

Read specific column and line

Additionally, using awk we can get a specific column(s) in of a given line as followsawk 'NR==12 {print $1, $2}' file.txtThe Code will return first and second columns form a line no. 12.