CentOS 7 released and checking out release notes

I'm always a big fan and user of CentOS. Started using CentOS 4 and it is very stables and secure for company servers usage. With the current release of CentOS 7, it is definitely worth while to check in out the release note.

It is definitely encourage to see that, for the first time, major upgrade between major CentOS is possible now.

The install media is splited such that it comes with the window managers. For server installation, you should really go using net install.

These are some major changes which should be noticeable. Like Wow!

• Kernel updated to 3.10.0


• Support for Linux Containers


• Open VMware Tools and 3D graphics drivers out of the box


• OpenJDK-7 as default JDK


• In Place Upgrade from 6.5 to 7.0 (as already mentioned)


• Switch to systemd, firewalld and GRUB2


• XFS as default file system


• iSCSI and FCoE in kernel space


• Support for PTPv2


• Support for 40G Ethernet Cards


• Supports installations in UEFI Secure Boot mode on compatible hardware

There are some known issues which you should really consider if you are doing upgrade and make sure you are well prepare.
network - Many people have complained that Ethernet interfaces are not started with the new default NetworkManager tool/have to be explicitly enabled during installation. See CentOS-7 FAQ#2.
installer memory usage - The installer needs at least 406MB of memory to work. On systems with less memory then 406MB the installation will terminate with a fatal error. 512MB is the minimum memory requirement for CentOS-7.
small screen - If your screen resolution is 800x600 or lower, parts of the images shown at the bottom during install are clipped. So watch up on the next,back and cancel buttons.
So I think this is a brief summary to get you started but you can find more at here.

How to prevent a package from upgrading in Debian

Often times, when you are having a server with a package installed, example database package and you want the rest of the packages in the system upgraded but not the database package. How can you do it?

With Debian, you can! You can set a package not to upgrade by setting flag to the package with dpkg. The followings are parameters that you need to be use.

--get-selections [package-name-pattern...]
      Get  list  of  package  selections, and write it to stdout. Without a pattern, non-installed packages (i.e. those which have been previously
      purged) will not be shown.

--set-selections
      Set package selections using file read from stdin. This file should be in the format 'package state', where state is one of  install,  hold,
      deinstall or purge. Blank lines and comment lines beginning with '#' are also permitted.

      The  available file needs to be up-to-date for this command to be useful, otherwise unknown packages will be ignored with a warning. See the
      --update-avail and --merge-avail commands for more information.

Let's check what is the state of a current package. To narrate better, I will use mongodb.

jason@localhost:~$ dpkg --get-selections mongodb-org
mongodb-org					install

If you decided to stop this package from the next upgrade, you should set flag hold to this package.

jason@localhost:~$ echo "mongodb-org hold" | sudo dpkg --set-selections
jason@localhost:~$ $  dpkg --get-selections mongodb-org
mongodb-org					hold

Notice that the package state has transitioned into hold. Okay, let's say we want the package to be upgrade in the next upgrade, then we can set install flag to this package.

jason@localhost:~$ echo "mongodb-org install" | sudo dpkg --set-selections
jason@localhost:~$ dpkg --get-selections mongodb-org
mongodb-org install

So, that's it, simple and quick.

Lightweight Java Game Library

Since childhood, gaming has been one of my favorite activities. If you are from 80s, Supermario should sound familiar to you. =) 30 years had passed, gaming development improve tremendously over the period.

In this article, we are going to explore gaming development. Most of the gaming is written in low level languages, example C and thus, it is very complicated. This certainly introduced steep learning curve if you are a beginner. Hence, we will choose a simple startup to learn about gaming development. A example of library that can be use is Lightweight Java Game Library or its acronym LWJGL.

What is Lightweight Java Game Library?

The Lightweight Java Game Library (LWJGL) is a solution aimed directly at professional and amateur Java programmers alike to enable commercial quality games to be written in Java. LWJGL provides developers access to high performance crossplatform libraries such as OpenGL (Open Graphics Library), OpenCL (Open Computing Language) and OpenAL (Open Audio Library) allowing for state of the art 3D games and 3D sound. Additionally LWJGL provides access to controllers such as Gamepads, Steering wheel and Joysticks. All in a simple and straight forward API.

Because nature of this library deal with graphic display, hence the hardware display driver must be setup correctly. For me, my workstation is using ati radeon, and using xserver-xorg-video-radeon and enable 3D acceleration with package libgl1-mesa-dri. We won't delve deep into graphic driver installation and configuration since our focus here is the gaming development. You can check if your drive is setup properly by running glxgears via a terminal. If a windows popup with three gears spinning, your driver install and setup should be fine to continue for this coding tutorial.

In the official wiki, it is well written and documented to get you started. With this, I have setup my eclipse environment in debian sid. The library needed to should be setup in the project build path so when you run your application, the library is detected. Because I'm running linux, the native library location is pointed to lwjgl-2.9.1/native/linux. These two library must be configured before any development begin. If you noticed, I've setup the source as well, it will be convienient to read the code if you need to be sure later down the road during coding phase.



There are many tutorials to pick from, as a start, I just pick the basics - LWJGL Basics 1 (The Display).  The source code should be in the link, and it is incredibly easy to create the display with few lines of codes and I got that window display with just initial try. Very impressive and promising.



It is pretty impressive what this library can do. There are many examples that come in the library and one of it is an example game. Just execute

java -cp .:res:jar/lwjgl.jar:jar/lwjgl_test.jar:jar/lwjgl_util.jar:jar/jinput.jar: -Djava.library.path=native/linux org.lwjgl.examples.spaceinvaders.Game

if you are running linux. Run fine in my environment and played the bundle game; amazing. Maybe in my next article, I'm gonna try to even complete this .

arch install/remove/search packages

Much like aptitude on debian, archlinux also use the same tools to add and remove packages.

pacman is the packages handler on arch, but as on debian there are more then one tool for the job, pacman is the stock like apt-get is on debian. The commands I list here is the minimum you have to know, to handel packages on archlinux. pacman is a command-line tool.

pacman <options> packag1 pack2

to update: pacman -Syu

to install: pacman -S package1

to remove: pacman -R package1

to search: pacman -Ss pack-x

Always good practice to run pacman -Sy before any install, to be sure you get the latest package

Like on debian all the dependencies are solved by pacman.

This is just the most common used, to get the full information on it, go to the archwiki or man pacman.

Changing default application that start a file

Sometime the default installation of an operating system configuration is not the way you wanted it to be, so with this article, it will show you how to change the application that open a file. Note that this article will work in gnome version 3.4.2 or later.

gui way
This is an easy way if you don't want to dig deep into how is the gnome mechanism file opening work. Just right click on the file, click on Properties. An pop up window associated with the file you selected appeared. On the tab Open With , locate the application you want the file to open with and then select it.

cli way
If you want to dig a little deeper, there are many configuration files located in /usr/share/applications. These files contain information about an application like application name and application exec command to trigger the start of an application. Once you located the application you want a file to be associated to start with, e.g. I would like csv file to be started with libreoffice-calc.desktop , then go to $HOME/.local/share/applications directory, locate another file, mimeapps.list. Open this file with an text editor, under the group [Default Applications] , add an entry like text/csv=libreoffice-calc.desktop. If you want to associated with more files, locate the general mime file at /etc/mime.types, and start to populate your environment with default application.

https://developer.gnome.org/integration-guide/stable/desktop-files.html.en

disk usage via command df

I'm pretty sure all of us have bunch of collections files like documents, audio and video in our computer and what is the simple way to check if the disk space usage is exceed the capacity that physical disk provided? For starter, I'm using a command called df, it cames from the package coreutils if you are using Fedora.

What is df?
df displays the amount of disk space available on the file system containing each file name argument.

Example of usage of df?

$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/vg_super-lv_root
                      51606140   9213992  39770708  19% /
tmpfs                  1977424      2348   1975076   1% /dev/shm
/dev/sda5               495844     68681    401563  15% /boot
/dev/mapper/vg_super-lv_home
                      92792824  60272440  27806708  69% /home


$ df /usr/share/man/man1/df.1.gz
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/vg_super-lv_root
                      51606140   9214012  39770688  19% /

df in techical
- Disk space is shown in 1K blocks by default, unless the environment variable POSIXLY_CORRECT is set, in which case 512-byte blocks are used.
- if the partition is not mounted, it will not shown in the df report.


Based on the example usage shown above, the output shown is not human readable, we have to add additional parameters to the command df to make the report much more readable, I summarize some of the parameters with description which I frequently used but if you want a full list, man df to get an all parameters available to command df.

-h, with this parameter, it output human readable size, such as KiB, MiB

-H, with this parameter, it output human readable size too but use power of 1000 not 1024. You could probably noticed that the hard disk normally use this unit to measure its capacity.

-T, with this parameter, it show additional column called type to shown the type of filesystem it is formatted.

--total, with this parameter, it give you a grand total of all the mounted filesystem in the report.

[debian] installing and removing with the same command

When you need to install a package and remove a package, you can do it with a single command than two separate commands. This can be achieve by appending a suffix to the package name. When the aptitude install command is used, a '-' to the suffix of the package name is to remove the package while an aptitude command remove with a '+' suffix to the package name is to install the package.

# aptitude install package1 package2-

# aptitude remove package1+ package2

what pages in memory context?

When a process uses some memory, the CPU is marking the RAM as used by that process. For efficiency, the CPU allocate RAM by chunks of 4K bytes (it's the default value on many platforms). Those chunks are named pages. Those pages can be swapped to disk, etc.