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Showing posts from 2020

A New Era of Spaceflight is Set to Begin | NASA

On May 27, NASA will launch their first flight with astronauts to the International Space Station from U.S. soil in nearly 10 years. This mission is the first launch of American astronauts aboard an American rocket and spacecraft as part of their Commercial Crew Program.
NASA astronauts Bob Behnken and Doug Hurley will fly on SpaceX’s Crew Dragon spacecraft, lifting off on a Falcon 9 rocket from Kennedy Space Center in Florida, for an extended stay at the space station for the Demo-2 mission. The specific duration of the mission is to be determined.
As the final flight test for SpaceX, this mission will validate the company’s crew transportation system, including the launch pad, rocket, spacecraft, and operational capabilities. This also will be the first time NASA astronauts will test the spacecraft systems in orbit.
Behnken and Hurley were among the first astronauts to begin working and training on SpaceX’s next-generation human space vehicle and were selected for their extensive test …

Mathematics for Computer Scientists | Integers

NumbersDefendit numerus: There is safety in numbers
We begin by talking about numbers. This may seen rather elementary but is does set the scene and introduce a lot of notation. In addition much of what follows is important in computing. Integers I will assume you are familiar with the integers. 1,2,3,4,. . .,101,102, . . . , n, . . . , 2^32582657 − 1, . . . sometime called the whole numbers. These are just the numbers we use for count- ing. To these integers we add the zero, 0, defined as: 0 + any integer n = 0 + n = n + 0 = n Once we have the integers and zero mathematicians create negative integers by defining (−n) as: the number which when added to n gives zero, so n + (−n) = (−n) + n = 0. Eventually we get fed up with writing n+(−n) = 0 and write this as n−n = 0. We have now got the positive and negative integers {. . . , −3, −2, −1, 0, 1, 2, 3, 4, . . .} You are probably used to arithmetic with integers which follows simple rules. To be on the safe side we itemize them, so for i…


Create a new file called with the following content and give it executable permissions with chmod +x 

Execute/Run via: ./

#!/usr/bin/env bash
# Note that spaces cannot be used around the `=` assignment operator
# Use printf to safely output the data
printf "Hello, %s\n" "$whom_variable"

#> Hello, World

This will print Hello, World to standard output when executed.
To tell bash where the script is you need to be very specific, by pointing it to the containing directory, normally with ./ if it is your working directory, where . is an alias to the current directory. If you do not specify the directory, bash tries to locate the script in one of the directories contained in the $PATH environment variable.

The following code accepts an argument $1 , which is the first command line argument, and outputs it in a formatted string, following Hello, .
Execute/Run via: ./ World

#!/usr/bin/env bash
printf "Hello, %s\n…


GNU Bash or simply Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell. First released in 1989, it has been used widely as the default login shell for most Linux distributions and Apple's macOS Mojave and earlier versionsInteractive ShellThe Bash shell is commonly used interactively: 
It lets you enter and edit commands, then executes them when you press the Return key. Many Unix-based and Unix-like operating systems use Bash as their default shell (notably Linux and macOS). The terminal automatically enters an interactive Bash shell process on startup.
Output Hello World by typing the following:

echo "Hello World"
#> Hello World # Output Example

Non-Interactive ShellThe Bash shell can also be run non-interactively from a script, making the shell require no human interaction. Interactive behavior and scripted behavior should be identical – an important design consideration of Unix V7 Bourne s…


File Compression with 'tar' command:Common Options -c --create 
Create a new archive.

-x --extract     
Extract files from an archive.

-t --list
List the contents of an archive.

-f --file=ARCHIVE
Use archive file or dir ARCHIVE.

-v --verbose 
Verbosely list files processed.
Compression Options - -a --auto-compress
Use archive suffix to determine the compression program.

-j --bzip2
Filter the archive through bzip2.

-J --xz --lzma 
Filter the archive through xz.

-z --gzip   
Filter the archive through gzip.
Compress a folderThis creates a simple archive of a folder : tar -cf ./my-archive.tar ./my-folder/
Verbose output shows which files and directories are added to the archive, use the -v option: tar -cvf ./my-archive.tar ./my-folder/
For archiving a folder compressed 'gzip', you have to use the -z option : tar -czf ./my-archive.tar.gz ./my-folder/
You can instead compress the archive with 'bzip2', by using the -j option: tar -cjf ./my-archive.tar.bz2 ./my-folder/
Or compress with 'xz&…


Database servers are used mainly to store databases and keep data into it in collaboration
with application or web application servers.

The most famous database servers are:
MySQL and Oracle DB 12c

Let us go ahead and install each one of these and we will start with:
To install MySQL server, you will need to run the following command: sudo apt-get install mysql-server

Some other people install maria db package and this will be done using the following command: sudo apt-get install maria-db
Then for security reasons, you may run the following command to get your server up and running securely: mysql_secure_install
The last thing, we need to install is oracle 12c database server express edition. INSTALLING AND CONFIGURING ORACLE DATABASE SERVER12C EXPRESS EDITION
Oracle Database Server 12c Express Edition is one of the mostly highly ranked database server in 2017 and it acts exactly as the data container that holds applications or web applications data.
Download the…


Web server is a kind of a container that contains a web application programmed by software
and web application programmer and the web application performs all of its functionality
using the web server.
First, let us get to install glassfish server as one of the most popular web application servers for java programming language as follows:
sudo apt-get update
sudo apt-get install default-jdk
glassfish4/bin/asadmin start-domain
Configuring Glassfish Server:
1. Open the URL http://localhost:4848 2. Then browse to the left side panel and select server-config 3. After that, go to Network Listeners, then 4. Set http-listener-1 to port 80 for default http listening
After that, we will move on to the next installation which is:
Apache web server is used as a stack bundled with PHP, and MySQL and PHP is the programming language …


Dealing with networking issues can be a very challenging task. A packet sent from a client to a server (or vice versa) might be dropped in various locations and for various reasons. Some of the most common cases are -
1. A firewall that is blocking the packet. It can be a firewall on the server side, on the client side or any firewall along the way between the server and the client. The blocking may be based on an ip (source and/or destination), ports (in case of tcp/udp) or any other layer 3,4 criteria.

2. Lack of layer 2 connectivity.

3. Routing configurations that are not set properly.

Fortunately, Linux command line tools supply significant information that might help us narrow down the problem. From my experience those are my top 8 tools that will help you overcome networking issues.
This is the most fundamental command when troubleshooting a networking issue. The ping command will test the network connectivity between your host and the server. This is definitely the first comman…

Magic SysRq key | Linux Rescue | Frozen Linux | LINUX SERIES

The magic SysRq key is a key combination which allows the user to perform various low-level commands regardless of the system's state. If your Linux machine freezes or you want to reboot your Linux machine without corrupting the filesystem you can use this magic SysRq key combination. This key combination provides access to powerful features and thus needs some knowledge to use it. Please do not proceed without knowing about these combinations as you may put your system in a critical state. Please note that this magic SysRq key cannot work under certain conditions, such as a kernel panic or a hardware failure preventing the kernel from running properly.
Commads:The key combination consists of Alt+SysRq and another key, which controls the command issued. SysRq may be released before pressing the command key, as long as Alt remains held down.
These combinations always assume the QWERTY keyboard layout.
1. Immediately reboot the system, without unmounting or syncing filesystems - b 2. Perfor…