A port is a logical entity which represents an endpoint of communication and is associated with a given process or service in an operating system. In previous articles, we explained how to find out list of all open ports in Linux and how to check if remote ports are reachable using ‘nc’ command.

In this short guide, we will show different ways of finding the process/service listening on a particular port in Linux.

1. Using netstat Command

netstat (network statistics) command is used to display information concerning network connections, routing tables, interface stats and beyond. It is available on all Unix-like operating systems including Linux and also on Windows OS.

In case you do not have it installed by default, use the following command to install it.

$ sudo yum install net-tools	#RHEL/CentOS 
$ sudo apt install net-tools	#Debian/Ubuntu
$ sudo dnf install net-tools	#Fedora 22+

Once installed, you can use it with grep command to find the process or service listening on a particular port in Linux as follows (specify the port).

$ netstat -ltnp | grep -w ':80' 

In the above command, the flags.

• l – tells netstat to only show listening sockets.
• t – tells it to display tcp connections.
• n – instructs it show numerical addresses.
• p – enables showing of the process ID and the process name.
• grep -w – shows matching of exact string (:80).

2. Using lsof Command

lsof command (LiSt Open Files) is used to list all open files on a Linux system. To install it on your system, type the command below.

$ sudo yum install lsof	        #RHEL/CentOS 
$ sudo apt install lsof		#Debian/Ubuntu
$ sudo dnf install lsof		#Fedora 22+

To find the process/service listening on a particular port, type (specify the port).

$ lsof -i :80

3. Using fuser Command

fuser command shows the PIDs of processes using the specified files or file systems in Linux.

You can install it as follows:

$ sudo yum install psmisc	#RHEL/CentOS 
$ sudo apt install psmisc	#Debian/Ubuntu
$ sudo dnf install psmisc	#Fedora 22+

You can find the process/service listening on a particular port by running the command below (specify the port).

$ fuser 80/tcp

Then find the process name using PID number with the ps command like so.

$ ps -p 2053 -o comm=
$ ps -p 2381 -o comm=

You can also check out these useful guides about processes in Linux.

1. All You Need To Know About Processes in Linux [Comprehensive Guide]
2. Limit CPU Usage of a Process in Linux with CPULimit Tool
3. How to Find and Kill Running Processes in Linux
4. Find Top Running Processes by Highest Memory and CPU Usage in Linux

That’s all! Do you know of any other ways of finding the process/service listening on a particular port in Linux, let us know via the comment form below.

Source

There are several ways of knowing the version of Linux you are running on your machine as well as your distribution name and kernel version plus some extra information that you may probably want to have in mind or at your fingertips.

Therefore, in this simple yet important guide for new Linux users, I will show you how to do just that. Doing this may seem to be relatively easy task, however, having a good knowledge of your system is always a recommended practice for a good number of reasons including installing and running the appropriate packages for your Linux version, for easy reporting of bugs coupled with many more.

Suggested Read: 5 Ways to Find Out Linux System is 32-bit or 64-bit

With that said, let us proceed to how you can figure out information about your Linux distribution.

Find Out Linux Kernel Version

We will use uname command, which is used to print your Linux system information such as kernel version and release name, network hostname, machine hardware name, processor architecture, hardware platform and the operating system.

To find out which version of Linux kernel you are running, type:

$ uname -or

In the preceding command, the option -o prints operating system name and -r prints the kernel release version.

You can also use -a option with uname command to print all system information as shown:

$ uname -a

Next, we will use /proc file system, that stores information about processes and other system information, it’s mapped to /proc and mounted at boot time.

Simply type the command below to display some of your system information including the Linux kernel version:

$ cat /proc/version

From the image above, you have the following information:

1. Version of the Linux (kernel) you are running: Linux version 4.5.5-300.fc24.x86_64
2. Name of the user who compiled your kernel: mockbuild@bkernel01.phx2.fedoraproject.org
3. Version of the GCC compiler used for building the kernel: gcc version 6.1.1 20160510
4. Type of the kernel: #1 SMP (Symmetric MultiProcessing kernel) it supports systems with multiple CPUs or multiple CPU cores.
5. Date and time when the kernel was built: Thu May 19 13:05:32 UTC 2016

Find Out Linux Distribution Name and Release Version

The best way to determine a Linux distribution name and release version information is using cat /etc/os-release command, which works on almost all Linux system.

---------- On Red Hat Linux ---------- 
$ cat /etc/redhat-release

---------- On CentOS Linux ---------- 
$ cat /etc/centos-release

---------- On Fedora Linux ---------- 
$ cat /etc/fedora-release

---------- On Debian Linux ---------- 
$ cat /etc/debian_version

---------- On Ubuntu and Linux Mint ---------- 
$ cat /etc/lsb-release

---------- On Gentoo Linux ---------- 
$ cat /etc/gentoo-release

---------- On SuSE Linux ---------- 
$ cat /etc/SuSE-release

In this article, we walked through a brief and simple guide intended to help new Linux user find out the Linux version they are running and also get to know their Linux distribution name and version from the shell prompt.

Perhaps it can also be useful to advanced users on one or two occasions. Lastly, to reach us for any assistance or suggestions you wish to offer, make use of the feedback form below.

Source

In this article, we will learn how to install, update, remove, find packages, manage packages and repositories on Linux systems using YUM (Yellowdog Updater Modified) tool developed by RedHat. The example commands shown in this article are practically tested on our CentOS 6.3 server, you can use these material for study purpose, certifications or just to explore ways to install new packages and keep your system up-to-date. The basic requirement of this article is, you must have a basic understanding of commands and a working Linux operating system, where you can explore and practice all the commands listed below.

What is YUM?

YUM (Yellowdog Updater Modified) is an open source command-line as well as graphical based package management tool for RPM (RedHat Package Manager) based Linux systems. It allows users and system administrator to easily install, update, remove or search software packages on a systems. It was developed and released by Seth Vidal under GPL (General Public License) as an open source, means anyone can allowed to download and access the code to fix bugs and develop customized packages. YUM uses numerous third party repositories to install packages automatically by resolving their dependencies issues.

1. Install a Package with YUM

To install a package called Firefox 14, just run the below command it will automatically find and install all required dependencies for Firefox.

# yum install firefox
Loaded plugins: fastestmirror
Dependencies Resolved

================================================================================================
 Package                    Arch        Version                    Repository            Size        
================================================================================================
Updating:
firefox                        i686        10.0.6-1.el6.centos     updates             20 M
Updating for dependencies:
 xulrunner                     i686        10.0.6-1.el6.centos     updates             12 M

Transaction Summary
================================================================================================
Install       0 Package(s)
Upgrade       2 Package(s)

Total download size: 32 M
Is this ok [y/N]: y
Downloading Packages:
(1/2): firefox-10.0.6-1.el6.centos.i686.rpm                                |  20 MB   01:10
(2/2): xulrunner-10.0.6-1.el6.centos.i686.rpm                              |  12 MB   00:52
------------------------------------------------------------------------------------------------
Total                                                           63 kB/s |  32 MB   02:04

Updated:
  firefox.i686 0:10.0.6-1.el6.centos

Dependency Updated:
  xulrunner.i686 0:10.0.6-1.el6.centos

Complete!

The above command will ask confirmation before installing any package on your system. If you want to install packages automatically without asking any confirmation, use option -y as shown in below example.

# yum -y install firefox

2. Removing a Package with YUM

To remove a package completely with their all dependencies, just run the following command as shown below.

# yum remove firefox
Loaded plugins: fastestmirror
Setting up Remove Process
Resolving Dependencies
--> Running transaction check
---> Package firefox.i686 0:10.0.6-1.el6.centos set to be erased
--> Finished Dependency Resolution

Dependencies Resolved

====================================================================================================
 Package                    Arch        Version                        Repository            Size        
====================================================================================================
Removing:
 firefox                    i686        10.0.6-1.el6.centos            @updates              23 M

Transaction Summary
====================================================================================================
Remove        1 Package(s)
Reinstall     0 Package(s)
Downgrade     0 Package(s)

Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Erasing        : firefox-10.0.6-1.el6.centos.i686                                                                                                                          1/1

Removed:
  firefox.i686 0:10.0.6-1.el6.centos

Complete!

Same way the above command will ask confirmation before removing a package. To disable confirmation prompt just add option -y as shown in below.

# yum -y remove firefox

3. Updating a Package using YUM

Let’s say you have outdated version of MySQL package and you want to update it to the latest stable version. Just run the following command it will automatically resolves all dependencies issues and install them.

# yum update mysql
Loaded plugins: fastestmirror
Dependencies Resolved

============================================================================================================
 Package            Arch                Version                    Repository                    Size
============================================================================================================
Updating:
 vsftpd             i386                2.0.5-24.el5_8.1           updates                       144 k

Transaction Summary
============================================================================================================
Install       0 Package(s)
Upgrade       1 Package(s)

Total size: 144 k
Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Updating       : vsftpd                                                                     1/2
  Cleanup        : vsftpd                                                                     2/2

Updated:
  vsftpd.i386 0:2.0.5-24.el5_8.1

Complete!

4. List a Package using YUM

Use the list function to search for the specific package with name. For example to search for a package called openssh, use the command.

# yum list openssh
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirror.neu.edu.cn
 * epel: mirror.neu.edu.cn
 * extras: mirror.neu.edu.cn
 * rpmforge: mirror.nl.leaseweb.net
 * updates: mirror.nus.edu.sg
Installed Packages
openssh.i386                                       4.3p2-72.el5_6.3                                                                      installed
Available Packages                                 4.3p2-82.el5                                                                          base

To make your search more accurate, define package name with their version, in case you know. For example to search for a specific version openssh-4.3p2 of the package, use the command.

# yum list openssh-4.3p2

5. Search for a Package using YUM

If you don’t remember the exact name of the package, then use search function to search all the available packages to match the name of the package you specified. For example, to search all the packages that matches the word .

# yum search vsftpd
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirror.neu.edu.cn
 * epel: mirror.neu.edu.cn
 * extras: mirror.neu.edu.cn
 * rpmforge: mirror.nl.leaseweb.net
 * updates: ftp.iitm.ac.in
============================== Matched: vsftpd ========================
ccze.i386 : A robust log colorizer
pure-ftpd-selinux.i386 : SELinux support for Pure-FTPD
vsftpd.i386 : vsftpd - Very Secure Ftp Daemon

6. Get Information of a Package using YUM

Say you would like to know information of a package before installing it. To get information of a package just issue the below command.

# yum info firefox
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirror.neu.edu.cn
 * epel: mirror.neu.edu.cn
 * extras: mirror.neu.edu.cn
 * rpmforge: mirror.nl.leaseweb.net
 * updates: ftp.iitm.ac.in
Available Packages
Name       : firefox
Arch       : i386
Version    : 10.0.6
Release    : 1.el5.centos
Size       : 20 M
Repo       : updates
Summary    : Mozilla Firefox Web browser
URL        : http://www.mozilla.org/projects/firefox/
License    : MPLv1.1 or GPLv2+ or LGPLv2+
Description: Mozilla Firefox is an open-source web browser, designed for standards
           : compliance, performance and portability.

7. List all Available Packages using YUM

To list all the available packages in the Yum database, use the below command.

# yum list | less

8. List all Installed Packages using YUM

To list all the installed packages on a system, just issue below command, it will display all the installed packages.

# yum list installed | less

9. Yum Provides Function

Yum provides function is used to find which package a specific file belongs to. For example, if you would like to know the name of the package that has the /etc/httpd/conf/httpd.conf.

# yum provides /etc/httpd/conf/httpd.conf
Loaded plugins: fastestmirror
httpd-2.2.3-63.el5.centos.i386 : Apache HTTP Server
Repo        : base
Matched from:
Filename    : /etc/httpd/conf/httpd.conf

httpd-2.2.3-63.el5.centos.1.i386 : Apache HTTP Server
Repo        : updates
Matched from:
Filename    : /etc/httpd/conf/httpd.conf

httpd-2.2.3-65.el5.centos.i386 : Apache HTTP Server
Repo        : updates
Matched from:
Filename    : /etc/httpd/conf/httpd.conf

httpd-2.2.3-53.el5.centos.1.i386 : Apache HTTP Server
Repo        : installed
Matched from:
Other       : Provides-match: /etc/httpd/conf/httpd.conf

10. Check for Available Updates using Yum

To find how many of installed packages on your system have updates available, to check use the following command.

# yum check-update

11. Update System using Yum

To keep your system up-to-date with all security and binary package updates, run the following command. It will install all latest patches and security updates to your system.

# yum update

12. List all available Group Packages

In Linux, number of packages are bundled to particular group. Instead of installing individual packages with yum, you can install particular group that will install all the related packages that belongs to the group. For example to list all the available groups, just issue following command.

# yum grouplist
Installed Groups:
   Administration Tools
   DNS Name Server
   Dialup Networking Support
   Editors
   Engineering and Scientific
   FTP Server
   Graphics
   Java Development
   Legacy Network Server
Available Groups:
   Authoring and Publishing
   Base
   Beagle
   Cluster Storage
   Clustering
   Development Libraries
   Development Tools
   Eclipse
   Educational Software
   KDE (K Desktop Environment)
   KDE Software Development

13. Install a Group Packages

To install a particular package group, we use option as groupinstall. Fore example, to install “MySQL Database“, just execute the below command.

# yum groupinstall 'MySQL Database'
Dependencies Resolved

=================================================================================================
Package								Arch      Version			 Repository        Size
=================================================================================================
Updating:
 unixODBC                           i386      2.2.11-10.el5      base              290 k
Installing for dependencies:
 unixODBC-libs                      i386      2.2.11-10.el5      base              551 k

Transaction Summary
=================================================================================================
Install       1 Package(s)
Upgrade       1 Package(s)

Total size: 841 k
Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing     : unixODBC-libs	1/3
  Updating       : unixODBC         2/3
  Cleanup        : unixODBC         3/3

Dependency Installed:
  unixODBC-libs.i386 0:2.2.11-10.el5

Updated:
  unixODBC.i386 0:2.2.11-10.el5

Complete!

14. Update a Group Packages

To update any existing installed group packages, just run the following command as shown below.

# yum groupupdate 'DNS Name Server'

Dependencies Resolved
================================================================================================================
 Package			Arch	        Version				Repository           Size
================================================================================================================
Updating:
 bind                           i386            30:9.3.6-20.P1.el5_8.2          updates              981 k
 bind-chroot                    i386            30:9.3.6-20.P1.el5_8.2          updates              47 k
Updating for dependencies:
 bind-libs                      i386            30:9.3.6-20.P1.el5_8.2          updates              864 k
 bind-utils                     i386            30:9.3.6-20.P1.el5_8.2          updates              174 k

Transaction Summary
================================================================================================================
Install       0 Package(s)
Upgrade       4 Package(s)

Total size: 2.0 M
Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Updating       : bind-libs            1/8
  Updating       : bind                 2/8
  Updating       : bind-chroot          3/8
  Updating       : bind-utils           4/8
  Cleanup        : bind                 5/8
  Cleanup        : bind-chroot          6/8
  Cleanup        : bind-utils           7/8
  Cleanup        : bind-libs            8/8

Updated:
  bind.i386 30:9.3.6-20.P1.el5_8.2                  bind-chroot.i386 30:9.3.6-20.P1.el5_8.2

Dependency Updated:
  bind-libs.i386 30:9.3.6-20.P1.el5_8.2             bind-utils.i386 30:9.3.6-20.P1.el5_8.2

Complete!

15. Remove a Group Packages

To delete or remove any existing installed group from the system, just use below command.

# yum groupremove 'DNS Name Server'

Dependencies Resolved

===========================================================================================================
 Package                Arch              Version                         Repository          Size
===========================================================================================================
Removing:
 bind                   i386              30:9.3.6-20.P1.el5_8.2          installed           2.1 M
 bind-chroot            i386              30:9.3.6-20.P1.el5_8.2          installed           0.0

Transaction Summary
===========================================================================================================
Remove        2 Package(s)
Reinstall     0 Package(s)
Downgrade     0 Package(s)

Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Erasing        : bind                                                   1/2
warning: /etc/sysconfig/named saved as /etc/sysconfig/named.rpmsave
  Erasing        : bind-chroot                                            2/2

Removed:
  bind.i386 30:9.3.6-20.P1.el5_8.2                                        bind-chroot.i386 30:9.3.6-20.P1.el5_8.2

Complete!

16. List Enabled Yum Repositories

To list all enabled Yum repositories in your system, use following option.

# yum repolist

repo id                     repo name                                            status
base                        CentOS-5 - Base                                      enabled:  2,725
epel                        Extra Packages for Enterprise Linux 5 - i386         enabled:  5,783
extras                      CentOS-5 - Extras                                    enabled:    282
mod-pagespeed               mod-pagespeed                                        enabled:      1
rpmforge                    RHEL 5 - RPMforge.net - dag                          enabled: 11,290
updates                     CentOS-5 - Updates                                   enabled:    743
repolist: 20,824

16. List all Enabled and Disabled Yum Repositories

The following command will display all enabled and disabled yum repositories on the system.

# yum repolist all

repo id                     repo name                                            status
C5.0-base                   CentOS-5.0 - Base                                    disabled
C5.0-centosplus             CentOS-5.0 - Plus                                    disabled
C5.0-extras                 CentOS-5.0 - Extras                                  disabled
base                        CentOS-5 - Base                                      enabled:  2,725
epel                        Extra Packages for Enterprise Linux 5 - i386         enabled:  5,783
extras                      CentOS-5 - Extras                                    enabled:    282
repolist: 20,824

17. Install a Package from Specific Repository

To install a particular package from a specific enabled or disabled repository, you must use –enablerepo option in your yum command. For example to Install PhpMyAdmin 3.5.2 package, just execute the command.

# yum --enablerepo=epel install phpmyadmin

Dependencies Resolved
=============================================================================================
 Package                Arch           Version            Repository           Size
=============================================================================================
Installing:
 phpMyAdmin             noarch         3.5.1-1.el6        epel                 4.2 M

Transaction Summary
=============================================================================================
Install       1 Package(s)

Total download size: 4.2 M
Installed size: 17 M
Is this ok [y/N]: y
Downloading Packages:
phpMyAdmin-3.5.1-1.el6.noarch.rpm                       | 4.2 MB     00:25
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : phpMyAdmin-3.5.1-1.el6.noarch             1/1
  Verifying  : phpMyAdmin-3.5.1-1.el6.noarch             1/1

Installed:
  phpMyAdmin.noarch 0:3.5.1-1.el6

Complete!

18. Interactive Yum Shell

Yum utility provides a custom shell where you can execute multiple commands.

# yum shell
Loaded plugins: fastestmirror
Setting up Yum Shell
> update httpd
Loading mirror speeds from cached hostfile
 * base: mirrors.sin3.sg.voxel.net
 * epel: ftp.riken.jp
 * extras: mirrors.sin3.sg.voxel.net
 * updates: mirrors.sin3.sg.voxel.net
Setting up Update Process
>

19. Clean Yum Cache

By default yum keeps all the repository enabled package data in /var/cache/yum/ with each sub-directory, to clean all cached files from enabled repository, you need to run the following command regularly to clean up all the cache and make sure that there is nothing unnecessary space is using. We don’t want to give the output of the below command, because we like to keep cached data as it is.

# yum clean all

20. View History of Yum

To view all the past transactions of yum command, just use the following command.

# yum history

Loaded plugins: fastestmirror
ID     | Login user               | Date and time    | Action(s)      | Altered
-------------------------------------------------------------------------------
    10 | root               | 2012-08-11 15:19 | Install        |    3
     9 | root               | 2012-08-11 15:11 | Install        |    1
     8 | root               | 2012-08-11 15:10 | Erase          |    1 EE
     7 | root               | 2012-08-10 17:44 | Install        |    1
     6 | root               | 2012-08-10 12:19 | Install        |    2
     5 | root               | 2012-08-10 12:14 | Install        |    3
     4 | root               | 2012-08-10 12:12 | I, U           |   13 E<
     3 | root               | 2012-08-09 13:01 | Install        |    1 >
     2 | root               | 2012-08-08 20:13 | I, U           |  292 EE
     1 | System            | 2012-08-08 17:15 | Install        |  560
history list

We have tried to cover all the basic to advance yum commands with their examples. If anything related to yum commands may have missed out. Please update us through our comment box. So, we keep updating the same based on feedback’s received.

Source

If you are in charge of managing a database server, from time to time you may need to run a query and inspect it carefully. While you can do that from the MySQL / MariaDB shell, but this tip will allow you to execute the MySQL/MariaDB Queries directly using the Linux command line AND save the output to a file for later inspection (this is particularly useful if the query return lots of records).

Let us look at some simple examples of running queries directly from the command line before we can move to a more advanced query.

To view all the databases on your server, you can issue the following command:

# mysql -u root -p -e "show databases;"

Next, to create a database table named tutorials in the database tecmintdb, run the command below:

$ mysql -u root -p -e "USE tecmintdb; CREATE TABLE tutorials(tut_id INT NOT NULL AUTO_INCREMENT, tut_title VARCHAR(100) NOT NULL, tut_author VARCHAR(40) NOT NULL, submissoin_date DATE, PRIMARY KEY (tut_id));"

We will use the following command and pipe the output to the tee command followed by the filename where we want to store the output.

Suggested Read: 20 MySQL/MariaDB Commands for Database Administration in Linux

For illustration, we will use a database named employees and a simple join between the employees and salaries tables. In your own case, just type the SQL query between the quotes and hit Enter.

Note that you will be prompted to enter the password for the database user:

# mysql -u root -p -e "USE employees; SELECT DISTINCT A.first_name, A.last_name FROM employees A JOIN salaries B ON A.emp_no = B.emp_no WHERE hire_date < '1985-01-31';" | tee queryresults.txt

View the query results with the help of cat command.

# cat queryresults.txt

With the query results in a plain text files, you can process the records more easily using other command-line utilities.

Summary

We have shared several Linux tips that you, as a system administrator, may find useful when it comes to automating your daily Linux tasks or performing them more easily.

Suggested Read: How to Backup and Restore MySQL/MariaDB Databases

Do you have any other tips that you would like to share with the rest of the community? If so, please do so using the comment form below.

Otherwise, feel free to let us your thoughts about the assortment of tips that we have looked at, or what we can add or possibly do to improve each of them. We look forward to hearing from you!

Source

ls command is one of the most frequently used command in Linux. I believe ls command is the first command you may use when you get into the command prompt of Linux Box.

We use ls command daily basis and frequently even though we may not aware and never use all the ls option available. In this article, we’ll be discussing basic ls command where we have tried to cover as much parameters as possible.

1. List Files using ls with no option

ls with no option list files and directories in bare format where we won’t be able to view details like file types, size, modified date and time, permission and links etc.

# ls

0001.pcap        Desktop    Downloads         index.html   install.log.syslog  Pictures  Templates
anaconda-ks.cfg  Documents  fbcmd_update.php  install.log  Music               Public    Videos

2 List Files With option –l

Here, ls -l (-l is character not one) shows file or directory, size, modified date and time, file or folder name and owner of file and its permission.

# ls -l

total 176
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos

3. View Hidden Files

List all files including hidden file starting with ‘.‘.

# ls -a

.                .bashrc  Documents         .gconfd          install.log         .nautilus     .pulse-cookie
..               .cache   Downloads         .gnome2          install.log.syslog  .netstat.swp  .recently-used.xbel
0001.pcap        .config  .elinks           .gnome2_private  .kde                .opera        .spice-vdagent
anaconda-ks.cfg  .cshrc   .esd_auth         .gtk-bookmarks   .libreoffice        Pictures      .tcshrc
.bash_history    .dbus    .fbcmd            .gvfs            .local              .pki          Templates
.bash_logout     Desktop  fbcmd_update.php  .ICEauthority    .mozilla            Public        Videos
.bash_profile    .digrc   .gconf            index.html       Music               .pulse        .wireshark

4. List Files with Human Readable Format with option -lh

With combination of -lh option, shows sizes in human readable format.

# ls -lh

total 176K
-rw-r--r--. 1 root root  683 Aug 19 09:59 0001.pcap
-rw-------. 1 root root 1.6K Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Documents
drwxr-xr-x. 4 root root 4.0K Aug 16 02:55 Downloads
-rw-r--r--. 1 root root  21K Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root  46K Jul 31 09:58 index.html
-rw-r--r--. 1 root root  48K Jul 31 02:17 install.log
-rw-r--r--. 1 root root  12K Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Music
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Public
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Templates
drwxr-xr-x. 2 root root 4.0K Jul 31 02:48 Videos

5. List Files and Directories with ‘/’ Character at the end

Using -F option with ls command, will add the ‘/’ Character at the end each directory.

# ls -F

0001.pcap        Desktop/    Downloads/        index.html   install.log.syslog  Pictures/  Templates/
anaconda-ks.cfg  Documents/  fbcmd_update.php  install.log  Music/              Public/    Videos/

6. List Files in Reverse Order

The following command with ls -r option display files and directories in reverse order.

# ls -r

Videos     Public    Music               install.log  fbcmd_update.php  Documents  anaconda-ks.cfg
Templates  Pictures  install.log.syslog  index.html   Downloads         Desktop    0001.pcap

7. Recursively list Sub-Directories

ls -R option will list very long listing directory trees. See an example of output of the command.

# ls -R

total 1384
-rw-------. 1 root     root      33408 Aug  8 17:25 anaconda.log
-rw-------. 1 root     root      30508 Aug  8 17:25 anaconda.program.log

./httpd:
total 132
-rw-r--r--  1 root root     0 Aug 19 03:14 access_log
-rw-r--r--. 1 root root 61916 Aug 10 17:55 access_log-20120812

./lighttpd:
total 68
-rw-r--r--  1 lighttpd lighttpd  7858 Aug 21 15:26 access.log
-rw-r--r--. 1 lighttpd lighttpd 37531 Aug 17 18:21 access.log-20120819

./nginx:
total 12
-rw-r--r--. 1 root root    0 Aug 12 03:17 access.log
-rw-r--r--. 1 root root  390 Aug 12 03:17 access.log-20120812.gz

8. Reverse Output Order

With combination of -ltr will shows latest modification file or directory date as last.

# ls -ltr

total 176
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap

9. Sort Files by File Size

With combination of -lS displays file size in order, will display big in size first.

# ls -lS

total 176
-rw-r--r--. 1 root root 48867 Jul 31 02:17 install.log
-rw-r--r--. 1 root root 46701 Jul 31 09:58 index.html
-rw-r--r--. 1 root root 21262 Aug 12 12:42 fbcmd_update.php
-rw-r--r--. 1 root root 11439 Jul 31 02:13 install.log.syslog
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Desktop
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Documents
drwxr-xr-x. 4 root root  4096 Aug 16 02:55 Downloads
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Music
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Pictures
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Public
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Templates
drwxr-xr-x. 2 root root  4096 Jul 31 02:48 Videos
-rw-------. 1 root root  1586 Jul 31 02:17 anaconda-ks.cfg
-rw-r--r--. 1 root root   683 Aug 19 09:59 0001.pcap

10. Display Inode number of File or Directory

We can see some number printed before file / directory name. With -i options list file / directory with inode number.

# ls -i

20112 0001.pcap        23610 Documents         23793 index.html          23611 Music     23597 Templates
23564 anaconda-ks.cfg  23595 Downloads            22 install.log         23612 Pictures  23613 Videos
23594 Desktop          23585 fbcmd_update.php     35 install.log.syslog  23601 Public

11. Shows version of ls command

Check version of ls command.

# ls --version

ls (GNU coreutils) 8.4
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Richard M. Stallman and David MacKenzie.

12. Show Help Page

List help page of ls command with their option.

# ls --help

Usage: ls [OPTION]... [FILE]...

13. List Directory Information

With ls -l command list files under directory /tmp. Wherein with -ld parameters displays information of /tmpdirectory.

# ls -l /tmp
total 408
drwx------. 2 narad narad   4096 Aug  2 02:00 CRX_75DAF8CB7768
-r--------. 1 root  root  384683 Aug  4 12:28 htop-1.0.1.tar.gz
drwx------. 2 root  root    4096 Aug  4 11:20 keyring-6Mfjnk
drwx------. 2 root  root    4096 Aug 16 01:33 keyring-pioZJr
drwx------. 2 gdm   gdm     4096 Aug 21 11:26 orbit-gdm
drwx------. 2 root  root    4096 Aug 19 08:41 pulse-gl6o4ZdxQVrX
drwx------. 2 narad narad   4096 Aug  4 08:16 pulse-UDH76ExwUVoU
drwx------. 2 gdm   gdm     4096 Aug 21 11:26 pulse-wJtcweUCtvhn
-rw-------. 1 root  root     300 Aug 16 03:34 yum_save_tx-2012-08-16-03-34LJTAa1.yumtx

# ls -ld /tmp/

drwxrwxrwt. 13 root root 4096 Aug 21 12:48 /tmp/

14. Display UID and GID of Files

To display UID and GID of files and directories. use option -n with ls command.

# ls -n

total 36
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Downloads
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Music
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Pictures
-rw-rw-r--. 1 500 500   12 Aug 21 13:06 tmp.txt
drwxr-xr-x. 2 500 500 4096 Aug  2 01:52 Videos

15. ls command and its Aliases

We have made alias for ls command, when we execute ls command it’ll take -l option by default and display long listing as mentioned earlier.

# alias ls="ls -l"

Note: We can see number of alias available in your system with below alias command and same can be unalias as shown below example.

# alias

alias cp='cp -i'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias mv='mv -i'
alias rm='rm -i'
alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

To remove an alias previously defined, just use the unalias command.

# unalias ls

Source

A jump host (also known as a jump server) is an intermediary host or an SSH gateway to a remote network, through which a connection can be made to another host in a dissimilar security zone, for example a demilitarized zone (DMZ). It bridges two dissimilar security zones and offers controlled access between them.

A jump host should be highly secured and monitored especially when it spans a private network and a DMZ with servers providing services to users on the internet.

A classic scenario is connecting from your desktop or laptop from inside your company’s internal network, which is highly secured with firewalls to a DMZ. In order to easily manage a server in a DMZ, you may access it via a jump host.

In this article, we will demonstrate how to access a remote Linux server via a jump host and also we will configure necessary settings in your per-user SSH client configurations.

Consider the following scenario.

In above scenario, you want to connect to HOST 2, but you have to go through HOST 1, because of firewalling, routing and access privileges. There is a number of valid reasons why jumphosts are needed..

Dynamic Jumphost List

The simplest way to connect to a target server via a jump host is using the -J flag from the command line. This tells ssh to make a connection to the jump host and then establish a TCP forwarding to the target server, from there (make sure you’ve Passwordless SSH Login between machines).

$ ssh -J host1 host2

If usernames or ports on machines differ, specify them on the terminal as shown.

$ ssh -J username@host1:port username@host2:port	  

Multiple Jumphosts List

The same syntax can be used to make jumps over multiple servers.

$ ssh -J username@host1:port,username@host2:port username@host3:port

Static Jumphost List

Static jumphost list means, that you know the jumphost or jumphosts that you need to connect a machine. Therefore you need to add the following static jumphost ‘routing’ in ~/.ssh/config file and specify the host aliases as shown.

### First jumphost. Directly reachable
Host vps1
  HostName vps1.example.org

### Host to jump to via jumphost1.example.org
Host contabo
  HostName contabo.example.org
  ProxyJump contabo

Now try to connect to a target server via a jump host as shown.

$ ssh -J vps1 contabo

The second method is to use the ProxyCommand option to add the jumphost configuration in your ~.ssh/config or $HOME/.ssh/config file as shown.

In this example, the target host is contabo and the jumphost is vps1.

Host vps1
	HostName vps1.example.org
	IdentityFile ~/.ssh/vps1.pem
	User ec2-user

Host contabo
	HostName contabo.example.org	
	IdentityFile ~/.ssh/contabovps
	Port 22
	User admin	
	Proxy Command ssh -q -W %h:%p vps1

Where the command Proxy Command ssh -q -W %h:%p vps1, means run ssh in quiet mode (using -q) and in stdio forwarding (using -W) mode, redirect the connection through an intermediate host (vps1).

Then try to access your target host as shown.

$ ssh contabo

The above command will first open an ssh connection to vps1 in the background effected by the ProxyCommand, and there after, start the ssh session to the target server contabo.

For more information, see the ssh man page or refer to: OpenSSH/Cookbxook/Proxies and Jump Hosts.

That’s all for now! In this article, we have demonstrated how to access a remote server via a jump host. Use the feedback form below to ask any questions or share your thoughts with us.

Source

We are pleased to announce our dedicated series of posts on Java Programming Language on the demand of our readers. In this series we are going to cover everything you need to know about Java.

Why Java?

Java is a General purpose, Object Oriented Programming Language written by James Gosling. It is known for many features which makes it different from other Programming languages. It is one of those Programming Language that has always remained in demand since the time of its initial release. It is one of the most powerful Programming Language which can do amazing things when combined with the power of Linux. Linux+Java is Future. Most talked about Features of Java are:

1. General Purpose Programming Language
2. Object Oriented approach
3. Friendly Syntax
4. Portability
5. Memory Management Feature
6. Architecture Neutral
7. Interpreted

This tutorial is for those, who have knowledge of any other programming and/or Scripting Language and want to learn Java from core level.

What you need to get started with Java

First thing is you need is to install Java Compiler and set path. The detailed instructions to install latest version of Java and set path is here [Install Java JDK/JRE in Linux]. Once the Java Compiler is Installed and Path is set, run

$ java -version

Sample Output

java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

Second thing is you need a text editor. You may use any text editor of your choice which may be command line based or a GUI Text editor. My favorite editors are nano (command_line) and gedit (GUI).

You may use any but make sure you don’t use an Integrated Development Environment (IDE). If you start using IDE at this level, you will never understand a few things, so say no to IDE.

The last things you need is a good step-by step guide (which I will be providing) and a never ending quest to learn Java.

Topics we are going to cover

This is an Ever expanding list of topics and there is nothing hard and fast associated with it. I will keep adding up topics to this section as we dive deep into Java. From here you may go anywhere but I suggest you to go through all the topics stepwise.

We always got the support of our readers and once again we seek the support of our beloved Readers to make popular Java Series Post on Tecmint. Fasten your seat belts and lets start. Keep following.

What is Java? A Brief History about Java

Java is a General Purpose, class based, object oriented, Platform independent, portable, Architecturally neutral, multithreaded, dynamic, distributed, Portable and robust interpreted Programming Language.

Why Java is a called:

General Purpose

Java capabilities are not limited to any specific application domain rather it can be used in various application domain and hence it is called General Purpose Programming Language.

Class based

Java is a class based/oriented programming language which means Java supports inheritance feature of object-oriented Programming Language.

Object oriented

Java is object-oriented means software developed in Java are combination of different types of object.

Platform Independent

A Java code will run on any JVM (Java Virtual Machine). Literally you can run same Java code on Windows JVM, Linux JVM, Mac JVM or any other JVM practically and get same result every time.

Architecturally Neutral

A Java code is not dependent upon Processor Architecture. A Java Application compiled on 64 bit architecture of any platform will run on 32 bit (or any other architecture) system without any issue.

Multithreaded
A thread in Java refers to an independent program. Java supports multithread which means Java is capable of running many tasks simultaneously, sharing the same memory.

Dynamic

Java is a Dynamic programming language which means it executes many programming behavior at Runtime and don’t need to be passed at compile time as in the case of static programming.

Distributed

Java Supports distributed System which means we can access files over Internet just by calling the methods.

Portable

A Java program when compiled produce bytecodes. Bytecodes are magic. These bytecodes can be transferred via network and can be executed by any JVM, hence came the concept of ‘Write once, Run Anywhere(WORA)’.

Robust

Java is a robust programming Language which means it can cope with error while the program is executing as well as keep operating with abnormalities to certain extent. Automatic Garbage collection, strong memory management, exception handling and type checking further adds to the list.

Interpreted

Java is a compiled programming Language which compiles the Java program into Java byte codes. This JVM is then interpreted to run the program.

Other than the above discussed feature, there are a few other remarkable features, like:

Security

Unlike other programming Language where Program interacts with OS using User runtime environment of OS, Java provides an extra layer of security by putting JVM between Program and OS.

Simple Syntax

Java is an improved c++ which ensures friendly syntax but with removed unwanted features and inclusion of Automatic Garbage collection.

High Level Programming Language

Java is a High Level Programming Language the syntax of which is human readable. Java lets programmer to concentrate on what to achieve and not how to achieve. The JVM converts a Java Program to Machine understandable language.

High Performance

Java make use of Just-In-Time compiler for high performance. Just-In-Time compiler is a computer program that turns Java byte codes into instructions that can directly be sent to compilers.

History of Java

Java Programming Language was written by James Gosling along with two other person ‘Mike Sheridan‘ and ‘Patrick Naughton‘, while they were working at Sun Microsystems. Initially it was named oak Programming Language.

Java Releases

1. Initial Java Versions 1.0 and 1.1 was released in the year 1996 for Linux, Solaris, Mac and Windows.
2. Java version 1.2 (Commonly called as java 2) was released in the year 1998.
3. Java Version 1.3 codename Kestrel was released in the year 2000.
4. Java Version 1.4 codename Merlin was released in the year 2002.
5. Java Version 1.5/Java SE 5 codename ‘Tiger’ was released in the year 2004.
6. Java Version 1.6/Java SE 6 Codename ‘Mustang’ was released in the year 2006.
7. Java Version 1.7/Java SE 7 Codename ‘Dolphin’ was released in the year 2011.
8. Java Version 1.8 is the current stable release which was released this year (2015).

Five Goals which were taken into consideration while developing Java:

1. Keep it simple, familiar and object oriented.
2. Keep it Robust and Secure.
3. Keep it architecture-neural and portable.
4. Executable with High Performance.
5. Interpreted, threaded and dynamic.

Why we call it Java 2, Java 5, Java 6, Java 7 and Java 8, not their actual version number which 1.2, 1.5, 1.6, 1.7 and 1.8?

Java 1.0 and 1.1 were Java. When Java 1.2 was released it had a lots of changes and marketers/developers wanted a new name so they called it Java 2 (J2SE), remove the numeric before decimal.

This was not the condition when Java 1.3 and Java 1.4 were released hence they were never called Java 3 and Java 4, but they were still Java 2.

When Java 5 was released, once again it was having a lots of changes for the developer/marketers and need a new name. The next number in sequence was 3, but calling Java 1.5 as Java 3 was confusing hence a decision was made to keep the naming as per version number and till now the legacy continues.

Places where Java is used

Java is implemented over a number of places in modern world. It is implemented as Standalone Application, Web Application, Enterprise Application and Mobile Application. Games, Smart Card, Embedded System, Robotics, Desktop, etc.

Keep connected we are coming up with “Working and code Structure of Java”.

How Java Works and Understanding Code Structure of Java – Part 2

In our last post ‘What is Java and History of Java‘ we had covered What is Java, features of Java in details, release history and its naming as well as places where Java is utilized.

Here in this post we will be going through working and code structure of Java Programming Language. Before we proceed let me remind you that Java was developed keeping in mind “Write Once Run Anywhere/Anytime (WORA)” means to ensure that the application developed should be architecturally neutral, Platform Independent and portable.

Working of Java

Having these goals in mind Java was developed with the below working model which can be classified into four stages.

Stage 1

Write the source file. This file contains all the procedure, method, class and objects within established protocol for Java Programming Language. The name of source file should be the name of the class or vice-versa. The source file name must have extension .java. Also the filename and class name are case sensitive.

Stage 2

Run the Java Source Code file through Java Compiler. Java Source code Compiler checks for error and syntax in the source file. It won’t let you compile your source code without satisfying Java compiler by fixing all errors and warning.

Stage 3

Compiler creates classfile. These classfile inherit the same name as the Source code file name, but the extension varies. The Source file name has extension 'filename.java', where as the extension of classfile created by compiler is 'filename.class'. This classfile is coded into bytecode – bytecodes are like magic.

Stage 4

This classfile created by Java Compiler is portable and architecturally neutral. You can port this classfile to run on any processor architecture and Platform/device. All you need is a Java Virtual Machine (JVM) to run this code no matter where.

Now understand the above four stages using an example. Here is a small sample Java Program code. Don’t worry if you don’t understand the code below. As of now just understand how it works.

public class MyFirstProgram
{
    public static void main(String[] args)
    {
        System.out.println("Hello Tecmint, This is my first Java Program");
    }
}

1. I wrote this program and defined class name MyFirstProgram. It is important to notice that this program must be saved as 'MyFirstProgram.java'.

Remember stage 1 above – The class name and file name must be same and filename must have extension .java. Also java is case sensitive hence if your classname is ‘MyFirstProgram‘, your source file name must be ‘MyFirstProgram.java‘.

You can not name it as ‘Myfirstprogram.java‘ or ‘myfirstprogram.java‘ or anything else. By convention it is a good idea to name your class based upon what the program is doing actually.

2. To compile this Java Source file, you need to pass it through Java compiler. Java compiler will essentially check the source code for any error and warning. It wont compile the source code until all the issues are solved. To compile java source code, you need to run:

$ javac MyFirstProgram.java

Where MyFirstProgram.java is the name of the source file.

3. On successful compilation you will notice that the Java compiler created a new file in the same directory the name of which is MyFirstProgram.class.

This class file is coded in bytecodes and can be run on any platform, any processor architecture any number of time. You may run the class file inside of JVM (Java Virtual Machine) on Linux or any other platform simply as:

$ java MyFirstProgram

So all you learnt above can be summarized as:

Java Source Code >> Compiler >> classfile/bytecode >> Various devices running JVM 

Understanding Code Structure in Java

1. Java source code file must contains a class definition. One Java Source file can contain only one public class/top-level class however it can contain lots of private class/inner-class.

The outer class/top class/public class can access all private class/inner class. The class must be within curly braces. Everything in Java is an object and class is a blueprint for object.

A demo of public/private class in Java:

public class class0
{
...
	private class1
	{
	…
	}

	private class 2
	{
	…
	}
...
}

2. Class contain one or more methods. Method must go within the curly braces of the class. A dummy example is:

public class class0
{
	public static void main(String[] args)
	{
	…..
	…..
	}
}

3. A method contain one or more statement/instruction. The instruction(s) must go within the curly braces of method. A dummy example is:

public class class0
{
	public static void main(String[] args)
	{
	System.out.println("Hello Tecmint, This is my first Java Program");
	System.out.println("I am Loving Java");
	…
	...
	}
}

Also important to mention at this point – Every Statement must end with semicolon. A dummy example is:

System.out.println("Hello Tecmint, This is my first Java Program");
...
...
System.out.println("I am Loving Java");

Writing your first Java Program with detailed description. The description is being put as comments here (//means commented out) in this example. You should write comments within a program.

Not only because this is a good habit but also because it makes the code readable ab you or anyone else at anytime later.

// Declare a Public class and name it anything but remember the class name and file name must be same, say class name is MyProg and hence file name must be MyProg.java
public class MyProg

// Remember everything goes into curly braces of class?
{
 

// This is a method which is inside the curly braces of class.
   public static void main(String[] args)

    // Everything inside a method goes into curly braces	
    {
        
    // Statement or Instruction inside method. Note it ends with a semicolon
    System.out.println("I didn't knew JAVA was so much fun filled");
    
    // closing braces of method
    }

// closing braces of class
}

A detailed technical description of the above simple Java Program.

public class MyProg

Here in the above name of class is MyProg and MyProg is a Public class which means everyone can access it.

public static void main(String[] args)

Here the method name is main which is a public method, means it can be accessed by anyone. The return type is void which means no return value. 'Strings[] args' means the arguments for the method main should be array which is to be called args. Don’t worry about the meaning of ‘static‘ as of now. We will be describing in details about it when required.

System.out.println("I didn't knew JAVA was so much fun filled");

System.out.ln ask JVM to print the output to standard output which is Linux command Line in our case. Anything that is in between braces of println statement gets print as it is, unless it is a variable. We will be going into details of variable later. The statement is ending with semicolon.

Even if something is not clear now you need not worry about this. Also you don’t need to memories anything. Just go through the post and understand terminologies and working even when the picture is not very clear.

That’s all for now. Provide us with your valuable feedback in the comments below. We are working on the next part “class and Main method in Java”and will be publishing soon.

Understanding Java Class, Main Method and Loops Control in Java – Part 3

In our last post ‘Working and code structure of Java‘ we emphasized in details of working of Java, Java Source File, Java Class File, Class (Public/Private), Method, Statement, Your first Java Program, Compilation and running of Java Program.

Here in this java learning series guide, we will understand how java class, main method and loops control works and also we will see basic codes using Java class with main method and loops control.

Everything in Java goes in a class

Everything in Java is an object and class is a blueprint of object. Every piece of code in Java is placed under the curly braces of class. When you compile a Java Program it produces a class file. When you run Java Program you are not running the Program file in actual but the class.

When you run a Program in Java Virtual Machine (JVM), it loads the required class and then goes directly to the main () method. The program continues to run till the closing braces of main () method. The program start executing just after the main() method. A class must have a main () method. Not all the class (Private class) requires a main () method.

What goes inside main () Method?

A main () method is the place where magic starts. You can ask JVM to do anything within main() method via statement/instructions and loops.

What is loop?

Loop is an instruction or a number of instructions in sequence that keeps on repeating till the condition is reached. Loops are the logical structure of a programming language. Loop logical structure are typically used to do a process, check the condition, do a process, check the condition,….. till condition requirements are met.

Loops in Java

There are three different loop mechanism in Java.

1. while Loop

while Loop in Java is a control structure which is used to perform a task repeatedly for a certain number of times, as defined in boolean expression, till the expression test result is true. If the boolean expression text result is false the while loop will be ignored completely without being executed even a single time.

Syntax of while loop:

while (boolean expression)
{
	statement/instructions
}

An example of while Loop in Java:

public class While_loop
{
    public static void main(String[] args)
    {
        int A = 100;
        while(A>0)
        {
            System.out.println("The Value of A = " +A);
            A=A-10;
        }
    }
}

Sample Output

$ java While_loop 

The Value of A = 100
The Value of A = 90
The Value of A = 80
The Value of A = 70
The Value of A = 60
The Value of A = 50
The Value of A = 40
The Value of A = 30
The Value of A = 20
The Value of A = 10

Anatomy of While_loop Program

// Public Class While_loop
public class While_loop
{
    // main () Method
    public static void main(String[] args)
    {
        // declare an integer variable named 'A' and give it the value of 100
        int A = 100;
        // Keep looping as long as the value of A is greater than 0. 'A>0' here is the boolean                 
           expression
        while(A>0)
        {
	 // Statement
            System.out.println("The Value of A = " +A);
            // Post Decrement (by 10)
	 A=A-10;
        }
    }
}

2. do..while Loop

do…while loop is very much similar to the while loop except the fact that it contains a do… before while to ensure that loop execute at least once.

Syntax of while loop:

do 
{
statement/instructions
}
while (boolean expression);

You may see the above syntax which clearly shows that the 'do..' part of the loop executed before checking the boolean expression, if it is true or false. Hence no matter what the result (true/false) of boolean expression, the loop executes. If true it will execute till the condition is satisfied. If false it will be executed once.

An Example of do…while Loop in Java:

public class do_while
{
    public static void main(String[] args)
    {
        int A=100;
        do
        {
            System.out.println("Value of A = " +A);
            A=A-10;
        }
        while (A>=50);
    }
}

Sample Output

$ java do_while 

Value of A = 100
Value of A = 90
Value of A = 80
Value of A = 70
Value of A = 60
Value of A = 50

Anatomy of do_while Program:

// public class do_while
public class do_while
{
    // main () Method
    public static void main(String[] args)
    {
        // Declare a Integer Variable 'A' and assign it a value = 100
        int A=100;
        // do...while loop starts
        do
        {
            // execute the below statement without checking boolean expression condition if true 
               or false
            System.out.println("Value of A = " +A);
            // Post Decrement (by 10)
            A=A-10;
        }
        // Check condition. Loop the execute only till the value of Variable A is greater than or 
           equal to 50.
        while (A>=50);
    }
}

3. for Loop

for_loop in Java is widely used for repetition control. It is used to iterate a task for specific number of times. For loop is used to control how many times the loop needs to execute to perform a task. for loop is only useful if you know how many times you need to execute the loop.

Syntax of for loop:

for (initialization; boolean-expression; update)
{
statement
}

An example of the for loop in Java

public class for_loop
{
    public static void main(String[] arge)
    {
        int A;
        for (A=100; A>=0; A=A-7)
        {
            System.out.println("Value of A = " +A);
        }
    }
}

Sample Output

$ java for_loop 

Value of A = 100
Value of A = 93
Value of A = 86
Value of A = 79
Value of A = 72
Value of A = 65
Value of A = 58
Value of A = 51
Value of A = 44
Value of A = 37
Value of A = 30
Value of A = 23
Value of A = 16
Value of A = 9
Value of A = 2

Anatomy of for_loop Program:

// public class for_loop
public class for_loop
{
    // main () Method
    public static void main(String[] arge)
    {
        // Declare a Integer Variable A
        int A;
        // for loop starts. Here Initialization is A=100, boolean_expression is A>=0 and update is 
           A=A-7
        for (A=100; A>=0; A=A-7)
        {
            // Statement        
            System.out.println("Value of A = " +A);
        }
    }
}

The Break and Continue keywords for loops in Java

1. The Break Keyword

As the name suggest the break keyword is used to stop the entire loop immediately. The break keyword must always be used inside the loop or switch statement. Once the loop breaks by using break; JVM starts executing the very next line of code outside of the loop. An example of break loop in Java is:

public class break
{
    public static void main(String[] args)
    {
        int A = 100;
        while(A>0)
        {
            System.out.println("The Value of A = " +A);
            A=A-10;
            if (A == 40)
            {
                break;
            }
        }
    }
}

Sample Output

$ java break 

The Value of A = 100
The Value of A = 90
The Value of A = 80
The Value of A = 70
The Value of A = 60
The Value of A = 50

The Continue Keyword

The continue keyword can be used with any loop in Java. Continue keyword ask the loop to jump to the next iteration immediately. However it is interpreted differently by for loop and while/do…while loop.

Continue Keyword in for loop jumps to the next update statement.

An example of continue in for loop:

public class continue_for_loop
{
    public static void main(String[] arge)
    {
        int A;
        for (A=10; A>=0; A=A-1)
        {
	    if (A == 2)
		{
	        continue;
		}
            System.out.println("Value of A = " +A);
        }
    }
}

Sample Output

$ java continue_for_loop

Value of A = 10
Value of A = 9
Value of A = 8
Value of A = 7
Value of A = 6
Value of A = 5
Value of A = 4
Value of A = 3
Value of A = 1
Value of A = 0

Did you noticed, it skipped Value of A = 2. It does so by dumping to the next update statement.

2. Continue Keyword in while loop or do…while loop jumps to the boolean expression.

Well you can do it yourself. Its too easy. Just follow the above steps.

That’s all for now from my side.

Understanding Java Compiler and Java Virtual Machine – Part 4

Till now we have gone through working and code structure of Java and Class, Main method & Loop Control in Java. Here in this post we will see What is Java Compiler and Java Virtual Machine. What are they meant for and their roles.

What is Java Compiler

Java is a strongly typed language which means variable must hold right kind of data. In a strongly typed language a variable can not hold wrong data type. This is a safety feature very well implemented in Java Programming Language.

Java compiler is responsible for through checking the variables for any violation in data-type holding. A few exception may arise at run-time which is compulsory for dynamic binding feature of Java. As Java program runs it may include new objects that were not existing before hence to have some degree of flexibility a few exceptions are allowed in data-type that a variable can hold.

Java Compiler set filter for those piece of code that won’t compile ever except for the comments. Compiler do not parse the comments and leave it as it is. Java code supports three kinds of comments within Program.

1. /* COMMENT HERE */
2. /** DOCUMENTATION COMMENT HERE */
3. // COMMENT HERE

Anything that is placed between /* and */ or /** and */ or after // is ignored by Java Compiler.

Java Compiler is responsible for strict checking any syntax violation. Java Compiler is designed to be a bytecode compiler ie., it create a class file out of actual program file written purely in bytecode.

Java Compiler is the first stage of security. It is the first line of defense where checking for incorrect data-type in variable is checked. A wrong data-type can cause damage to the program and outside it. Also compiler check if any piece of code trying to invoke restricted piece of code like private class. It restrict unauthorized access of code/class/critical data.

Java Compiler produce bytecodes/class file that are platform and architecturally neutral that requires JVM to run and it will literally run on any device/platform/architecture.

What is Java Virtual Machine (JVM)

Java Virtual Machine is the next line of security which put an extra layer between Java Application and OS. Also it check the class file that has been security checked and compiled by Java Compiler, if someone tampered the class file/bytecode to restrict access to unauthorized critical data.

Java Virtual Machine interprets the bytecode by loading the class file to machine Language.

JVM is responsible for functions like Load and Store, Arithmetic calculation, Type conversion, Object Creation, Object Manupulation, Control Transfer, Throwing exception, etc.

The working model of Java in which Java Compiler compiles the code into calssfile/bytecodes and then Java Virtual Machine run the classfile/bytecode. This model ensures that code run at fast speed and the additional layer ensures security.

So what do you think – Java Compiler or Java Virtual Machine perform more important task? A Java program has to run through both the surface (Compiler and JVM) essentially.

This post sums the role of Java Compiler and JVM. All your suggestions are welcome in the comments below. We are working on the next post “object oriented approach of Java”. Till then stay tuned and connected to TecMint. Like and share us and help us get spread.

Object Oriented Approach of Java Programming and Encapsulation – Part 5

Since the beginning of this series (and even before that) you knew Java is an Object Oriented Programming Language. The object oriented Programming Language is based upon the concept of “objects”, which contains data as attributes in methods.

Every object in Java has state and behavior which are represented by instance variables and methods. Each instance of a class can have unique value for it’s instance variable.

For example,

Machine A may be powered up with Debian and have 8GB of RAM while Machine B can have installed Gentoo with 4GB of RAM. Also it is obvious that managing Machine that have installed Gentoo requires more knowledge – A behavior acting on its state. Here method is using instance variable values.

The JVM when parse a class, it make object of that kind. When you are writing a class, in actual you acting like a compiler telling your class what the object should know and how it should act. Every object of a particular type can have different value for same instance variable.

Every Instance of a class has the same method but it possible that all of them behave differently.

The OS class has 3 Instance variables namely OS Name, OS Type, OS Category.

OS

OS_Name OS_Type OS_Category

Boot() Reboot() scan()

The Boot() method boots one OS which is represented by OS Name for that instance. So if you boot() on one instance you will boot into Debian while on another instance you will boot into Gentoo. The method code, remains the same in either case.

Void Boot() 
	{
	bootloader.bootos(OS_Name);
	}

You are already aware that the program starts to execute just after the main() method. You can pass values into you method.

For example you would like to tell you OS what services to start at boot as:

You are already aware that the program starts to execute just after the main() method. You can pass values into you method. For example you would like to tell you OS what services to start at boot as:

OS.services(apache2);

What you pass into methods are called arguments. You can use a variable with a type and a name inside a method. It is important to pass values with parameter if a method takes a parameter.

OS deb = debian();
deb.reboot(600);

Here the reboot method on the OS passes the value of 600 (reboot machine after 600 sec) as an argument to the method. Till now we have seen method always returning void, which means it don’t return you anything, simply as:

void main()
	{
	…
	…
	}

However you can ask your compiler to get exactly what you are desiring and your compiler won’t return you wrong types. You may simply do like:

int Integer()
	{
	…
	…
	return 70;
	}

You can send more than one value value to a method. You can do this by calling two parameter methods and sending it to arguments. Note variable type and parameter type must always match.

void numbers(int a, int b)
	{
	int c = a + b;
	System.out.print(“sum is” +c);
	}

Declare and Initialize Instance Variables

1. When you don’t know the value to initialize.

int a;
float b;
string c;

2. When the know the value to Initialize.

int a = 12;
float b = 11.23;
string c = tecmint;

Note: A instance variables are often confused with local variables, however there is a very thin line between them to differentiate.

3. Instance Variables are declared inside a class unlike local variables that are declared within a method.

4. Unlike Instance Variables, local variables must initialize before it can be used. The compiler will report error if you use local variable before it is initialized.

Encapsulation

You might have heard about encapsulation. It is a feature of most of the object oriented programming language which makes it possible to bind data and functions into a single component. Encapsulation is supported by class and protects codes from accidental damage by creating a wall around objects and hides their properties and methods, selectively.

We will expand encapsulation in details in the right tutorial when it is required. As of now it is sufficient for you to know What encapsulation is? What it does? And how it does?

That’s all for now.

Day to Day: Learning Java Programming Language – Part I

In 1995 when c++ programming language were widely used. An employee of Sun Microsystem working on a platform called ‘Green‘ Developed a programming language and named it as ‘oak‘.

The name was inspired by an oak tree which he use to see outside of his office windows. Later the name oakwas replaced by Java.

Java Programming language was developed by James Gosling and hence James Gosling has been honoured as the Father of Java Programming Language.

Now the question is, if there already was such a functional programming language (c++) available, why Mr. Gosling and his team needed a different programming language.

Java was intended with the Features:

1. Write once, run anywhere
2. Cross Platform Program Development i.e., Architecturally Neutral
3. Security
4. Class based
5. Object oriented
6. Support for web technologies
7. Robust
8. Interpreted
9. Inheritance
10. Threaded
11. Dynamic
12. High Performance

Before Java was developed, The program written on a computer or for an architecture won’t run on another computer and architecture, hence while developing Java the team focus mainly on cross platform functionality and from there the concept of write once, run anywhere came, which remains the quote of sun microsystem for a long time.

Java program runs inside JVM (Java Virtual Machine) which adds an extra layer between System and program, which further means extra security. Other programming language prior to Java was not having such feature which means a code being run could be malicious can infect a system or other systems attached to it, however Java maintained to overcome this issue using JVM.

Java is a OOP (Object Oriented Programming) Language. By object oriented feature, it means all the entity is a object which further suggest Real World Object.

When Java was being developed at Sun, coincidentally web technologies has started to take take shape and the Java development was highly influenced with this, and even today web world uses Java more than any other language. Java is strictly an interpreted language, which means Java executes the source code directly by translating the source code in an intermediate form.

Java is robust in nature i.e., it can cope up with errors be in input or calculation. When we say Java is dynamic programming language, we mean to say that it is capable of breaking complex problems into simple problems and then execute them independently.

Java supports threading. Threads are small processes that can be managed independently by operating system scheduler.

Java Support Inheritance, which means relation can be established between classes.

No doubt! Java was developed as a successor to ‘c‘ and ‘c++‘ programming Language hence it inherit a number of features from its predecessor viz., c and c++ with a number of new features.

Learning Java from the point of view of carrier is highly appreciated and one of the most sought after technology. The best way to learn any programming language is to start programming.

Before we go to programming, one more thing we need to know is: the class name and program name should be same, however it can be different in certain condition but by convention it is always a good idea to rename the program as it’s class name.

Javac is the compiler of Java Programming Language. Obviously you should have Java installed and environment variable set. Installing Java on RPM based system is just a click away as on Windows and more or less on Debian based system.

However Debian Wheezy don’t have Java in its repo. And it is a little messy to install Java in Wheezy. Hence a quick step to install on debian is as below:

Installing Java in Debian Wheezy

Download correct Java version for your System and architecture from here:

1. http://www.oracle.com/technetwork/java/javase/downloads/index.html

Once you’ve downloaded , use the following commands to install in Debian Wheezy.

# mv /home/user_name/Downloads /opt/
# cd /opt/
# tar -zxvf jdk-7u3-linux-x64.tar.gz
# rm -rf jdk-7u3-linux-x64.tar.gz
# cd jdk1.7.0_03
# update-alternatives --install /usr/bin/java java /opt/jdk1.7.0_03/bin/java 1
# update-alternatives --install /usr/bin/javac javac /opt/jdk1.7.0_03/bin/javac 1
# update-alternatives --install /usr/lib/mozilla/plugins/libjavaplugin.so mozilla-javaplugin.so /opt/jdk1.7.0_03/jre/lib/amd64/libnpjp2.so 1
# update-alternatives --set java /opt/jdk1.7.0_03/bin/java
# update-alternatives --set javac /opt/jdk1.7.0_03/bin/javac
# update-alternatives --set mozilla-javaplugin.so /opt/jdk1.7.0_03/jre/lib/amd64/libnpjp2.so

For RHEL, CentOS and Fedora users can also install latest version of Java by going to below url.

1. Install Java in RHEL, CentOS and Fedora

Let’s move to programming section to learn few basic Java programs.

Program 1: hello.java

class hello{
public static void main (String args[]){
System.out.println("Sucess!");
}
}

Save it as: hello.java. And Compile it and run as shown.

# javac hello.java
# java hello

Sample Output

Sucess!

Program 2: calculation.java

class calculation { 
public static void main(String args[]) { 
int num; 
num = 123;
System.out.println("This is num: " + num); 
num = num * 2; 
System.out.print("The value of num * 2 is "); 
System.out.println(num); 
} 
}

Save it as: calculation.java. And Compile it and run as shown.

# javac calculation.java
# java calculation

Sample Output

This is num: 123
The value of num * 2 is 246

Do it Yourself:

1. Write a program that ask for your first name and last name and then address you with your last name.
2. Write a program with three Integer values and perform addition, Substraction, Multiplication and Divisionand gets the custom output.

Note: This way of learning will make you know and learn something. However if you face problem in writing programs of ‘Do it Yourself‘ you can come up with your codes and problems in comments.

Day to Day: Learning Java Programming Language – Part 2

Moving a step ahead of the previous article on Day-to-Day: Java Programming Part – I. Here in this very post we will be learning control statements and loops in Java, which is very useful in developing an application.

if statement

The if statement in Java works similar to if statement in any other programming language of the world including shell scripting.

Program 3: compare.java

class compare{ 
public static void main(String args[]){ 
int a,b; 
a=10; 
b=20; 
if(a < b)  
System.out.println("a(" +a+ ")is less than b(" +b+")");  
a=a*2;  
if(a==b)  
System.out.println("a(" +a+ ")is equal to b(" +b+")");  
a=a*2;  
if(a>b) 
System.out.println("a(" +a+ ")is greater than b(" +b+")"); 
} 
}

Save it as: compare.java. And Compile it and run as shown.

# javac compare.java
# java compare

Sample Output

a(10)is less than b(20) 
a(20)is equal to b(20) 
a(40)is greater than b(20)

Note: In the above program

1. A class namely compare is defined.
2. Two Integers are declared with the initial value of 10 and 20 respectively.
3. The if statement checks the condition and act according to the statement. Syntax of if statement is if (condition) statement;
4. System.out.println prints anything and everything that is placed between double quotes. Anything within the quotes are printed as it is, and outside of quotes are treated as variable.
5. + is a concatenation, which is used to concatenate two parts of a statement.

for loop

If you have any programming experience, sure you would be aware of the importance of loop statements. Here again the for loop statement works similar to the for statement in any language.

Program4: forloop.java

class forloop{ 
public static void main(String args[]){ 
int q1; 
for (q1=0; q1<=10; q1++) 
System.out.println("The value of interger: "+q1); 
} 
}

Save it as: forloop.java. And Compile it and run as shown.

# javac forloop.java
# java forloop

Sample Output

Output:
The value of interger: 0 
The value of interger: 1 
The value of interger: 2 
The value of interger: 3 
The value of interger: 4 
The value of interger: 5 
The value of interger: 6 
The value of interger: 7 
The value of interger: 8 
The value of interger: 9 
The value of interger: 10

Note: In the above program all the statements and codes are more or less identical to the above program, except the for statement.

1. The above for statement is a loop, which continues to execute again and again till the conditions are satisfied.
2. The for loop, generally is divided in three chunks of codes separated by semicolon, each of which is very meaningful.
3. The first part (q1=0, in the above program) is called initialiser. i.e., the above integer, q1 is forced to start with ‘0‘.
4. The second part (q1<=10, in the above program) is called condition. i.e., the above integer is permitted to go up-to the value of 10 or less than 10, which ever is correct for the given situation.
5. The Third and the last part (q1++, in the above code, which may be written as q+1) is called iteration.i.e., the above integer value is asked to increase with a value of ‘+1‘ every time the loop is executed, till the condition is satisfied.

Well the above program has only one linked statement to the ‘for loop‘. But in larger and more sophisticated program the loop statement could be linked to more than one statement or say a block of codes.

Program 5: loopblock.java

class loopblock{ 
	public static void main(String args[]){ 
		int x, y=20;		 
		for(x=0;x<20;x=x+2) 
		{ 
		System.out.println("x is: "+x); 
		System.out.println("y is: "+y); 
		y=y-2; 
} 
} 
}

Save it as: loopblock.java. And Compile it and run as shown.

# javac loopblock.java
# java loopblock

Sample Output

x is: 0 
y is: 20 
x is: 2 
y is: 18 
x is: 4 
y is: 16 
x is: 6 
y is: 14 
x is: 8 
y is: 12 
x is: 10 
y is: 10 
x is: 12 
y is: 8 
x is: 14 
y is: 6 
x is: 16 
y is: 4 
x is: 18 
y is: 2

Note: The above program is almost the same as the previous program, except it uses a block of codes linked with for loop. To execute more than one statement/block, we need to put all the statement as “{….codes/block..}” else the code won’t compile correctly.

Yeah we can use ‘x- –‘ or ‘x-1‘ for decrease statement in for loop where required.

After getting a glimpse of whole lot of codes, we need to know a little theory which will be helpful in the later stage of coding’s.

What we have seen till now is: Java programs are a collection of Whitespaces, identifiers, comments, literals, operators, separators and keywords.

Whitespace

Java is a free form language, you need not follow any indentation rule. You could write all the codes on a single line with one whitespace between each token and it will execute correctly. However it will be difficult to understand.

Identifiers

In Java identifiers are class name, method name or variable name. It could be uppercase, lowercase, their sequence or a combination of all of these with special characters like ‘$‘. However identifiers should never start with a numerical values.

Examples of valid identifiers in Java:

s4, New#class, TECmint_class, etc.

Literals

A constant value in Java is created using literals. e.g., ‘115′ is an integer literal. ‘3.14‘ is a float literal, ‘X‘ is a character constant and “tecmint is the best online site dedicated to foss technology” is a string literal.

Comment

comment has nothing to do with the execution of codes in Java or any other language, however comment in between the codes make them readable and human understandable. It is a good practice to write comments in between the lines of code, where required.

In Java anything between /** and **/ is meant for documentation and is a comment.

Certain separators are defined in Java.

1. Parenthesis ()
2. Braces {}
3. Brackets []
4. Semicolon ;
5. comma ,
6. Period .

Note: Each separator has a meaning and needs to be used where required, You can’t use one in place of other. We will discuss them in details, in the later phase of codes itself.

Keywords

There are 50 reserved keywords defined in Java. These keywords can not be used as names for a variable, class or method as these keywords has predefined meaning.

abstract	continue	for	          new	        switch
assert	        default	        goto	          package	synchronized
boolean	        do	        if	          private	this
break   	double	        implements	  protected	throw
byte	        else	        import	          public	throws
case	        enum	        instanceof	  return	transient
catch	        extends	        int	          short	        try
char	        final	        interface	  static	void
class	        finally	        long	          strictfp	volatile
const	        float	        native	          super	        while

The keyword cons and keywords are reserved but not used. Feeling nervous with all these stuffs. You actually don’t need to be nervous, neither you need to memorise all these stuffs. You will be used to all these when you start living Java.

That’s all for now from me.

Source

Installing Netbeans and Java JDK in Ubuntu and Setting Up a Basic HTML5 Project

In this 4-article mobile web development series, we will walk you through setting up Netbeans as an IDE (also known as Integrated Development Environment) in Ubuntu 14.04.2 LTS Trusty Tahr to start developing mobile-friendly and responsive HTML5 web applications.

Following are the 4-article series about HTML5 Mobile Web Development:

A well-polished work environment (as we will later see), autocompletion for supported languages, and its seamless integration with web browsers are, in our opinion, some of Netbeans, most distinguishing features.

Let us also remember that the HTML 5 specification brought many advantages for developers – to name a few examples: cleaner code thanks to many new elements), built-in video and audio playback capabilities (which replaces the need for Flash), cross-compatibility with major browsers, and optimization for mobile devices.

Although we will initially test our applications on our local development machine, we will eventually move our web site to a LAMP server and turn it into a dynamic tool.

Along the way we will make use of jQuery (a well-known cross-platform Javascript library that greatly simplifies client-side scripting), and of Bootstrap (the popular HTML, CSS, and JavaScript framework for developing responsive websites). You will see in coming articles how easy it is to set up a mobile-friendly application using these HTML 5 tools.

After you go through this brief series, you will be able to:

1. use the tools described herein to create basic HTML5 dynamic applications, and
2. go on to learn more advanced web development skills.

However, please note that even though we will be using Ubuntu for this series, the instructions and procedures are perfectly valid for other desktop distributions as well (Linux Mint, Debian, CentOS, Fedora, you name it).

To that end, we have chosen to install the necessary software (Netbeans and the Java JDK, as you will see in a minute) using a generic tarball (.tar.gz) as installation method.

That being said – let’s get started with Part 1.

Installing Java JDK and NetBeans

This tutorial assumes that you already have an Ubuntu 14.04.2 LTS Trusty Tahr desktop installation in place. If you don’t, please refer to Ubuntu 14.04 Desktop Installation article, written by our colleague Matei Cezar before proceeding further.

Since the Netbeans version that is available for download from the Ubuntu official repositories (7.0.1) is a little outdated, we will download the package from the Oracle website to get a newer version (8.0.2).

To do this, you have two choices:

1. Choice 1: download the bundle that includes Netbeans + JDK, or
2. Choice 2: install both utilities separately.

In this article we will choose #2 because that not only means a download that is a bit smaller (as we will only install Netbeans with support for HTML5 and PHP), but also will allow us to have a standalone JDK installer should we need it for another setting that does not require Netbeans nor involve web development (mostly related to other Oracle products).

To download JDK 8u45, go to the Oracle Technology Network site and navigate to the Java → Java SE → Downloads section.

When you click on the image highlighted below, you will be asked to accept the license agreement and then you will be able to download the necessary JDK version (which in our case is the tarball for 64-bit machines). When prompted by your web browser, choose to save the file instead of opening it.

When the download is complete, go to ~/Downloads and extract the tarball to /usr/local/bin:

$ sudo tar xf jdk-8u45-linux-x64.tar.gz -C /usr/local/bin

To install Netbeans with support for HTML5 and PHP, go to https://netbeans.org/downloads/ and click Download as indicated in the following image:

This will cause your browser to either open the installation shell script or save it to your computer. Choose Save File, then OK:

Once done, turn the .sh into an executable file and then run the shell script with administrative privileges:

$ cd ~/Downloads
$ chmod 755 netbeans-8.0.2-php-linux.sh
$ sudo ./netbeans-8.0.2-php-linux.sh --javahome /usr/local/bin/jdk1.8.0_45

From then on, follow the on-screen instructions to complete the installation leaving the default values:

and wait for the installation to complete.

!DOCTYPE html>
<html>
	<head>
    	<title>TODO supply a title</title>
    	<meta charset="UTF-8">
    	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	</head>
	<body>
    	<header style="background-color: #6699CC">THIS IS A HEADER</header>
    	<main>
        	<article>
            	<p>This is some sample text.</p>
            	<p>Another line of sample text for this HTML 5 article</p>
                	<aside>
                    	<figure>
                        	<img src="https://www.w3.org/html/logo/downloads/HTML5_Logo_256.png" alt="HTML 5 logo" />
                        	<figcaption>Figure 1: The HTML 5 logo</figcaption>
                    	</figure>
                        	<h2>Web development basics series at Tecmint.com</h2>
                        	<h3><a href="http://dev.w3.org/html5/html-author/">This is HTML 5!</a></h3>
                        	<p>Some text here</p>
                	</aside>
        	</article>
    	</main>
    	<footer style="background-color: #CC6699">THIS IS A FOOTER</footer>
	</body>
</html>

Adding References

That sure looks good, but we still haven’t told our index.html file to use any of those files. For the sake of simplicity, we will replace the contents of that file with a barebones html file first:

<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<title>jQuery and Bootstrap</title>
</head>
<body>
 
   <!-- // Your code will appear here. -->

</body>
</html>

Then, just drag and drop each file from the project navigator section to the code, inside the </head> tags, as you can see in the following screencast. Make sure that the reference to jQuery appears before the reference to Bootstrap because the latter depends on the former:

That’s it – you have added the references to both jQuery and Bootstrap, and can now start writing code.

Writing Your First Responsive Code

Let’s now add a navigation bar and place it at the top of our page. Feel free to include 4-5 links with dummy text and don’t link it to any document for the time being – just insert the following code snippet inside the body of the document.

Don’t forget to spend some time become acquainted with the auto-completion feature in Netbeans, which will show you the classes made available by Bootstrap as you start typing.

At the heart of the code snippet below is the Bootstrap container class, which is used to place content inside of a horizontal container which will automatically resize depending on the size of the screen where it is being viewed. Not less important is the container-fluid class, which will ensure that the content within will occupy the entire width of the screen.

<div class="container">
  	<nav class="navbar navbar-default">
    	<div class="container-fluid">
      	<div class="navbar-header">
        	<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
          	<span class="sr-only">Toggle navigation</span>
          	<span class="icon-bar"></span>
          	<span class="icon-bar"></span>
          	<span class="icon-bar"></span>
        	</button>
        	<a class="navbar-brand" href="#">Project name</a>
      	</div>
      	<div id="navbar" class="navbar-collapse collapse">
        	<ul class="nav navbar-nav">
          	<li class="active"><a href="#">Home</a></li>
          	<li><a href="#">About</a></li>
          	<li><a href="#">Contact</a></li>
          	<li class="dropdown">
            	<a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
            	<ul class="dropdown-menu">
              	<li><a href="#">Action</a></li>
              	<li><a href="#">Another action</a></li>
              	<li><a href="#">Something else here</a></li>
              	<li role="separator" class="divider"></li>
              	<li class="dropdown-header">Nav header</li>
              	<li><a href="#">Separated link</a></li>
              	<li><a href="#">One more separated link</a></li>
            	</ul>
          	</li>
        	</ul>
      	</div><!--/.nav-collapse -->
    	</div><!--/.container-fluid -->
  	</nav>
</div>

Another distinguishing feature of Bootstrap is that it eliminates the need for tables in HTML code. Instead, it uses a grid system to layout content and make it look properly both on large and small devices (from phones all the way to big desktop or laptop screens).

In Bootstrap’s grid system, the screen layout is divided in 12 columns:

A typical setup consists of using the 12-column layout divided into 3 groups of 4 columns each, as follows:

To indicate this fact in code, and in order to have it displayed that way starting in medium-size devices (such as laptops) and above, add the following code below the closing </nav> tag:

...
    </nav>
   	 <div class="row">
   	 	<div class="col-md-4">This is the text in GROUP 1</div>
   	 	<div class="col-md-4">This is the text in GROUP 2</div>
   	 	<div class="col-md-4">This is the text in GROUP 3</div>
   	 </div>
</div> <!--Closing tag of the container class -->

You must have probably noticed that the column classes in the Bootstrap grid indicate the starting layout for a specific device size and above, as md in this example stands for medium (which also covers lg, or large devices).

For smaller devices (sm and xs), the content divs gets stacked and appears one above the next.

In the following screencast you can see how your page should look by now. Note that you can resize your browser’s window to simulate different screen sizes after launching the project using the Run project button as we learned in Part 1.

Summary

Congratulations! You must have written a simple, yet functional, responsive page by now. Don’t forget to check the Bootstrap website in order to become more familiar with the almost-limitless functionality of this framework.

As always, in case you have a question or comment, feel free to contact us using the form below.

Creating a Dynamic HTML5 Web Application and Deploying on Remote Web Server Using Filezilla

In the previous two articles of this series, we explained how to set up Netbeans in a Linux desktop distribution as an IDE to develop web applications. We then proceeded to add two core components, jQuery and Bootstrap, in order to make your pages mobile-friendly and responsive.

1. Install Netbeans and Java to Create a Basic HTML5 Application – Part 1
2. Creating Mobile-Friendly and Responsive Web Application Using jQuery and Bootstrap – Part 2

As you will seldom deal with static content as a developer, we will now add dynamic functionality to the basic page that we set up in Part 2. To begin, let us list the prerequisites and address them before moving forward.

Prerequisites

In order to test a dynamic application in our development machine before deploying it to a LAMP server, we will need to install some packages. Since we are using a Ubuntu 14.04 desktop to write this series, we assume that your user account has already been added to the sudoers file and granted the necessary permissions.

Installing Packages and Configuring Access to the DB Server

Please note that during the installation you may be prompted to enter a password for the MySQL root user. Make sure you choose a strong password and then continue.

Ubuntu and derivatives (also for other Debian-based distributions):

$ sudo aptitude update && sudo aptitude install apache2 php5 php5-common php5-myqsql mysql mysql-server filezilla

Fedora / CentOS / RHEL:

$ sudo yum update && sudo yum install httpd php php-common php-mysql mysql mysql-server filezilla

When the installation is complete, it is strongly recommended that you run mysql_secure_installation to, not surprisingly, secure your database server. You will be prompted for the following information:

1. Change the root password? [Y/n]. If you already set a password for the MySQL root user, you can skip this step.
2. Remove anonymous users? [Y/n] y.
3. Disallow root login remotely? [Y/n] y (Since this is your local development environment, you will not need to connect to your DB server remotely).
4. Remove test database and access to it? [Y/n] y
5. Reload privilege tables now? [Y/n] y.

Creating a sample Database and Loading test Data

To create a sample database and load some test data, log on to your DB server:

$ sudo mysql -u root -p

You will be be prompted to enter the password for the MySQL root user.

At the MySQL prompt, type

CREATE DATABASE tecmint_db;

and press Enter:

Now let’s create a table:

USE tecmint_db;
CREATE TABLE articles_tbl(
   Id INT NOT NULL AUTO_INCREMENT,
   Title VARCHAR(100) NOT NULL,
   Author VARCHAR(40) NOT NULL,
   SubmissionDate TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
   PRIMARY KEY ( Id )
);

and populate it with sample data:

INSERT INTO articles_tbl (Title, Author) VALUES ('Installing Filezilla in CentOS 7', 'Gabriel Canepa'), ('How to set up a LAMP server in Debian', 'Dave Null'), ('Enabling EPEL repository in CentOS 6', 'John Doe');

Adding symbolic links in the Web Server directory

Since Netbeans, by default, stores projects in the current user’s home directory, you will need to add symbolic links that point to that location. For example,

$ sudo ln -s /home/gabriel/NetBeansProjects/TecmintTest/public_html /var/www/html/TecmintTest

will add a soft link called TecmintTest that points to /home/gabriel/NetBeansProjects/TecmintTest/public_html.

For that reason, when you point your browser to http://localhost/TecmintTest/, you will actually see the application that we set up in Part 2:

Setting up a remote FTP and Web server

Since you can easily set up a FTP and Web server with the instructions provided in Part 9 – Install and Configure Secure FTP and Web Server of the RHCSA series in Tecmint, we will not repeat them here. Please refer to that guide before proceeding further.

Turning our application into a Dynamic One

You will probably think that we can’t do much with the sample data that we added to our database earlier, and you are right, but it will be enough to learn the basics of embedding PHP code and the results of queries to a MySQL DB in your HTML5 pages.

First off, we will need to change the extension of the main document of our application to .php instead of html:

# mv /var/www/html/TecmintTest/index.html /var/www/html/TecmintTest/index.php

Then let’s open the project in Netbeans and start doing some modifications.

1. Add a folder to the project named includes where we will store backend php applications.

2. Create a file named dbconnection.php inside includes and insert with the following code:

<?php
    $host = "localhost";
    $username = "root";
    $password = "MyFancyP4ssw0rd";
    $database = "tecmint_db";

    //Establish a connection with MySQL server
    $mysqli = new mysqli($host, $username, $password,$database);

    /* Check connection status. Exit in case of errors. */
    if (mysqli_connect_errno()) {
        printf("Connect failed: %s\n", mysqli_connect_error());
        exit();
    }
    $mysqli -> query("SET character_set_results = 'utf8', character_set_client = 'utf8', character_set_connection = 'utf8', character_set_database = 'utf8', character_set_server = 'utf8'");

    $records = array();
    $query = "SELECT Title, Author, SubmissionDate FROM articles_tbl;";
    $result = $mysqli->query($query) or die($mysqli->error);
    $data = array();

    while ( $row = $result->fetch_assoc() ){
        $data[] = json_encode($row);
    }
    echo json_encode( $data );
?>

as indicated in the following image:

This file will be used to connect to the database server, to query it, and to return the results of that query in a JSON-like string to be consumed by the frontend application with a slight modification.

Note that typically you would use separate files to perform each of these operations, but we chose to include all of that functionality in one file for sake of simplicity.

3. In index.php, add the following snippet just beneath the opening body tag. That is the jQuery way of calling an external PHP app when the web document is ready, or in other words, each time it loads:

<script>
    $(document).ready(function(){
        $.ajax({
            url: 'includes/dbconnection.php',
            datatype: 'json',
            type: 'POST',
            success: function(data){
                var output = $.parseJSON(data);
                for(var i =0;i < output.length;i++)
                {
                  var item = output[i];
                  $("#title").append("<br>"+item.Title);
                  $("#author").append("<br>"+item.Author);
                  $("#submissiondate").append("<br>"+item.SubmissionDate);
                }
            }}
        );
    });
</script>

4. Now, add an unique id (same as inside the for loop above) to each line in the div with class row at the bottom of index.php:

<div class="row">
    <div class="col-md-4" id="title" style="text-align: center"><strong>Titles</strong></div>
    <div class="col-md-4" id="author" style="text-align: center"><strong>Authors</strong></div>
    <div class="col-md-4" id="submissiondate" style="text-align: center"><strong>Published on:</strong></div>
</div>

If you now click Run Project, you should see this:

Which is essentially the same as the information returned when we ran the query from our MySQL client prompt earlier.

Deploying to a LAMP server using Filezilla

Launch Filezilla from the Dash menu and enter the IP of the remote FTP server and your credentials. Then click Quickconnect to connect to the FTP server:

Navigate to /home/gabriel/NetBeansProjects/TecmintTest/public_html/, select its contents, right click on them and select Upload.

This, of course, assumes that the remote user indicated in Username has write permissions on the remote directory. When the upload is complete, point your browser to the desired location and you should see the same page as before (please note that we have not set up the local MySQL database to the remote host, but you can easily do so following the steps from the beginning of this tutorial).

Summary

In this article we have added dynamic functionality to our web application using jQuery and a little JavaScript. You can refer to the official jQuery docs for more information, which will be very helpful if you decide to write more complex applications. Wrapping up, we have also deployed our application to a remote LAMP server using a FTP client.

We are excited to hear your opinion about this article – feel free to contact us using the form below.

Tuning Dynamic HTML5 Web Apps Using Open Source Online Tools

As I begin the last article in this series, it is my hope that you have been able to grasp the importance of HTML 5 and mobile-friendly / responsive web development. Regardless of your desktop distribution of choice, Netbeansis a powerful IDE and when used together with basic Linux command-line skills and the tools discussed in Part 3, can help you to create outstanding applications without much hassle.

However, please note that we have only covered the basics of HTML 5 and web development in this series and assumed that you are somewhat familiar with HTML, but the WWW is full of great resources – some of them are FOSS – to expand on what we’ve shared here.

In this last guide we will talk about some of those tools and show you how to use them to add to the existing page we have been working on Beautifying our UI (user interface).

You will recall from Part 2 of this series (“Adding jQuery and Bootstrap to Write a HTML5 Web Application“) that the Bootstrap zip file comes with a directory named fonts. We saved its contents into a folder with the same name inside our project’s SiteRoot:

As you probably can tell from the above image, Bootstrap includes a set of elements called glyphicons, which are no more and no less the built-in components that provide nice-looking icons for buttons and menus in your applications. The complete list of glyphicons included in Bootstrap is available at http://getbootstrap.com/components/.

To illustrate the use of glyphicons, let’s add some to the navigation bar in our main page. Modify the navigation bar menus as follows. Please note the space between each closing span tag and the menu text:

<li class="active"><a href="#"><span class="glyphicon glyphicon-home" aria-hidden="true"></span> Home</a></li>
<li><a href="#"><span class="glyphicon glyphicon-info-sign" aria-hidden="true"></span> About</a></li>
<li><a href="#"><span class="glyphicon glyphicon-envelope" aria-hidden="true"></span> Contact</a></li>

(by the way, the span tags are used here to prevent the icons from getting mixed with other components).

And here’s the result:

Glyphicons, though useful, are also limited. And here’s where Font Awesome enters the scene. Font Awesome is an icon / font / css complete toolkit that has the potential to seamlessly integrate with Bootstrap.

Not only you can add a whole lot of other icons to your pages, but can also resize them, cast shadows, change color, and a many other options using CSS. However, since dealing with CSS is out of the scope of this series, we will only deal with the default-sized icons but encourage you at the same time to “dig a little deeper” to discover how far this tool can take you.

To download Font Awesome and incorporate it into your project, execute the following commands (or feel free to go directly to the project’s web site and download the zip file through your browser and decompress it using GUI tools):

# wget http://fortawesome.github.io/Font-Awesome/assets/font-awesome-4.3.0.zip

(yes, the domain name is actually FortAwesome, with an R, so that is not a typo).

# unzip font-awesome-4.3.0.zip
# cp font-awesome-4.3.0/css/font-awesome.min.css /home/gabriel/NetBeansProjects/TecmintTest/public_html/styles
# cp font-awesome-4.3.0/fonts/* /home/gabriel/NetBeansProjects/TecmintTest/public_html/fonts

And add the .css file to the references list at the top of our page, just like we did with jQuery and Bootstrapearlier (remember that you don’t have to type everything – just drag the file from the Projects tab into the code window):

Let’s take the dropdown list in our navigation bar, for example:

Nice, right? All it takes is replacing the contents of the existing ul class named dropdown-menu at the bottom of index.php with:

<li><a href="#"><i class="fa fa-pencil fa-fw"></i> Edit</a></li>
<li><a href="#"><i class="fa fa-trash-o fa-fw"></i> Delete</a></li>
<li><a href="#"><i class="fa fa-ban fa-fw"></i> Ban</a></li>
<li class="divider"></li>
<li><a href="#"><i class="i"></i> Make admin</a></li>

Believe me – investing your time in learning how to use these tools will be a very rewarding experience.

Where to Ask for Help

As an IT person, you must be well acquainted with the many resources for help the Internet has made available. Since doing web development is not an exception, here are a few resources that we’re sure you will find useful while tuning your applications.

When dealing with Javascript code (for example, when working with jQuery as we did in Part 2), you will want to use JSHint, an online Javascript quality code checker that aims at helping developers to detect errors and potential problems. When those pitfalls are found, JSHint indicates the line number where they are located and gives you hints to fix them:

That surely looks great, but even with this great automated tool, there will be times when you will need someone else to take a look at your code and tell you how to fix it or otherwise improve it, which implies sharing it somehow.

JSFiddle (an online Javascript / CSS / HTML code tester) and Bootply (same as JSFiddle but specialized in Bootstrap code) let you save code snippets (also known as fiddles) and provide you a link to share them very easily over the Internet (either via email with your friends, using your social network profiles, or in forums).

Summary

In this article we have provided you with a few tips to tune your web applications and shared some resources that will come in handy if you get stuck or want another pair of eyes (and not just one, but many) to take a look at your code to see how it can be improved. Chances are that you may know of other resources as well. We hope that this series have given you a glimpse of the vast possibilities of mobile-friendly and responsive web development.

Source

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Here at SUSE, we’re very excited to let you know that the latest version of SUSE OpenStack Cloud is due to be released later this month. In fact, you might say that we’re on cloud 9.

The main event

Now that I have that dreadful pun out of the way, let me tell you a little bit about this release. Designed and engineered to take the pain out of implementing a software-defined infrastructure, SUSE OpenStack Cloud enables customers to transform their IT infrastructure so they can meet the business challenges of today and tomorrow. Based on the OpenStack Rocky release, SUSE OpenStack Cloud 9 delivers enhanced agility and improved time to value, while simplifying the transition of traditional workloads. New features and improved functionality include:

• Improved agility with a new user interface that simplifies post-deployment cloud operations. The new day two UI, CLM Admin Console simplifies post-deployment cloud operations for customers choosing the CLM installation of SUSE OpenStack Cloud, through its familiar interface.
• Simplifying the transition of traditional workloads to OpenStack private cloud and maximizing the value of your existing IT investments. Businesses can now choose to customize their bare metal servers for specific workload performance and use case needs, further simplifying the transition of traditional or business-critical workloads to their OpenStack private cloud.
• Improved time to value by reducing the complexity of delivering an enterprise-grade software-defined infrastructure. Powered by the OpenStack Rocky release and built on SUSE Linux Enterprise 12 SP4, SUSE OpenStack Cloud takes the complexity out of deploying a mature, stable and robust private cloud that has been designed for mission-critical workloads.

Moving to a software-defined infrastructure is a key part of many organizations’ digital transformation, and increasing numbers of businesses are choosing SUSE OpenStack Cloud to power their software-defined infrastructure. It gives them the business agility they need to compete in today’s market, as well as enabling them to make better use of their existing IT infrastructure for cost benefit, while taking advantage of new business opportunities and rapidly evolving technology trends such as DevOps, containers and AI.

The winning combination

Combining SUSE OpenStack Cloud with SUSE Enterprise Storage makes it easier for businesses to build a software-defined infrastructure with cost-effective, near-infinite storage scalability. Those companies concerned about an IT skills gap delaying their digital transformation may be interested in SUSE Select Services. Offering a 12-month, fixed-price solution combining implementation, consulting, knowledge transfer and premium support to jumpstart their private cloud investment and help them to realize the benefits sooner.

Gonna fly now

One thing’s for sure, SUSE OpenStack Cloud 9 powered by OpenStack Rocky can keep your business going the distance through their IT transformation, and with SUSE Select Services to support them, your internal IT teams won’t be holding out for a hero with a burning heart. Before the final countdown commences though, I’m gonna fly now as there’s no easy way out of these terrible attempts at puns. What’s your favourite tune from the Rocky series of films? In my view, they’re all great and the perfect addition to my workout playlist, but Gonna Fly Now always helps me push a little bit harder on the treadmill!

If you’re visiting Nashville for SUSECON, then please come and say hello to hear more about this upcoming release. We’ll also be at the inaugural Open Infrastructure Summit in Denver at the end of April, so come along there to hear more about SUSE OpenStack Cloud 9 and to hear about how it could help you to take the stress out of your software-defined infrastructure.

Note: OpenStack Rocky is not affiliated with, supported or endorsed by Rocky Balbao. No copyright infringement intended. Other Rockys are available – see Rocky Mountains, Rocky Road, Rocky the Flying Rooster, Rocky and Bullwinkle and Rocky Horror for more details.

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11 Basic Linux Interview Questions and Answers

Theories apart, we are proud to announce a new section on Tecmint, dedicated to Linux Interview. Here we will bring to you Linux Interview Questions and all other aspects of Linux, which is must for a professional in this cut-throat competition world.

A new article in this section (Linux Interview) will be posted on every weekend. The initiative taken by Tecmint is first of it’s kind among other Linux Dedicated websites, along with quality and unique articles.

We will start with Basic Linux Interview Question and will go advance article by article, for which your response is highly appreciated, which put us on a higher note.

Q.1: What is the core of Linux Operating System?

1. Shell
2. Kernel
3. Command
4. Script
5. Terminal

Q.2: What Linus Torvalds Created?

1. Fedora
2. Slackware
3. Debian
4. Gentoo
5. Linux

Q.3: Torvalds, Wrote most of the Linux Kernel in C++ programming Language, do you agree?

The remaining Lines of code 1,972,615 is written in C++, Assembly, Perl, Shell Script, Python, Bash Script, HTML, awk, yacc, lex, sed, etc.

Note : The Number of Lines of codes varies on daily basis and an average of more than 3,509 lines are being added to Kernel.

Q.4: Linux initially was developed for intel X86 architecture but has been ported to other hardware platform than any other Operating System. Do you agree?.

Q.5: Is it legal to edit Linux Kernel?

Q.6: What is the basic difference between UNIX and Linux Operating System.

Q. 7: Choose the odd one out.

1. HP-UX
2. AIX
3. OSX
4. Slackware
5. Solaris

Q.8: Is Linux Operating system Virus free?

Q.9: Linux is which kind of Operating System?

1. Multi User
2. Multi Tasking
3. Multi Process
4. All of the above
5. None of the above

Q.10: Syntax of any Linux command is:

1. command [options] [arguments]
2. command options [arguments]
3. command [options] [arguments]
4. command options arguments

Q.11: Choose the odd one out.

1. Vi
2. vim
3. cd
4. nano

That’s all for now. How much you learned for the above questions? How it helped you in your Interview? We would like to hear all these from you in our comment section. Wait till the next weekend, for new set of questions.

Basic Linux Interview Questions and Answers – Part II

Continuing the Interview Series, we are giving 10 Questions here, in this article. These questions and the questions in the future articles doesn’t necessarily means they were asked in any interview. We are presenting you an interactive learning platform through these kind of posts, which surely will be helpful.

Upon the analysis of comments in different forums on last article 11 Basic Linux Interview Questions of this series, it is important to mention here that to bring up a quality article to our readers. We give our time and money, and in return what we expect from you? Nothing. If you can’t praise our work, please don’t demoralize us from your negative comments.

If you find nothing new in a post, don’t forget that for someone it was helpful, and for that he/she was thankful. We can’t make everyone happy in each of our article. Hope you readers would take pain to understand this.

Q.1: Which command is used to record a user login session in a file?

1. macro
2. read
3. script
4. record
5. sessionrecord

Let’s record the user’s login session with script command as shown.

[root@tecmint ~]# script my-session-record.txt

Script started, file is my-session-record.txt

The content of log file ‘my-session-record.txt’ can be views as:

[root@tecmint ~]# nano my-session-record.txt

script started on Friday 22 November 2013 08:19:01 PM IST
[root@tecmint ~]# ls
^[[0m^[[01;34mBinary^[[0m ^[[01;34mDocuments^[[0m ^[[01;34mMusic^[[0m $
^[[01;34mDesktop^[[0m ^[[01;34mDownloads^[[0m my-session-record.txt ^[[01;34$

Q.2: The kernel log message can be viewed using which of the following command?

1. dmesg
2. kernel
3. ls -i
4. uname
5. None of the above

[root@tecmint ~]# dmesg

Initializing cgroup subsys cpuset
Initializing cgroup subsys cpu
Linux version 2.6.32-279.el6.i686 (mockbuild@c6b9.bsys.dev.centos.org) (gcc version 4.4.6 20120305 (Red Hat 4.4.6-4) (GCC) ) #1 SMP Fri Jun 22 10:59:55 UTC 2012
KERNEL supported cpus:
  Intel GenuineIntel
  AMD AuthenticAMD
  NSC Geode by NSC
  Cyrix CyrixInstead
  Centaur CentaurHauls
  Transmeta GenuineTMx86
  Transmeta TransmetaCPU
  UMC UMC UMC UMC
Disabled fast string operations
BIOS-provided physical RAM map:
...

Q.3: Which command is used to display the release of Linux Kernel?

1. uname -v
2. uname -r
3. uname -m
4. uname -n
5. uname -o

[root@tecmint ~]# uname -r

2.6.32-279.el6.i686

Q.4: Which command is used to identify the types of file?

1. type
2. info
3. file
4. which
5. ls

[root@tecmint ~]# file wtop

wtop: POSIX shell script text executable

Q.5: Which command locate the binary, source and man page of a command?

[root@tecmint ~]# whereis /usr/bin/ftp

ftp: /usr/bin/ftp /usr/share/man/man1/ftp.1.gz

Q.6: When a user login, which files are called for user profile, by default??

[root@tecmint ~]# ls -al
-rw-r--r--.  1 tecmint     tecmint            176 May 11  2012 .bash_profile
-rw-r--r--.  1 tecmint     tecmint            124 May 11  2012 .bashrc

Q.7: The ‘resolv.conf’ file is a configuration file for?

[root@tecmint ~]# cat /etc/resolv.conf

nameserver 172.16.16.94

Q.8: Which command is used to create soft link of a file?

1. ln
2. ln -s
3. link
4. link -soft
5. None of the above

[root@tecmint ~]# ln -s /etc/httpd/conf/httpd.conf httpd.original.conf

Q.9: The command ‘pwd’ is an alias of command ‘passwd’ in Linux?

[root@tecmint ~]# pwd

/home/tecmint

[root@tecmint ~]# passwd
Changing password for user root.
New password:
Retype new password:

Q.10: How will you check pci devices vendor and version on a Linux?

[root@tecmint ~]# lspci

00:00.0 Host bridge: Intel Corporation 5000P Chipset Memory Controller Hub (rev b1)
00:02.0 PCI bridge: Intel Corporation 5000 Series Chipset PCI Express x8 Port 2-3 (rev b1)
00:04.0 PCI bridge: Intel Corporation 5000 Series Chipset PCI Express x8 Port 4-5 (rev b1)
00:06.0 PCI bridge: Intel Corporation 5000 Series Chipset PCI Express x8 Port 6-7 (rev b1)
00:08.0 System peripheral: Intel Corporation 5000 Series Chipset DMA Engine (rev b1)
...

That’s all for now. I hope these above questions might be very helpful to you. In our next weekend we again come-up with some new set of questions.

10 Linux Interview Questions and Answers for Linux Beginners – Part 3

Continuing the Interview Questions series, with a big thanks for the nice feedback on last two articles of this series, we are here presenting 10 questions again for interactive learning.

1. 11 Basic Linux Interview Questions and Answers – Part 1
2. 10 Basic Linux Interview Questions and Answers – Part II

1. How will you add a new user (say, tux) to your system.?

1. useradd command
2. adduser command
3. linuxconf command
4. All of the above
5. None of the above

2. How many primary partition is possible on one drive?

1. 1
2. 2
3. 4
4. 16

3. The default port for Apache/Http is?

1. 8080
2. 80
3. 8443
4. 91
5. None of the above.

4. What does GNU stand for?

1. GNU’s not Unix
2. General Unix
3. General Noble Unix
4. Greek Needed Unix
5. None of the above

5. You typed at shell prompt “mysql” and what you got in return was “can’t connect to local MySQL server through socket ‘/var/mysql/mysql.sock’”, what would you check first.

Note:The above error message can be the result of ill configured my.cnf or mysql user permission. If mysql service starting doesn’t help, you need to see into the above said issues.

6. How to Mount a windows ntfs partition on Linux?

7. From the following which is not an RPM based OS.?

1. RedHat Linux
2. Centos
3. Scientific Linux
4. Debian
5. Fedora

8. Which command can be used to rename a file in Linux.?

1. mv
2. ren
3. rename
4. change
5. None of the Above

9. Which command is used to create and display file in Linux?

1. ed
2. vi
3. cat
4. nano
5. None of the above

10. What layer protocol is responsible for user and the application program support such as passwords, resource sharing, file transfer and network management?

1. Layer 4 protocols
2. Layer 5 protocols
3. Layer 6 protocols
4. Layer 7 protocols
5. None of the above

That’s all for now. I will be writing on another useful topic soon.

15 Basic MySQL Interview Questions for Database Administrators

Prior to This Article, three articles has already been published in ‘Linux Interview‘ Section and all of them were highly appreciated by our notable readers, however we were receiving feedback to make this interactive learning process, section-wise. From idea to action, we are providing you 15 MySQL Interview Questions.

1. How would you check if MySql service is running or not?

root@localhost:/home/avi# service mysql status

/usr/bin/mysqladmin  Ver 8.42 Distrib 5.1.72, for debian-linux-gnu on i486
Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Server version 5.1.72-2
Protocol version 10
Connection Localhost via UNIX socket
UNIX socket /var/run/mysqld/mysqld.sock
Uptime: 1 hour 22 min 49 sec

Threads: 1  Questions: 112138  Slow queries: 1  Opens: 1485  Flush tables: 1  Open tables: 64  Queries per second avg: 22.567.

2. If the service is running/stop how would you stop/start the service?

root@localhost:/home/avi# service mysql stop

Stopping MySQL database server: mysqld.

root@localhost:/home/avi# service mysql start

Starting MySQL database server: mysqld.

Checking for corrupt, not cleanly closed and upgrade needing tables..

3. How will you login to MySQL from Linux Shell?

root@localhost:/home/avi# mysql -u root -p 
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g. 
Your MySQL connection id is 207 
Server version: 5.1.72-2 (Debian) 

Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved. 

Oracle is a registered trademark of Oracle Corporation and/or its 
affiliates. Other names may be trademarks of their respective 
owners. 

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. 

mysql>

4. How will you obtain list of all the databases?

mysql> show databases; 
+--------------------+ 
| Database           | 
+--------------------+ 
| information_schema | 
| a1                 | 
| cloud              | 
| mysql              | 
| phpmyadmin         | 
| playsms            | 
| sisso              | 
| test               | 
| ukolovnik          | 
| wordpress          | 
+--------------------+ 
10 rows in set (0.14 sec)

5. How will you switch to a database, and start working on that?

mysql> use cloud; 
Reading table information for completion of table and column names 
You can turn off this feature to get a quicker startup with -A 

Database changed 
mysql>

6. How will you get the list of all the tables, in a database?

mysql> show tables; 
+----------------------------+ 
| Tables_in_cloud            | 
+----------------------------+ 
| oc_appconfig               | 
| oc_calendar_calendars      | 
| oc_calendar_objects        | 
| oc_calendar_repeat         | 
| oc_calendar_share_calendar | 
| oc_calendar_share_event    | 
| oc_contacts_addressbooks   | 
| oc_contacts_cards          | 
| oc_fscache                 | 
| oc_gallery_sharing         | 
+----------------------------+ 
10 rows in set (0.00 sec)

7. How will you get the Field Name and Type of a MySql table?

mysql> describe oc_users; 
+----------+--------------+------+-----+---------+-------+ 
| Field    | Type         | Null | Key | Default | Extra | 
+----------+--------------+------+-----+---------+-------+ 
| uid      | varchar(64)  | NO   | PRI |         |       | 
| password | varchar(255) | NO   |     |         |       | 
+----------+--------------+------+-----+---------+-------+ 
2 rows in set (0.00 sec)

8. How will you delete a table?

mysql> drop table lookup; 

Query OK, 0 rows affected (0.00 sec)

9. What about database? How will you delete a database?

mysql> drop database a1; 

Query OK, 11 rows affected (0.07 sec)

10. How will you see all the contents of a table?

mysql> select * from engines; 
+------------+---------+----------------------------------------------------------------+--------------+------+------------+ 
| ENGINE     | SUPPORT | COMMENT                                                        | TRANSACTIONS | XA   | SAVEPOINTS | 
+------------+---------+----------------------------------------------------------------+--------------+------+------------+ 
| InnoDB     | YES     | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        | 
| MRG_MYISAM | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         | 
| BLACKHOLE  | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         | 
| CSV        | YES     | CSV storage engine                                             | NO           | NO   | NO         | 
| MEMORY     | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         | 
| FEDERATED  | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       | 
| ARCHIVE    | YES     | Archive storage engine                                         | NO           | NO   | NO         | 
| MyISAM     | DEFAULT | Default engine as of MySQL 3.23 with great performance         | NO           | NO   | NO         | 
+------------+---------+----------------------------------------------------------------+--------------+------+------------+ 
8 rows in set (0.00 sec)

11. How will you see all the data in a field (say, uid), from table (say, oc_users)?

mysql> select uid from oc_users; 
+-----+ 
| uid | 
+-----+ 
| avi | 
+-----+ 
1 row in set (0.03 sec)

12. Say you have a table ‘xyz’, which contains several fields including ‘create_time’ and ‘engine’. The field ‘engine’ is populated with two types of data ‘Memory’ and ‘MyIsam’. How will you get only ‘create_time’ and ‘engine’ from the table where engine is ‘MyIsam’?

12. mysql> select create_time, engine from xyz where engine="MyIsam";

+---------------------+--------+ 
| create_time         | engine | 
+---------------------+--------+ 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-12-15 13:43:27 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
| 2013-10-23 14:56:38 | MyISAM | 
+---------------------+--------+ 
132 rows in set (0.29 sec)

13. How will you show all the records from table ‘xrt’ where name is ‘tecmint’ and web_address is ‘tecmint.com’?

mysql> select  * from xrt where name = "tecmint" and web_address = “tecmint.com”;
+---------------+---------------------+---------------+ 
| Id                  | name                   | web_address | 
+---------------+---------------------+----------------+ 
| 13                 |  tecmint               | tecmint.com  |
+---------------+---------------------+----------------+ 
| 41                 |  tecmint               | tecmint.com  |
+---------------+---------------------+----------------+

14. How will you show all the records from table ‘xrt’ where name is not ‘tecmint’ and web_address is ‘tecmint.com’?

mysql> select * from xrt where name != ”tecmint” and web_address = ”tecmint.com”;

+---------------+---------------------+---------------+ 
| Id            | name                | web_address   | 
+---------------+---------------------+----------------+ 
| 1173          |  tecmint            | tecmint.com   |
+---------------+---------------------+----------------+

15. You need to know total number of row entry in a table. How will you achieve it?

mysql> select count(*) from Tables; 

+----------+ 
| count(*) | 
+----------+ 
|      282 | 
+----------+ 
1 row in set (0.01 sec)

Read Also : 10 MySQL Database Interview Questions Intermediates

That’s all for now. How you feel about this ‘Linux Interview Question‘ section. Don’t forget to provide us with your valuable feedback in our comment section.

25 Apache Interview Questions for Beginners and Intermediates

We are very thankful to All our readers for the response we are getting for our new Linux Interview section. And now we have started section wise learning for Interview questions and continuing with the same today’s article focuses on Basic to Intermediate Apache interview Questions that will help you to prepare yourself.

In this section, we have covered some interesting 25 Apache Job Interview Questions along with their answers so that you can easily understand some new things about Apache that you might never known before.

Before you read this article, We strongly recommend you to don’t try to memorize the answers, always first try to understand the scenarios on a practical basis.

1. What is Apache web server?

2. How to check Apache and it’s version?

[root@tecmint ~]# rpm -qa | grep httpd

httpd-devel-2.2.15-29.el6.centos.i686
httpd-2.2.15-29.el6.centos.i686
httpd-tools-2.2.15-29.el6.centos.i686

[root@tecmint ~]# httpd -v

Server version: Apache/2.2.15 (Unix)
Server built:   Aug 13 2013 17:27:11

3. Apache runs as which user? and location of main config file?.

4. On which port Apache listens http and https both?

[root@tecmint ~]# netstat -antp | grep http

tcp        0      0 :::80                       :::*                        LISTEN      1076/httpd          
tcp        0      0 :::443                      :::*                        LISTEN      1076/httpd

5. How do you install Apache Server on your Linux machine?

[root@tecmint ~]# yum install httpd

[root@tecmint ~]# apt-get install apache2

6. Where you can find all configuration directories of Apache Web Server?

[root@tecmint ~]# cd /etc/httpd/
[root@tecmint httpd]# ls -l
total 8
drwxr-xr-x. 2 root root 4096 Dec 24 21:44 conf
drwxr-xr-x. 2 root root 4096 Dec 25 02:09 conf.d
lrwxrwxrwx  1 root root   19 Oct 13 19:06 logs -> ../../var/log/httpd
lrwxrwxrwx  1 root root   27 Oct 13 19:06 modules -> ../../usr/lib/httpd/modules
lrwxrwxrwx  1 root root   19 Oct 13 19:06 run -> ../../var/run/httpd

[root@tecmint ~]# cd /etc/apache2
[root@tecmint apache2]# ls -l
total 84
-rw-r--r-- 1 root root  7113 Jul 24 16:15 apache2.conf
drwxr-xr-x 2 root root  4096 Dec 16 11:48 conf-available
drwxr-xr-x 2 root root  4096 Dec 16 11:45 conf.d
drwxr-xr-x 2 root root  4096 Dec 16 11:48 conf-enabled
-rw-r--r-- 1 root root  1782 Jul 21 02:14 envvars
-rw-r--r-- 1 root root 31063 Jul 21 02:14 magic
drwxr-xr-x 2 root root 12288 Dec 16 11:48 mods-available
drwxr-xr-x 2 root root  4096 Dec 16 11:48 mods-enabled
-rw-r--r-- 1 root root   315 Jul 21 02:14 ports.conf
drwxr-xr-x 2 root root  4096 Dec 16 11:48 sites-available
drwxr-xr-x 2 root root  4096 Dec  6 00:04 sites-enabled

7. Can Apache be secured with TCP wrappers?

8. How to change default Apache Port and How Listen Directive works in Apache?

Suppose you have multiple IPs assigned to your Linux machine and want Apache to receive HTTP requests on a special Ethernet port or Interface, even that can be done with Listen directive.

To change the Apache default port, please open your Apache main configuration file httpd.conf or apache2.conffile with VI editor.

[root@tecmint ~]# vi /etc/httpd/conf/httpd.conf

[root@tecmint ~]# vi /etc/apache2/apache2.conf

Search for the word ”Listen”, comment the original line and write your own directive below that line.

# Listen 80
Listen 8080

OR

Listen 172.16.16.1:8080

Save the file and restart the web server.

[root@tecmint ~]# service httpd restart

[root@tecmint ~]# service apache2 restart

9. Can we have two Apache Web servers on a single machine?

10. What do you mean by DocumentRoot of Apache?

11. How to host files in different folder and what is Alias directive?

To use Alias directive, Its the part of mod_alias module of Apache. The default syntax of Alias directive is:

Alias /images /var/data/images/

Here in above example, /images url prefix to the /var/data/images prefix that mean clients will query for “http://www.example.com/images/sample-image.png” and Apache will pick up the “sample-image.png” file from /var/data/images/sample-image.png on the server. It’s also known as URL Mapping.

12. What do you understand by “DirectoryIndex”?

The default setting of DirectoryIndex is .html index.html index.php, if you have different names of your first file, you need to make the changes in httpd.conf or apache2.conf for DirectoryIndex value to display that to your client browser.

#
# DirectoryIndex: sets the file that Apache will serve if a directory
# is requested.
#
# The index.html.var file (a type-map) is used to deliver content-
# negotiated documents.  The MultiViews Option can be used for the
# same purpose, but it is much slower.
#
DirectoryIndex index.html index.html.var index.cgi .exe

13. How to disable Directory listing when an index file is missing?

To stop Apache directory listing, you can set the following rule in the main configuration file globally or in .htaccess file for a particular website.

<Directory /var/www/html>
   Options -Indexes
</Directory>

14. What are different log files of Apache Web Server?

15. What do you understand by “connection reset by peer” in error logs?

16. What is Virtual Host in Apache?

You are free to add as many directives you require for your domain, but the two minimal entries for a working website is the ServerName and DocumentRoot. We usually define our Virtual Host section at the bottom of httpd.conf file in Linux machines.

Sample VirtualHost

<VirtualHost *:80>
   ServerAdmin webmaster@dummy-host.example.com
   DocumentRoot /www/docs/dummy-host.example.com
   ServerName dummy-host.example.com
   ErrorLog logs/dummy-host.example.com-error_log
   CustomLog logs/dummy-host.example.com-access_log common
</VirtualHost>

1. ServerAdmin : Its usually the email address of the website owner, where the error or notification can be sent.
2. DocumentRoot : location where the web files are located in the server(Necessary).
3. ServerName : Its the domain name which you want to access from your web browser(Necessary).
4. ErrorLog : Its the location of the log file where all the domain related logs are being recorded.

17. What’s the difference between <Location> and <Directory>?

18. What is Apache Virtual Hosting?

For more information, read on How to Create Name/IP based Virtual Hosts in Apache.

19. What do you understand by MPM in Apache?

20. What is the difference between Worker and Prefork MPM?

1. Basic difference between Worker and MPM is in their process of spawning the child process. In the Prefork MPM, a master httpd process is started and this master process starts manages all other child processes to serve client requests. Whereas, In the worker MPM one httpd process is active, and it uses different threads to serve client requests.
2. Prefork MPM uses multiple child processes with one thread each, where worker MPM uses multiple child processes with many threads each.
3. Connection handling in the Prefork MPM, each process handles one connection at a time, whereas in the Worker mpm each thread handles one connection at a time.
4. Memory footprints Prefork MPM Large memory footprints, where Worker has smaller memory footprints.

21. What’s the use of “LimitRequestBody” and how to put limit on your uploads?

For example: I want to put limits of 100000 Bytes in the folder /var/www/html/tecmin/uploads. So, you need to add following directive in Apache configuration file.

<Directory "/var/www/html/tecmint/uploads">
LimitRequestBody 100000
</Directory>

22. What is mod_perl and mod _php?

23. What is Mod_evasive?

For more information, read the article that guides you how to install and configure mod_evasive in Apache.

24. What is Loglevel debug in httpd.conf file?

25. What’s the use of mod_ssl and how SSL works with Apache?

How SSL works with Apache

Whenever an https requests comes, these three steps Apache follows:

1. Apache generates its private key and converts that private key to .CSR file (Certificate signing request).
2. Then Apache sends the .csr file to the CA (Certificate Authority).
3. CA will take the .csr file and convert it to .crt (certificate) and will send that .crt file back to Apache to secure and complete the https connection request.

These are just most popular 25 questions being asked these days by Interviewers, please provide some more interview questions which you have faced in your recent interview and help others via our Comment section below.

We are also recommend you to read our previous articles on Apache.

1. 13 Apache Web Server Security and Hardening Tips
2. How to Sync Two Apache Web Servers/Websites Using Rsync

10 MySQL Database Interview Questions for Beginners and Intermediates

In our last article, we’ve covered 15 Basic MySQL Questions, again we are here with another set interview questions for intermediate users. As we said earlier these questions can be asked in Job Interviews. But some of our critics on the last article said, that I don’t give response to my critics and the questions are very basic and will never be asked in any Database Administrator Interview.

To them we must admit all the articles and question can not be composed keeping all the flock in mind. We are coming from basic to expert level step by step. Please Co­operate with us.

1. Define SQL?

2. What is RDBMS? Explain its features?

3. What is Data Mining?

4. What is an ERD?

5. What is the difference between Primary Key and Unique Key?

6. How to store picture file in the database. What Object type is used?

7. What is Data Warehousing?

8. What are indexes in a Database. What are the types of indexes?

9. How many TRIGGERS are possible in MySql?

10. What is Heap table?

That’s all for now on MySQL questions, I will be coming up with another set of questions soon. Don’t forget to provide your valuable feedback in comment section.

10 Core Linux Interview Questions and Answers

Again its time to read some serious content in light mood, Yup! Its another article on Interview question and here we are presenting 10 Core Linux Questions, which surely will add to your knowledge.

1. You need to define a macro, a key binding for the existing command. How would you do it?

[root@localhost ~]# bind '"\e[24~":"date"'

Note : Different types of terminals or terminal emulators can emit different codes for the same key.

2. A user is new to Linux and he wants to know full list of available commands, what would you suggest him?

[root@localhost ~]$ compgen -c

l.
ll
ls
which
if
then
else
elif
fi
case
esac
for
select
while
until
do
done
...

3. Your assistant needs to print directory stack, what would you suggest?

[root@localhost ~]# dirs

/usr/share/X11

4. You have lots of running jobs, how would you remove all the running processes, without restarting the machine?

5. What does the command ‘hash’ is used for in bash Shell?

[root@localhost ~]# hash

hits    command
   2    /bin/ls
   2    /bin/su

6. Which built­in Linux command performs arithmetic operation of Integers in Bash?

#! /bin/bash

...

...

let c=a+b

...

...

7. You have a large text file, and you need to see one page at a time. What will you do?

[root@localhost ~]# cat file_name.txt | more

8. Who own the data dictionary?

9. How to know a command summary and useability in Linux?

Assume you came across a command in /bin directory, which you are completely unaware of, and have no idea what it does. What will you do to know its useability?

[root@localhost ~]# whatis zcat

zcat [gzip]          (1)  - compress or expand files

10. What command should you use to check the number of files and disk space used by each user’s defined quotas?

That’s all for now. Provide your valuable Feedback in our comment section. Stay tuned for more Linux and Foss posts.

10 VsFTP (Very Secure File Transfer Protocol) Interview Questions and Answers

FTP stands for ‘File Transfer Protocol‘ is one of the most widely used and standard protocol available over Internet. FTP works in a Server/­Client architecture and is used to transfer file. Initially FTP client were command-­line based. Now most of the platform comes bundled with FTP client and server program and a lot of FTP Client/Server Program is available. Here we are presenting 10 Interview Questions based on Vsftp (Very Secure File Transfer Protocol) on a Linux Server.

1. What is the Difference between TFTP and FTP Server?

Note: Briefly you can say FTP usages port 21 by default when clarification between Data and Control is not required.

2. How to Restrict Users and Disallow browsing beyond their Home Directories? How?

chroot_local_user=YES

3. How would you manage number of FTP clients that connect to your FTP server?

4. How to limit the FTP login attempts to fight against botnet/illegal login attempts?

5. How to enable file upload from anonymous users to FTP Server?

6. How would you disabled downloads from FTP server?

7. How to enable and permit FTP login to local users?

8. Is it Possible to maintain log of FTP requests and responses?

Note: In order to create and maintains logs successfully, the parameter ‘xferlog_std_format’ must be enabled.

9. How to disable the login for few seconds, in case of failed login. How will you achieve this?

10. How to display certain text message before a client connects to FTP server. How would you get this done?

FTP is a very Useful tool and is vast yet very interesting. Moreover it is useful from Interview Point of View. We have taken the pain to bring these questions to you and will cover more of these questions in our future article. Till then stay tuned and connected to Tecmint.

Read Also: 10 Advance VsFTP Interview Questions and Answers – Part II

10 Advance VsFTP Interview Questions and Answers – Part II

We were overwhelmed with the response we’ve received on our last article. Where we’ve presented 10 wonderful question on Very Secure File Transfer Protocol. Continuing VSFTP interview article we are here presenting you yet another 10 Advance Interview Questions which surely will help you.

1. 10 Basic Vsftp Interview Question/Answers – Part I

Please note vsftpd.conf file is used to control various aspects of configuration as specified in this article. By default, the vsftpd searches for the configuration file under /etc/vsftpd/vsftpd.conf. However, the format of file is very simple and it contains comment or directive. Comment lines begins with a ‘#‘ are ignored and a directive line has the following format.

option=value

Before we start the Question and their well explained Answer we would like to answer a question “Who is going to attend FTP Interview?”. Well no one. Perhaps no one would be attending FTP interview. But we are presenting subject wise questions to maintain a systematic approach so that in any Interview, you wont get a new question which you wont be knowing on any of the topics/subjects we covered here.

11. How would you block an IP which is acting malicious on your internal private VSFTP network?

Block IP using host.deny file

Open ‘/etc/hosts.deny’ file.

# vi /etc/hosts.deny

Append the following line at the bottom of the file with the IP address that you want to block access to FTP.

#
# hosts.deny    This file contains access rules which are used to
#               deny connections to network services that either use
#               the tcp_wrappers library or that have been
#               started through a tcp_wrappers-enabled xinetd.
#
#               The rules in this file can also be set up in
#               /etc/hosts.allow with a 'deny' option instead.
#
#               See 'man 5 hosts_options' and 'man 5 hosts_access'
#               for information on rule syntax.
#               See 'man tcpd' for information on tcp_wrappers
#
vsftpd:172.16.16.1

Block IP using iptables rule

To block FTP access to particular IP address, add the following drop rule to iptables INPUT chain.

iptables -A RH-Firewall-1-INPUT -p tcp -s 172.16.16.1 -m state --state NEW -m tcp --dport 21 -j DROP

12. How to allow secured SSL connections to Anonymous users? How would you do?

# Add this line to enable secured SSL connection to anonymous users.
allow_anon_ssl=YES

13. How to allow Anonymous users to create new directory and write to that directory?

# Uncomment this to enable any form of FTP write command.
write_enable=YES

# Uncomment this if you want the anonymous FTP user to be able to create
# new directories.
anon_mkdir_write_enable=YES

14. How to enable Anonymous downloads, but disable permission to write?

# Add this line to enable read only permission to anonymous users.
allow_anon_ssl=YES

15. How to CHMOD all Anonymous uploads automatically. How would you do?

# Add this line to chmod all anonymous uploads automatically.
chmod_enable=YES

16. How to disable directory listing in a FTP server?

# Add this line to disable directory listing.
dirlist_enable=NO

17. How to maintain sessions for logins of VSFTP. How will you do?

# Add this line to maintain session logins.
session_support=YES

18. How to display time in local time zone, when listing the contents of directory?

# Add this line to display directory listing in local time zone.
usr_localtime=YES

19. How will you limit the maximum transfer rate from VSFTP server?

# Add this line to limit the ftp transfer rate.
anon_max_rate=0 # 0 means unlimited

20. How will you timeout the idle session of VSFTP?

# Add this line to set the ftp timeout session.
idle_session_timeout=300

That’s all for now. We will be coming up with next article very soon, till then stay tuned and connected and don’t forget to provide us with your valuable feedback in our comment section.

10 Useful Random Linux Interview Questions and Answers

To a little surprise this time we are not presenting Interview question on any specific subject but on random topics. These question will surely help you in cracking Interviews beside adding to your Knowledge.

1. Let’s say you maintains a backup on regular basis for the company you are working. The backups are maintained in Compressed file format. You need to examine a log, two months old. What would you suggest without decompressing the compressed file?

# zcat ­f phpshell­2.4.tar.gz

2. You need to track events on your system. What will you do?

Running ‘syslogd‘ application in terminal generates log file at the location ‘/var/log/syslog‘. The syslogd application is very useful in troubleshooting Linux sytems. A sample log file looks similar to below.

3. How will you restrict IP so that the restricted IP’s may not use the FTP Server?

Block IP Address

Open ‘/etc/hosts.deny’ file.

# vi /etc/hosts.deny

Add the IP address that you want to block at the bottom of the file.

#
# hosts.deny    This file contains access rules which are used to
#               deny connections to network services that either use
#               the tcp_wrappers library or that have been
#               started through a tcp_wrappers-enabled xinetd.
#
#               The rules in this file can also be set up in
#               /etc/hosts.allow with a 'deny' option instead.
#
#               See 'man 5 hosts_options' and 'man 5 hosts_access'
#               for information on rule syntax.
#               See 'man tcpd' for information on tcp_wrappers
#
vsftpd:172.16.16.1

4. Tell us the difference between Telnet and SSH?

6. You need to stop your X server. When you tries to kill your X server, You got an error message that you cannot quit X server. What will you do?

6. What is the difference between command ‘ping’ and ‘ping6’?

7. You want to search for all the *.tar files in your Home directory and wants to delete all at once. How will you do it?

# find /home/ ­name '*.tar' | xargs rm ­rf

8. What is the difference between locate and slocate command?

9. You need to search for the string “Tecmint” in all the “.txt” files in the current directory. How will you do it?

# find -­name “*.txt” | xargs grep “Tecmint”

10. You want to send a message to all connected users as “Server is going down for maintenance”, what will you do?

# echo please save your work, immediately. The server is going down for Maintenance at 12:30 Pm, sharply. | wall

That’s all for now. Don’t Forget to give your valuable feedback in comment section below.

10 Useful Interview Questions on Linux Services and Daemons

Daemon is a computer program that runs as a background process and generally do not remains under the direct control of user. The parent process of a daemon in most cases are init, but not always.

In Linux, a Service is an application that runs in a background carrying out essential task or waiting for its execution.

Generally, there is no difference between a Daemon and a Service. Daemon is Service but service may be bigger than Daemon. Daemon provide some services and services may contain more than one Daemon.

Here in this series of Interview Article, we would be covering Services and Daemons in Linux.

1. What is Exim Service? What is the purpose of this Service?

2. What is NIS server? What is the purpose of NIS Server?

3. What will you prefer for a reverse proxy in Linux?

4. You are getting following codes (2xx, 3xx, 4xx, 5xx) in Apache, at some point of time. What does this means?

5. You are asked to stop Apache Service through its control Script. What will you do?

# apachectl stop		[On Debian based Systems]
# /etc/inid.t/httpd stop	[On Red Hat based Systems]

6. How is ‘apachectl restart’ different from ‘apachectl graceful’

7. How will you configure the nfs mounts to export it, from your local machine?

8. You are supposed to create a new Apache VirtualHost configuration for the host www.Tecmint.com that is available at /home/Tecmint/public_html/ and maintains log at /var/log/httpd/ by default.

<VirtualHost *:80>
DocumentRoot /home/Tecmint/public_html
ServerName www.Tecmint.com
Server Alias Tecmint.com
CustomLog /var/log/httpd/Tecmint.com.log combined
ErrorLog /var/log/httpd/Tecmint.com.error.log
</VirtualHost>

9. You are supposed to dump all the packets of http traffic in file http.out. What will you suggest?

# tcpdump tcp port 80 -s0 -w http.out

10. How will you add a service (say httpd) to start at INIT Level 3?

chkconfig –level 3 httpd on

That’s all for now. I’ll be here again with another interesting article very soon.

10 Useful SSH (Secure Shell) Interview Questions and Answers

SSH stands for Secure Shell is a network protocol, used to access remote machine in order to execute command-line network services and other commands over a Network. SSH is Known for its high security, cryptographic behavior and it is most widely used by Network Admins to control remote web servers primarily.

Here in this Interview Questions series article, we are presenting some useful 10 SSH (Secure Shell) Questionsand their Answers.

1. SSH is configured on what Port Number, by default? How to change the port of SSH?

We can check port number of SSH by running the below one liner script, directly on terminal.

# grep Port /etc/ssh/sshd_config		[On Red Hat based systems]

# grep Port /etc/ssh/ssh_config		        [On Debian based systems]

To change the port of SSH, we need to modify the configuration file of SSH which is located at ‘/etc/ssh/sshd_config‘ or ‘/etc/ssh/ssh_config‘.

# nano /etc/ssh/sshd_config	[On Red Hat based systems]

# nano /etc/ssh/ssh_config		[On Debian based systems]

Searh for the Line.

Port 22

And replace ‘22‘ with any UN-engaged port Number say ‘1080‘. Save the file and restart the SSH service to take the changes into effect.

# service sshd restart					[On Red Hat based systems]

# service ssh restart					[On Debian based systems]

2. As a security implementation, you need to disable root Login on SSH Server, in Linux. What would you suggest?

To disable SSH root login, open the configuration file located at ‘/etc/ssh/sshd_config‘ or ‘/etc/ssh/ssh_config‘.

# nano /etc/ssh/sshd_config			[On Red Hat based systems]

# nano Port /etc/ssh/ssh_config			[On Debian based systems]

Change the parameter ‘PermitRootLogin‘ to ‘no‘ and restart the SSH service as show above.

3. SSH or Telnet? Why?

4. Is it possible to login to SSH server without password? How

Create ssh-keygen using the command below.

$ ssh-keygen

Copy public keys to remote host using the command below.

$ ssh-copy-id -i /home/USER/.ssh/id_rsa.pub REMOTE-SERVER

Note: Replace USER with user name and REMOTE-SERVER by remote server address.

The next time we try to login to SSH server, it will allow login without asking password, using the keygen. For more detailed instructions, read how to login remote SSH server without password.

5. How will you allows users and groups to have access to SSH Sever?

Here again we need to edit the configuration file of SSH service. Open the configuration file and add users and groups at the bottom as show below and then, restart the service.

AllowUsers Tecmint Tecmint1 Tecmint2
AllowGroups group_1 group_2 group_3

6. How to add welcome/warning message as soon as a user login to SSH Server?

# nano /etc/issue

And add your custom message in this file. See, below a screen grab that shows a custom message as soon as user logged into server.

7. SSH has two protocols? Justify this statement.

Again, we need to open the SSH configuration file and add/edit the lines as shown below.

# protocol 2,1

to

Protocol 2

Save the configuration file and restart the service.

8. Is it possible to trace unauthorized login attempts to SSH Server with date of Intrusion along with their corresponding IP.

# cat /var/log/secure | grep “Failed password for”

Note: The grep command can be tweaked in any other way to produce the same result.

9. Is it possible to copy files over SSH? How?

A dummy SCP command in action is depicted below:

$ scp text_file_to_be_copied Your_username@Remote_Host_server:/Path/To/Remote/Directory

For more practical examples on how to copy files/folders using scp command, read the 10 SCP Commands to Copy Files/Folders in Linux.

10. Is it possible to pass input to SSH from a local file? If Yes! How?

# ssh username@servername < local_file.txt

SSH is a very hot topic from interview point, of all times. The above questions would have surely added to your knowledge.

That’s all for now. I’ll soon be here with another interesting article.

10 Interview Questions and Answers on Various Commands in Linux

Our last article, “10 Useful SSH Interview Questions” was highly appreciated on various Social Networking sites as well as on Tecmint. This time we are presenting you with “10 Questions on various Linux commands“. These questions will prove to be brainstorming to you and will add to your knowledge which surely will help you in day-to-day interaction with Linux and in Interviews.

Q1. You have a file (say virgin.txt). You want this file to be alter-proof so that no one can edit or delete this file, not even root. What will you do?

The Syntax of command chattr, for the above purpose is:

# chattr +i virgin.txt

Now try to remove the file using normal user.

$ rm -r virgin.txt 

rm: remove write-protected regular empty file `virgin.txt'? Y 
rm: cannot remove `virgin.txt': Operation not permitted

Now try to remove the file using root user.

# rm -r virgin.txt 

cannot remove `virgin.txt': Operation not permitted

Q2. If several users are using your Linux Server, how will you find the usage time of all the users, individually on your server?

# apt-get install acct

# ac -p 

(unknown)                     14.18 
server                             235.23 
total      249.42

Q3. Which is preferred tool to create Network Statistics for your server?

# apt-get install mrtg

Q4. It is possible to send query to BIOS from Linux Command Line?

# biosdecode 

# biosdecode 2.11 

ACPI 2.0 present. 
	OEM Identifier: LENOVO 
	RSD Table 32-bit Address: 0xDDFCA028 
	XSD Table 64-bit Address: 0x00000000DDFCA078 
SMBIOS 2.7 present. 
	Structure Table Length: 3446 bytes 
	Structure Table Address: 0x000ED9D0 
	Number Of Structures: 89 
	Maximum Structure Size: 184 bytes 
PNP BIOS 1.0 present. 
	Event Notification: Not Supported 
	Real Mode 16-bit Code Address: F000:BD76 
	Real Mode 16-bit Data Address: F000:0000 
	16-bit Protected Mode Code Address: 0x000FBD9E 
	16-bit Protected Mode Data Address: 0x000F0000 
PCI Interrupt Routing 1.0 present. 
	Router ID: 00:1f.0 
	Exclusive IRQs: None 
	Compatible Router: 8086:27b8 
	Slot Entry 1: ID 00:1f, on-board 
	...
	Slot Entry 15: ID 02:0c, slot number 2

Q5. Most of the Linux Server are headless, i.e., they run in command mode only. No GUI is installed. How will you find hardware description and configuration of your box?

# dmidecode

The output of dmidecode is extensive. It will be a nice idea to redirect its output to a file.

# dmidecode > /path/to/text/file/text_file.txt

Q6. You need to know all the libraries being used and needed by a binary, say ‘/bin/echo’. How will you achieve desirable output?

$ ldd /bin/echo 

linux-gate.so.1 =>  (0xb76f1000) 
libc.so.6 => /lib/i386-linux-gnu/i686/cmov/libc.so.6 (0xb7575000) 
/lib/ld-linux.so.2 (0xb76f2000)

Q7. You are working for the country’s army. You have a file (say “topsecret.txt”) which contains confidential and country’s security Information, Nuclear missiles, etc. What will be your preferred method to delete this file?

# shred -n 15 -z topsecret.txt

shread – overwrite a file to hide its contents, and optionally delete it.

1. -n – Overwrites the files n times
2. -z – Add a final overwrite with zeros to hide shredding.

Note: The above command overwrites the file 15 times before overwriting with zero, to hide shredding.

Q8. Is it possible to mount an NTFS partition on Linux?

For more information, read article on how to monitor an NTFS Partition on Linux.

Q9. What and where you need to edit so that the default desktop at login will be KDE, which at present is GNOME.

DESKTOP=”KDE”
DISPLAYMANAGER=”KDE”

Save the file with above content. Next time when machine boots, it automatically will load KDE as default display manager.

Q10. What does an intrid image file refers to?

That’s all for now. I’ll be here again with another interesting topic, worth knowing.

10 Useful ‘Interview Questions and Answers’ on Linux Shell Scripting

Greeting of the day. The vastness of Linux makes it possible to come up with a unique post every time. 

We have lots of tutorials on Shell Scripting language and Interview Questions for readers of all kind, here are the links to those articles.

1. Shell Scripting Series
2. Interview Question and Answer Series

Adding to the shell scripting posts here, in this article we will be going through questions related to Linux Shell from interview point of view.

1. How will you abort a shell script before it is successfully executed?

For example, create a following shell script as ‘anything.sh‘.

#!/bin/bash
echo "Hello"
exit -1
echo "bye"

Save the file and execute it.

# sh anything.sh

Hello
exit.sh: 3: exit: Illegal number: -1

From the above script, it is clear that the execution went well before exit -1 command.

2. How to remove the headers from a file using command in Linux?

Here it the exact command to remove headers from a file (or first line of a file).

# sed '1 d' file.txt

The only problem with above command is that, it outputs the file on standard output without the first line. In order to save the output to file, we need to use redirect operator which will redirects the output to a file.

# sed '1 d' file.txt > new_file.txt

Well the built in switch ‘-i‘ for sed command, can perform this operation without a redirect operator.

# sed -i '1 d' file.txt

3. How will you check the length of a line from a text file?

A ‘sed –n ‘n p’ file.txt‘, where ‘n‘ represents the line number and ‘p‘ print out the pattern space (to the standard output). This command is usually only used in conjunction with the -n command-line option. So, how to get the length count? Obviously! we need to pipeline the output with ‘wc‘ command.

# sed –n 'n p' file.txt | wc –c

To get the length of line number ‘5’ in the text file ‘tecmint.txt‘, we need to run.

# sed -n '5 p' tecmint.txt | wc -c

4. Is it possible to view all the non-printable characters from a text file on Linux System? How will you achieve this?

How to show non-printable characters in ‘vi‘ editor?

1. Open vi editor.
2. Go to command mode of vi editor by pressing [esc] followed by ‘:’.
3. The final step is to type execute [set list] command, from command interface of ‘vi’ editor.

Note: This way we can see all the non-printable characters from a text file including ctrl+m (^M).

5. You are a Team-Leader of a group of staffs working for a company xyz. The company ask you to create a directory ‘dir_xyz’, such that any member of the group can create a file or access a file under it, but no one can delete the file, except the one created it. what will you do?

# mkdir dir_xyz
# chmod g+wx dir_xyz
# chmod +t dir_xyz

The first line of command create a directory (dir_xyz). The second line of command above allow group (g) to have permission to ‘write‘ and ‘execute‘ and the last line of the above command – The ‘+t‘ in the end of the permissions is called the ‘sticky bit‘. It replaces the ‘x‘ and indicates that in this directory, files can only be deleted by their owners, the owner of the directory or the root superuser.

6. Can you tell me the various stages of a Linux process, it passes through?

Here are the 4 stages of Linux process.

1. Waiting: Linux Process waiting for a resource.
2. Running : A Linux process is currently being executed.
3. Stopped : A Linux Process is stopped after successful execution or after receiving kill signal.
4. Zombie : A Process is said to be ‘Zombie’ if it has stopped but still active in process table.

7. What is the use of cut command in Linux?

For example, extract first 10 columns of a text file ‘txt_tecmint‘.

# cut -c1-10 txt_tecmint

To extract 2nd, 5th and 7th column of the same text file.

# cut -d;-f2 -f5 -f7 txt_tecmint

8. What is the difference between commands ‘cmp’ and ‘diff’?

The ‘diff‘ command reports the changes one should make so that both the files look the same. Whereas ‘cmp‘ command compares the two files byte-by-byte and reports the first mismatch.

9. Is it possible to substitute ‘ls’ command with ‘echo’ command?

10. You might have heard about inodes. can you describe inode briefly?

That’s all for now. We will be coming up with another interesting and knowledgeable Interview questions, in the next article.

Practical Interview Questions and Answers on Linux Shell Scripting

#!/bin/bash 
echo "Hello, $LOGNAME" 
echo "Current date is `date`" 
echo "User is `who i am`" 
echo "Current directory `pwd`"

# chmod 755 userstats.sh 
# ./userstats.sh

Hello, avi 
Current date is Sat Jun  7 13:05:29 IST 2014 
User is avi      pts/0        2014-06-07 11:59 (:0) 
Current directory /home/avi/Desktop

#!/bin/bash 
# The Shebang

if [ $# -ne 2 ] 
# If two Inputs are not received from Standard Input

then 
# then execute the below statements

    echo "Usage - $0   x    y" 
    # print on standard output, how-to use the script (Usage - ./1.sh   x    y )

    echo "        Where x and y are two nos for which I will print sum" 
    # print on standard output, “Where x and y are two nos for which I will print sum ”

    exit 1 
    # Leave shell in Error Stage and before the task was successfully carried out.

fi 
# End of the if Statement.

    echo "Sum of $1 and $2 is `expr $1 + $2`"
    # If the above condition was false and user Entered two numbers as a command Line Argument,   
       it will show the sum of the entered numbers.

# chmod 755 two-numbers.sh

# ./two-numbers.sh

Usage - ./two-numbers.sh   x    y 
        Where x and y are two nos for which I will print sum

$ ./two-numbers.sh 4 5 

Sum of 4 and 5 is 9

#!/bin/bash 
if [ $# -ne 1 ] 
then 
    echo "Usage: $0   number" 
    echo "       I will find reverse of given number" 
    echo "       For eg. $0 0123, I will print 3210" 
    exit 1 
fi 

n=$1 
rev=0 
sd=0 

while [ $n -gt 0 ] 
do 
    sd=`expr $n % 10` 
    rev=`expr $rev \* 10  + $sd` 
    n=`expr $n / 10` 
done 
    echo  "Reverse number is $rev"

# chmod 755 numbers.h

./numbers.sh

Usage: ./numbers.sh  number 
       I will find reverse of given number 
       For eg. ./2.sh 123, I will print 321

$ ./numbers.sh 10572 

Reverse number is 27501

$ echo 7.56 + 2.453 | bc

10.013

# pi 100 

3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067

mysql> SELECT VERSION(), DATABASE();

+-------------------------+------------+
| VERSION()               | DATABASE() |
+-------------------------+------------+
| 5.5.34-0ubuntu0.13.10.1 | NULL       |
+-------------------------+------------+
1 row in set (0.06 sec)

mysql> use Tecmint;

Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed

mysql> select VERSION(), DATABASE();

+-------------------------+------------+
| VERSION()               | DATABASE() |
+-------------------------+------------+
| 5.5.34-0ubuntu0.13.10.1 | tecmint    |
+-------------------------+------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM Tecmint WHERE user !=SAM;

+---------------------+---------+---------+---------+---------+-------+ 
| date                | user    | host 	 | root     | local   | size  | 
+---------------------+---------+---------+---------+---------+-------+ 
| 2001-05-14 14:42:21 | Anthony | venus  | barb     | venus   | 98151 | 
| 2001-05-15 08:50:57 | TIM     | venus  | phil     | venus   | 978   | 
+---------------------+---------+---------+---------+---------+-------+

mysql> SELECT * FROM mail WHERE user = SAM AND root = phil

mysql> SELECT * FROM mail WHERE user != SAM OR root != phil

+---------------------+---------+---------+---------+---------+-------+ 
| date                | user    | host    | root    | local   | size  | 
+---------------------+---------+---------+---------+---------+-------+ 
| 2001-05-14 14:42:21 | Anthony | venus   | barb    | venus   | 98151 | 
+---------------------+---------+---------+---------+---------+-------+

mysql> SELECT name, IFNULL(id,'Unknown') AS 'id' FROM taxpayer;

+---------+---------+ 
| name 	  | id      | 
+---------+---------+ 
| bernina | 198-48  | 
| bertha  | Unknown | 
| ben     | Unknown | 
| bill    | 475-83  | 
+---------+---------+

mysql> SELECT * FROM name LIMIT 1;

+----+------+------------+-------+----------------------+------+ 
| id | name | birth      | color | foods                | cats | 
+----+------+------------+-------+----------------------+------+ 
| 1  | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 0    | 
+----+------+------------+-------+----------------------+------+

mysql> SELECT * FROM profile LIMIT 5;

+----+------+------------+-------+-----------------------+------+ 
| id | name | birth      | color | foods                 | cats | 
+----+------+------------+-------+-----------------------+------+ 
| 1  | Fred | 1970-04-13 | black | lutefisk,fadge,pizza  | 0    | 
| 2  | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3    | 
| 3  | Brit | 1957-12-01 | red   | burrito,curry,pizza   | 1    |   
| 4  | Carl | 1973-11-02 | red   | eggroll,pizza         | 4    | 
| 5  | Sean | 1963-07-04 | blue  | burrito,curry         | 5    | 
+----+------+------------+-------+-----------------------+------+

mysql> SELECT * FROM profile ORDER BY birth LIMIT 1;

+----+------+------------+-------+----------------+------+ 
| id | name | birth      | color | foods          | cats | 
+----+------+------------+-------+----------------+------+ 
| 9  | Dick | 1952-08-20 | green | lutefisk,fadge | 0    | 
+----+------+------------+-------+----------------+------+

mysql> SELECT CURRENT_DATE();

+----------------+
| CURRENT_DATE() |
+----------------+
| 2014-06-17     |
+----------------+

mysql -u USER_NAME –xml -e 'SELECT * FROM table_name' > table_name.xml

mysql> show index from user;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | 
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| user  |          0 | PRIMARY  |            1 | Host        | A         |        NULL |     NULL | NULL   |      | BTREE      |         |               | 
| user  |          0 | PRIMARY  |            2 | User        | A         |           4 |     NULL | NULL   |      | BTREE      |         |               | 
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
2 rows in set (0.00 sec)

# cat /var/log/messages

OR 

# dmesg

# nano /etc/squid/squid.conf

http_port 3128

# service squid restart

# touch /etc/squid/blacklist

# nano /etc/squid/blacklist

.facebook.com
.twitter.com
.gmail.com
.yahoo.com
...

# nano /etc/squid/squid.conf

acl BLACKLIST dstdom_regex -i “/etc/squid/blacklist”
http_access deny blacklist

# service squid restart

# service squid stop
# rm -rf /var/lib/squid/cache/*<

# squid -z

# nano /etc/squid/squid.conf

acl ALLOW_TIME time M T W H F 16:00-19:00
shttp_access allow ALLOW_TIME

# service squid restart

82.175.219.112

 2005:3200:230:7e:35dl:2874:2190

# ls

# echo *

# echo */

# ls -a

# ls -A

# ls -l

drwxr-xr-x  5 avi tecmint      4096 Sep 30 11:31 Binary

# ls -la

# ls -lA

# ls --author -l

# ls -b

# ls --block-size=M -l
# ls --block-size=K -l

# ls -B

# ls -cl

# ls -ltc

# ls --color=never
# ls --color=auto
# ls --color=always

# ls -d

# ls -a
# nano .bashrc
# ll >> ll.txt
# nano ll.txt

# ls -l

# ls -l --author

# ls -g

# ls -Gl

# ls -hl

# ls -hs

# ls -si

# ls -si -l

# ls -m

# ls -ml

# ls -r

# ls -rl

# ls -R

# ls -S

# ls -Sr

# ls -1

# ls -Q

# ls --group-directories-first

# rpm -qa iptables

iptables-1.4.21-13.el7.x86_64

# yum install iptables-services

# service iptables status			[On CentOS 6/5]
# systemctl status iptables			[On CentOS 7]

---------------- On CentOS 6/5 ---------------- 
# chkconfig --level 35 iptables on
# service iptables start

---------------- On CentOS 7 ---------------- 
# systemctl enable iptables 
# systemctl start iptables 

# lsmod | grep ip_tables

# iptables -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

 
# iptables --flush OUTPUT

# iptables --flush

# iptables -A INPUT -s 192.168.0.7 -j ACCEPT 

# iptables -A INPUT -s 192.168.0.7/24 -j ACCEPT 
# iptables -A INPUT -s 192.168.0.7/255.255.255.0 -j ACCEPT

# iptables -A INPUT -s -p tcp --dport 22 -j ACCEPT 

# iptables -A INPUT -s -p tcp --dport 22 -j REJECT

 
# iptables -A INPUT -s -p tcp --dport 22 -j DENY

 
# iptables -A INPUT -s -p tcp --dport 22 -j DROP

# iptables -A INPUT -s 192.168.0.6 -p tcp -m multiport --dport 21,22,23,80 -j DROP

# iptables -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             state NEW tcp dpt:ssh
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited
DROP       tcp  --  192.168.0.6          anywhere             multiport dports ssh,telnet,http,webcache

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

# rpm -qa nano
OR
# rpm -qa | grep -i nano

nano-2.3.1-10.el7.x86_64

# vi
# vim

# rpm -qa vim*

vim-minimal-7.4.160-1.el7.x86_64

# rpm -ivh peazip-1.11-1.el6.rf.x86_64.rpm

Preparing...                          ################################# [100%]
Updating / installing...
   1:peazip-1.11-1.el6.rf             ################################# [100%]

# rpm -ql httpd

/etc/httpd
/etc/httpd/conf
/etc/httpd/conf.d
...

# rpm -qa postfix*

postfix-2.10.1-6.el7.x86_64

# rpm -ev postfix-2.10.1-6.el7.x86_64

Preparing packages...
postfix-2:3.0.1-2.fc22.x86_64

# rpm -qa openssh

[root@tecmint tecmint]# rpm -qi openssh
Name        : openssh
Version     : 6.8p1
Release     : 5.fc22
Architecture: x86_64
Install Date: Thursday 28 May 2015 12:34:50 PM IST
Group       : Applications/Internet
Size        : 1542057
License     : BSD
....

# rpm -qc httpd

/etc/httpd/conf.d/autoindex.conf
/etc/httpd/conf.d/userdir.conf
/etc/httpd/conf.d/welcome.conf
/etc/httpd/conf.modules.d/00-base.conf
/etc/httpd/conf/httpd.conf
/etc/sysconfig/httpd

# rpm -qd httpd

/usr/share/doc/httpd/ABOUT_APACHE
/usr/share/doc/httpd/CHANGES
/usr/share/doc/httpd/LICENSE
...

# rpm -qL openssh

/usr/share/licenses/openssh/LICENCE

# rpm -qf /usr/share/alsa/cards/AACI.conf
alsa-lib-1.0.28-2.el7.x86_64

# rpm -qa --last

# rpm -qa --last | grep -i sqlite

sqlite-3.8.10.2-1.fc22.x86_64                 Thursday 18 June 2015 05:05:43 PM IST

# rpm -qa --last | head

# rpm -qa --last | head -n 2

# rpm -qpR gedit-3.16.1-1.fc22.i686.rpm 

/bin/sh
/usr/bin/env
glib2(x86-32) >= 2.40.0
gsettings-desktop-schemas
gtk3(x86-32) >= 3.16
gtksourceview3(x86-32) >= 3.16
gvfs
libX11.so.6
...

# yum repolist
or
# dnf repolist

Last metadata expiration check performed 0:30:03 ago on Mon Jun 22 16:50:00 2015.
repo id                                                                           repo name                                                                                             status
*fedora                                                                           Fedora 22 - x86_64                                                                                    44,762
ozonos                                                                            Repository for Ozon OS                                                                                    61
*updates                                                                          Fedora 22 - x86_64 - Updates   

# yum repolist all
or
# dnf repolist all

Last metadata expiration check performed 0:29:45 ago on Mon Jun 22 16:50:00 2015.
repo id                                                                         repo name                                                                                      status
*fedora                                                                         Fedora 22 - x86_64                                                                             enabled: 44,762
fedora-debuginfo                                                                Fedora 22 - x86_64 - Debug                                                                     disabled
fedora-source                                                                   Fedora 22 - Source                                                                             disabled
ozonos                                                                          Repository for Ozon OS                                                                         enabled:     61
*updates                                                                        Fedora 22 - x86_64 - Updates                                                                   enabled:  5,018
updates-debuginfo                                                               Fedora 22 - x86_64 - Updates - Debug

# yum list available
or
# dnf list available

ast metadata expiration check performed 0:34:09 ago on Mon Jun 22 16:50:00 2015.
Available Packages
0ad.x86_64                               0.0.18-1.fc22                   fedora 
0ad-data.noarch                          0.0.18-1.fc22                   fedora 
0install.x86_64                          2.6.1-2.fc21                    fedora 
0xFFFF.x86_64                            0.3.9-11.fc22                   fedora 
2048-cli.x86_64                          0.9-4.git20141214.723738c.fc22  fedora 
2048-cli-nocurses.x86_64                 0.9-4.git20141214.723738c.fc22  fedora 
....

# yum list installed
or
# dnf list installed

Last metadata expiration check performed 0:34:30 ago on Mon Jun 22 16:50:00 2015.
Installed Packages
GeoIP.x86_64                            1.6.5-1.fc22                     @System
GeoIP-GeoLite-data.noarch               2015.05-1.fc22                   @System
NetworkManager.x86_64                   1:1.0.2-1.fc22                   @System
NetworkManager-libnm.x86_64             1:1.0.2-1.fc22                   @System
aajohan-comfortaa-fonts.noarch          2.004-4.fc22                     @System
....

# yum list
or
# dnf list

Last metadata expiration check performed 0:32:56 ago on Mon Jun 22 16:50:00 2015.
Installed Packages
GeoIP.x86_64                             1.6.5-1.fc22                    @System
GeoIP-GeoLite-data.noarch                2015.05-1.fc22                  @System
NetworkManager.x86_64                    1:1.0.2-1.fc22                  @System
NetworkManager-libnm.x86_64              1:1.0.2-1.fc22                  @System
aajohan-comfortaa-fonts.noarch           2.004-4.fc22                    @System
acl.x86_64                               2.2.52-7.fc22                   @System
....

# yum install nano

# yum groupinstall 'haskell'

# yum update nano

# yum groupupdate 'haskell'

# yum distro-sync      [On CentOS/RHEL]
or
# dnf distro-sync      [On Fedora 20 Onwards]

# yum install deltarpm  python-deltarpm  createrepo

# mkdir /home/$USER/rpm
# cp /path/to/rpm/on/DVD/*.rpm /home/$USER/rpm

# createrepo -v /home/$USER/rpm

cd /etc/yum.repos.d && cat << EOF > abc.repo
[local-installation]name=yum-local
baseurl=file:///home/$USER/rpm
enabled=1
gpgcheck=0
EOF