USERS IN GNU/LINUX 

The first thing we need to know is that, in GNU/LINUX there are different types of users already predefined and each of them have different characteristics for the correct functioning of the system. The first of these is the root user.

Root user

The Root user is the administrator of the operating system, with all options and permissions to perform operations and changes in the system. Its main features are:

• It is the only user account with system-wide privileges.
• In charge of controlling the administration of user accounts.
• It is the only user that can stop the system.
• The root user can install, uninstall and update basic system programs and libraries.

In other words, the root user is the all-powerful user of the system.

Special Users 

They are also called users without login. Their roles in the system are determined by the addition of installed programs. They are distinguished by:

• They are created (usually) automatically at the time of Linux or application installation.
• It does not have all the privileges of the root user, but depending on the account they assume different root privileges.
• They are created when installing the system or any of its extra components.
• Some of them are: bin, daemon, adm, lp, sync, shutdown, mail, operator, squid, apache

Common Users 

These last users are us. That is, they are accounts that are created to use all accessible parts of the system, such as assigned programs and directories. They have a Home directory where they can easily access their documents and files.

• Such users have only full privileges in their working directory or HOME.
• They are used for individual users.
• These users do not have privileges to manage the system or install software. 

How to Create or Add USERS TO Linux Systems?

The adduser command is a “new” alternative to the useradd binary that allows us to add common users to our GNU/LINUX distribution. It is much more “friendly” and intuitive than useradd and allows us to add additional information about the new user. Here we will learn how to create user on Linux system.

In order to use the adduser command we must be root users, for that reason write the following command in a terminal

#Connect using su.
su

You need to write the root password then we can use adduser. The most basic way to use the adduser command is:

adduser new_user 

where new_user is the name of user account. With this “basic” form a new user is created and by giving Enter we are asked to enter the new password for this new user account.

Type the password for your new user account and press Enter. Prompt will ask you to reenter the same password. Re-enter the password and press enter button.

After that you will be asked for any additional information such as Full Name, phone number etc.

At the end, prompt will ask us to confirm the information provided. Type Y to proceed if all information is correct.

Now, as mentioned above, that is the most basic way to create a user but adduser also has options and alternatives to create users. These alternatives are given by the options that can be added to the command, some of them are:

adduser [--home DIR] [--shell SHELL] [--no-create-home] [--uid ID]
[--firstuid ID] [--lastuid ID] [--gecos GECOS] [--ingroup GROUP | --gid ID]
[--disabled-password] [--disabled-login] [--add_extra_groups] USER

You can see various options available with adduser command in above screenshot. Let’s look at example. If you would like to add a user named angelob, and set their home directory to /opt/angelob/ the the command will be as follows.

adduser --home /opt/angelob/ angelob

And then it will ask us for the new password and basic user information. This way you can add user to Linux system.

How to Delete Linux User Accounts? 

As in above section we have learnt how to create user on Linux system. Here i will explain how remove user from Linux system. Removing users from the Linux system is simple and the command to do so is userdel and has many fewer options to make the task. You can see the descriptions for this command in below image.

The easiest way to delete a user is to remove it by running below command. Here user is the name of user account.

#user is the name of user account.
userdel user

We can also delete your entire personal directory with the -r option. Run below command to do this.

userdel -r user

How to Modify Linux User Accounts? 

I have shown how to remove Linux user  in above section. Here, i will explain how to modify a Linux user. To modify users in the GNU/LINUX system we use the command usermod. We can see in the following image the possibilities shown in command:

For example, if we wanted to change the password of a created user, it would be as follows.

#user is the name of Linux user account.
usermod -p ‘new_password’ user

Obviously this method is not recommended because it leaves the password visible. Be careful. Read below article if you want to change your Linux user account password.

• How to Reset or Change User Password on RHEL?

Another useful example is to move the personal folder of the desired user to another one, for it we must combine the options -d and -m in the following way.

usermod -d /opt/angelob1/ -m angelob

With the rest of the options, we only have to place them according to our needs.

In conclusion we can say that the administration of GNU/LINUX users is a matter for system administrators, the subject is wide and with many options to take into account for the creation of them.

Adduser simplifies the creation of user accounts and usermod allows us to modify them in a very fast and dynamic way. And if a user has already completed his life cycle in the system, we can remove it with userdel.

I hope this small guide has been useful to understand a little bit about GNU/LINUX users, clarifying that the commands explained in the article can be used in all popular distributions such as Ubuntu, Red Hat, Suse, Debian and others.
I hope you like this article. Please follow our Facebook page and Twitter handle to get latest updates.

Read More:

• RHEL Interview Questions and Answers
• How to Change Hostname of RHEL system?
• How to Install and Use Htop to Monitor Linux Processes?
• Install Debian 9 Stretch in Step by Step Process

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Red Hat Linux Interview Questions and Answers

1. What is Red Hat Enterprise Linux?

Answer – It is a GNU/LINUX distribution created and developed by the multinational enterprise oriented company Red Hat, being the basis for deploying cloud applications, servers or data center.

2. Which is the package manager used by RHEL?

Answer – The package manager in RHEL is YUM.

3. How do I install a program in RHEL?

Answer – Installing a package or program in RHEL is simple thanks to YUM. With root permission we type in a terminal.

yum install package_name

4. How can I update the packages in RHEL?

Answer – Run below command to update all packages in your RHEL system.

yum upgrade

5. How can I remove the packages in RHEL?

Answer – Run below command to remove any package from RedHat Linux system.

#Make sure to change the name of package that needs to be removed.
yum remove package_name

6. How can I create a user in RHEL?

Answer – We can create a new user from the terminal with the command:

#Pass the user name in place of user_name 
useradd user_name

7. How can I change the user password?

Answer – The command that allows us to change the password of the active user is passwd. By entering the command in the terminal, you will be prompted for the current password and twice for the new password. Read below article to learn step by step process to change user as well as sudo passwords.

• How to Reset or Change User Password on RHEL?

8. How can I change the system language?

Answer- In RHEL, the system language can be changed using the following command:

localectl set-locale LANG=”xx_XX.utf8”

For example, if the system is in English and we want to change to German

localectl set-locale LANG=de_DE.utf8

Then simply restart the active session (not the system)

9. What is a GNU/LINUX service?

Answer – A service is a program that runs in the background, outside the interactive control of the users of the system.

10. How can I list the services that are running in RHEL?

Answer – This is very useful when managing systems in GNU/LINUX. We can list all the services that are running with the command:

systemctl -t service --state=active

If you want to check specific service running on RHEL then you can pass service name in above command. For example, i want to check SQL Server service running on my RHEL system then i will run below command. This command will show the output for only this service.

systemctl status mssql-server

11. How can I start or stop a service?

Answer – During the administration of a GNU/LINUX based server we will have to initialize and also stop services.

To initialize:

systemctl start name.service 

To Stop:

systemctl stop name.service

To Restart:

systemctl restart name.service

12. How do I know which version of the kernel I am using?

Answer – To know which version of the Linux Kernel we are using, just use the command:

uname –r

13. How can I add and remove kernel modules?

Answer – The kernel modules are small programs or libraries that are added to the kernel so that it can manage hardware devices. To add or remove it we use the command modprobe.

For example, to add a module:

modprobe module_name

And to remove a module

modprobe –r module_name

14. What is the file system RHEL uses?

Answer – The file system used by RHEL is XFS

15. How do I know how much space is available on my hard disk with RHEL?

Answer – This is useful to know when we are reaching the hard disk capacity. With the command:

df –h

16. Can I change hostname in RHEL?

Answer – Yes. There are multiple ways to change hostname of a RHEL system. For example if we want hostname to be example. com, we must write the following command in a terminal:

hostnamectl set-hostname example.com

I have explained it in below article in very details way. Have a look at this article to learn the process to rename or change the hostname of RHEL system.

• How to Change Hostname of RHEL system?

17. How do I know the IP address of the equipment in RHEL?

Answer – With the command:

nmcli dev show

We will not only be able to see the ip address, but also the complete information of all the network interfaces. You can read below article to learn how to check and change the IP address, subnet mask etc of a RHEL system.

• How to Find and Change IP Address, Subnet Mask and Default Gateway of RHEL system?

18. Can I install a Web server in RHEL?

Answer- Of course, we can install the web server we want: Apache, nGix or other.

19. After installing Apache2, where is the default Document Root located?

Answer – By default the apache2 DocumentRoot in RHEL sets it to /var/www/html/

20. How do I know which version of Red Hat Enterprise Linux is running the server?

Answer – Run the command in a terminal to get the RHEL version running on your system.

/etc/redhat-release

I hope you like this set of Linux Interview Questions and Answers. Please follow our Facebook page and Twitter handle to get latest updates.

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HTOP is a process viewer on the terminal created for the entire Unix family but has great popularity within GNU/LINUX. The main advantages of this tool are that we can monitor processes, finalize and explore them all from the terminal which guarantees a minimum expenditure of resources.

How to Install Htop on Various Linux Systems?

We can install Htop from your sources or simply from pre-compiled packages. However, Htop is found in most GNU/LINUX distributions through its main repositories. Please have a look at the step by step process to install Htop on Debian, RHEL, Ubuntu and other Linux systems.

Install Htop on Debian 8 Jessie and Debian 9 Stretch

The first thing we need to do is login as root. Type su on your terminal window and press enter followed by typing its password.

su

Once you will be connected using su account, run below command to install Htop on Debian systems.

apt install htop

Install Htop on Ubuntu 14.04  and Ubuntu 16.04

Here, I will show you how to install Htop on Ubuntu servers. Installation of htop on Ubuntu is very much similar to installing it on Debian as we did in above section. In the case of Ubuntu, we must be root user to install htop. Type su and press enter followed by its password.

su

Once we will be connected using root user, we will proceed to install htop on Ubuntu by running below commands. A set of execution will start and execution will finish with its installation.

apt install htop

Install Htop on Fedora 26 and Fedora 27

There is little different way to install Htop on Fedora systems as we use dnf command to install it on fedora. Fedora users can easily install htop as following below steps. First we connect to root privileges using su account. Type su and its password.

su

Once we will be connected using su account, we can invoke dnf which is Fedora package manager. Run below dnf command to install htop on Fedora systems. A set of execution will start and finish with htop installation.

dnf install htop

Install Htop on Gentoo

For Gentoo we must also be root users. Write in a terminal:

su

Once we connected with root user permissions, we can install Htop by invoking Gentoo’s own package manager emerge. Run below command to start Htop installation on Gentoo system. A series of execution will start and finish with its installation.

emerge sys-process/htop

Install Htop on CentOS 7 and Red Hat Linux Enterprise

In the case of CentOS 7, we must first add the EPEL repository and then install it normally. First we logged in as root user to get installation privileges. Connect using su account by typing below command and its password.

su

Once logged in as root user, we proceed to download the package containing the EPEL repository:

wget http://mirror.pnl.gov/epel/7/x86_64/e/epel-release-7-5.noarch.rpm

And then install the downloaded package to add the EPEL repository:

rpm -Uvh epel-release-7-5.noarch.rpm

The next step is to “validate” the newly added repository by running below command.

yum repolist

And Finally we will install htop on RHEL and Centos by running below command.

yum install htop 

Basic use of HTOP

We have installed Htop on various Linux systems in above section. We must write the command htop To launch it. Here, it is not necessary to do it as root user. You can see i wrote htop on terminal window and press enter to get the Htop task manager screen in below image.

You will get below screen once you execute above command on your terminal screen.

It’s the initial screen. As we can see, it is very simple and easy to understand. We can have fun in three parts that we will study. In this “first part” we see the use of the processor and RAM by all processes along with other details. On the right side a small summary of them: how many tasks are running and the total active time.

“Second part” also you can say middle part is the main view where all the processes are showing. You can see this in below image.

This is where we have the information through columns that contain details about these processes. The columns are:

• PID: Displays the process identifier number.
• User: The user tells us that he has activated this process.
• PRI: It shows the priority with which the process is executed.
• NI: Shows the priority level
• VIRT: The number of virtual memory used by the process.
• RES: Used ram memory (Megabytes)
• SHR: Shared memory used
• S: Process status (Running, sleeping or zombie)
• CPU%: percentage of CPU used by the process
• MEM%: percentage of ram memory used by the process
• TIME+: Active life-time of the process
• Command: The command that executes the process.

So with this we have all the information of the processes that are running at that time.

The “last part” displays the tools provided by Htop for navigation. For example, allows us to search, sort the processes and the best, for me, the “kill” processes by using shortcut keys.

To kill a process we must navigate through the processes with the up and down keys, select it, press the F9 key, and choose the number that corresponds to the option of killing that serves us. I almost always use the 9th showing in below screen.

In conclusion we can say that Htop is a great tool to visualize the active and non-active processes in our Linux systems. Being terminal-based, it has the advantage of consuming virtually nothing in resources and helps us to close very heavy or unnecessary processes.

Here i have shown you how to install Htop on Red Hat, Ubuntu, Fedora and Debian systems and its uses. I hope you like this article. Please follow our Facebook page and Twitter handle to get latest updates.

Read More:

• How to Install Debian 9 Stretch?
• How to Install Zebbix on Ubuntu 16.4?
• How to Install RedHat Enterprise Linux?
• Installing Ubuntu 16.04 in Step by Step Process

The post How to Install Htop and Use it to Monitor Linux Processes? appeared first on Techyaz.com.

Related Articles

• How to Change Hostname of RedHat Linux Server?
• How to Change IP, Subnet Mask and Default Gateway of RHEL Machine?
• Step by Step Process to Install RHEL 7.3

Change User Password in Linux

We will use passwd command to change our own password. Type passwd without specifying a username if you are changing your password. You will be prompted to enter your old password for verification, and then a new password. Below is the high level steps to follow password change process.

1. Open Linux terminal or connect to your server using PuTTY.
2. Type passwd and press Enter.
3. Type your current password and press Enter.
4. Now type your new password and press Enter.
5. Make sure your password is complex and not similar to older one otherwise you will get BAD PASSWORD error as shown in below screenshot.
6. Retype your new password and press Enter.
7. Now you will get notify that your password has been changed.

Change root Password

The “root” account is the most privileged account on a Linux system. When using this account, it is crucial to be very careful. Follow below steps to reset or change the password for your root account.

1. Open Linux terminal or connect to your server using PuTTY.
2. Type su at the command prompt, and press Enter.
3. Type the current root password, then press Enter.
4. Type passwd and press Enter.
5. Type a new password and press Enter.
6. Retype the new password and press Enter.
7. Your password is changed here.

Change other User’s Account Password

To change a password for a user, you need to login using “root” account. Follow below steps to change password on behalf of any user.

1. Open Linux terminal or connect to your server using PuTTY.
2. Type su at the command prompt, and press Enter.
3. Enter current root password, then press Enter.
4. Type passwd LoginName and press Enter. If login account is Manvendra. Type passwd Manvendra.
5. Enter a new password and press Enter.
6. Retype the new password and press Enter.
7. Your password is changed here.

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Related Articles:

• How to Change IP Address, Subnet Mask & Default Gateway of Linux Server
• Fix Error “User is not in sudoers file.
• Installing SQL Server 2017 on Red Hat Linux Server

There are three methods that i will explain here to rename or change hostname of any Redhat Linux server. These three methods that we will use in this article are given below.

1. Edit etc/hostname.
2. nmcli utility.
3. hostnamectl utility.

Change Hostname of RedHat Linux Server by editing etc/hostname

Connect to your Redhat Linux server and check the hostname by running below command.

#Check hostname using any command.
hostname

#You can run below command as well to check the hostname.
cat /etc/hostname

We can see the hostname of this RedHat Linux server is SQLServerRHEL.localdomain. Now we will change this name to TechyazRHEL. If you want to change hostname with fully qualified name then you can write fully qualified name in below command.

#Connect to RHEL as root.
Sudo su

#Change Hostname using /etc/hostname. Make sure to write your new hostname in double quote.
echo "TechyazRHEL" > /etc/hostname

You can see when we check hostname of this linux server using hostname command, it is still showing older name SQLServerRHEL.localdomain whereas when we check using cat /etc/hostname it is showing newly changed hostname.

To reflect new host name, we need to restart service systemd-hostnamed. Once you will restart this service, the output of hostname command will also be same as new hostname as shown in above image.

#Restart service systemd-hostnamed
service systemd-hostnamed restart
 or
systemctl restart systemd-hostnamed 

You can see new hostname after restarting above service in above screenshot. One thing you might notice that command prompt is still showing as older hostname i.e. root@SQLServerRHEL. This will disappear if you close this session and connect to new session. Let’s try this by launching new session, you can see new session is showing new hostname in below image.

Rename Hostname of RedHat Linux Server using nmcli

The nmcli (NetworkManager Command Line Interface) command-line utility is used for controlling Network Manager and reporting network status. nmcli is used to create, display, edit, delete, activate, and deactivate network connections, as well as control and display network device status. Here, we will use nmcli utility to change the hostname of redhat linux server.

Connect to your RedHat Linux server for which you want to change hostname. Run below commands to change hostname of your linux server.

#Connect to RedHat Linux server using root access.
sudo su

#Check existing hostname of RHEL server
hostname

#Change hostname of Redhat Linux server to techyazRHEL-nmCLI. 
#Change your new hostname in place of techyazRHEL-nmCLI.
nmcli general hostname techyazRHEL-nmCLI

#You can also check the hostname of Redhat linux server using nmcli utility.
nmcli general hostname

You can see hostname of RHEL server has been changed to techyazRHEL-nmCLI in above screenshot. Your PuTTY terminal will display new hostname once you open new session.

If new hostname is not showing after running above command then you should restart service systemd-hostnamed to reflect changes as we did in first section. You can see new hostname while launching new session in below screenshot.

Rename Hostname of RedHat Linux Server using hostnamectl

The hostnamectl tool is provided for administering the host names in use on a given system. First we will check the existing hostname of this server by running hostname command. We can also check the hostname using hostnamectl utility.

Connect to your Redhat Linux server using root and check the existing hostname.

#Connect to Linux server using root.
sudo su

#Check existing hostname.
hostname
or 
hostnamectl status

#Change hostname to techyazRHEL.
hostnamectl set-name techyazRHEL

You can check the hostname post renaming it. You can see new hostname in above image. Putty terminal will display new hostname once you open new session as shown in below image.

We can set all three kind of hostnames (Static, Pretty and Transient) using hostnamectl utility. Static name is mandatory whereas remaining two is optional. If you will not pass any hostname type in above command, same name will be set for all three hostnames. I haven’t used any hostname option in above command so all hostname options will have same name. Let’s check this in below image.

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Read More on SQL Server Installation on Linux Servers

• Installing SQL Server 2017 on Ubuntu Server
• How to Install SQL Server on Redhat Linux without Internet – Offline Installation
• How to Install SQL Server on Ubuntu without Internet access – Offline Installation
• Installing SQL Tools on RedHat Linux Server

Uninstall SQL Server 2017 on RHEL

Uninstalling SQL Server 2017 that is running on Linux server is totally different process as compared to its windows based SQL Server. Follow below step by step process to uninstall SQL Server 2017 from your RHEL Linux server.

1- Connect to your RHEL server where SQL Server 2017 is running and that needs to be uninstalled from the server. Check whether SQL Server is installed on this machine or not by checking SQL Server service. You can see SQL Server is running on this server.

#Run below command to check SQL Server Service.
systemctl status mssql-server

2- Check SQL Server files along with the database files before removing SQL Server. We will validate SQL Server removal by checking these files post uninstallation.

#Check SQL Server binaries & database files
sudo ls -lrt /var/opt/mssql
sudo ls -lrt /var/opt/mssql/data

Note: Make sure to run full backup of all your databases and keep them in a safer place for future needs.

3- Now, we will uninstall SQL Server by running below command.

#Remove SQL Server from your Linux machine
sudo yum remove mssql-server

You can see details of SQL Server packages that are going to be removed from this machine. These packages are SQL Server and SQL Server Agent that are going to be removed here. Above command will ask us to type y to proceed with the uninstallation and type N to quit this uninstallation. Type y to proceed with the SQL Server removal.

You can see SQL Server and SQL Server Agent both packages have been removed from this machine.

4- Now, remove all SQL Server binaries and database files from its installed location that we checked in second screenshot by running below command.

#Remove database files and SQL binaries.
sudo rm -rf /var/opt/mssql

Here, your SQL Server has been removed from this linux machine. You can check SQL Server binaries and database files by running below commands. We can see no files in these locations because they have been removed now.

#Check database files.
sudo ls -lrt /var/opt/mssql/data

#Check SQL Server files & folders.
sudo ls -lrt /var/opt/mssql

5- Now you can check SQL Server service again by executing below command.

#Check SQL Server Service post SQL Server Removal.
systemctl status mssql-server 

We can see SQL Server is not loaded this time but service is showing as active.

When you will try to restart this service, it will throw you an error that unit not found as shown in below image.

6- Now we need to reboot the server to update SQL Server service status on RHEL. I restarted this linux machine and again checked the SQL Server service. You can see this time it says service could not found.

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Linux Error: User is not in the sudoers file. This incident will be reported.

Sudoers file determines which users can run administrative tasks, those requiring superuser privileges. I was failed to perform certain tasks because my username was not part of this file and returned with below error.

Linux Error: User is not in the sudoers file. This incident will be reported.

Someone who has super user (su) access can fix this issue by adding the impacted user to the Wheel group. The wheel group is a special user group used on Linux systems to control access to the sudo command, which allows a user to behave as super user. Once we will add our user into this group, we will be able to use sudo command.

Adding a Linux User to Sudoers File

Connect to your Linux server using superuser account.

#Connect using su
su

We have now connected to Linux server using superuser account. Now we can add our username to the Wheel group.

#Add your user to the Wheel group. 
#Change techyaz to your user name.
usermod -G wheel techyaz

Now we see that command executed successfully. If you attempt to execute the sudo command now, you might get the same error message unless you completely logout your user and re-login again. Next you can exit from superuser account by typing exit on the prompt.

#Exit su account
exit

Your account or user added to sudoers file now by adding it to wheel group. You can now connect to Linux server from your PuTTY terminal or on the local server and try to run sudo command post completely logout your user and re-login on the server.

#Connect to Linux server using your user name that was added to wheel group.
#Run sudo su
sudo su

You can see we have access to run anything using sudo command in above screenshot. Now your username is part of sudoers file so you can perform your activities using sudo command now.

Read more:

• How to Install Red Hat Linux 7.3 on Virtual Machine
• Fixing PuTTY connection issue while connecting to Red Hat Linux Server
• Installing SQL Server 2017 on Red Hat Linux Server

Here, we have fixed Linux error “User is not in the sudoers file. This incident will be reported.” I hope you like this article. Please follow us on our facebook page and on Twitter handle to get latest updates.

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