The kernel is the main component of most computer operating systems. It acts as a bridge between the applications and the actual data processing done at the hardware level. The kernel runs once when the booting process of the computer starts.
The kernel is a simple program inside the OS that is responsible for setting the memory, abstracting the hardware, schedules, and runs the process.
In simple terms, it acts as a mediator between the program and the hardware.
1. Monolithic Kernel
In this type of kernel, all operating system services operate in kernel space. It has huge lines of code which is complex.
Unix, Linux, Open VMS, XTS-400, etc.
2. MicroKernel –
In this type of kernel, there is virtual memory and thread scheduling. It has fewer services in the kernel space and fewer services in the userspace.
Mach, L4, AmigaOS, Minix, K42 etc.
3. Hybrid Kernel –
It is the combination of both monolithic kernel and microkernel. It has the stability and the speed compared to the monolithic and microkernel.
Windows NT, Netware, BeOS, etc.
4. Exo Kernel –
This type of kernel follows the end-to-end principle. It allocates physical resources to the applications.
Nemesis, ExOS, etc.
5. Nano Kernel –
It is the type of kernel that offers hardware abstraction but without system services.
A run level is a current condition of an operating system. Each of the run levels is represented by a single digit. The default run level is 5
Main Run levels are :
Other Run levels are :
To run a system level for example 0, the command is:
To get the current run level of the system, the command is:
The boot process sequence changes for CentOS/Redhat 7 and above. The service manager
systemd controls the boot process in the CentOS/Redhat 7. It is backward compatible with SysV init scripts used by prior versions of Redhat Linux with Redhat 6.
It is very important to know the boot process to troubleshoot quickly.
Linux Boot Process consists of the following steps one by one :
The message of the day is the first message to be displayed once we login into the Linux system. The message of the day file location is in
To set the message, run the command:
Once the editor opens, move to the insert mode and type the message.
Save and quit.
Once we login into the system again the message gets displayed.
To customize the message of the day follows the steps:
/etc/profile.d/motd.sh, with the command:
#!/bin/bash # echo -e " ################################## # # Welcome to `hostname` # This system is running `cat /etc/redhat-release` # kernel is `uname -r` # "
/etc/ssh.sshd_configwith the command:
/PrintMotdand set the parameter:
systemctl restart sshd.service
Myself Debomita Bhattacharjee, an IT employee with 6+ years of experience in Software industry. My area of interest is Automation testing and Front End Development.