Init vs systemd and runlevels vs targets in RHEL 6/7/8/9

 RHEL 6 uses SysVinit, which is a traditional init system that was widely used in Linux distributions before the introduction of systemd.

SysVinit is a simple, yet effective init system that uses a series of scripts to manage system services and startup processes. However, it has some limitations, such as:

- No parallelization of startup processes

- No automatic dependency management

RHEL 7 uses systemd, provides a more modern and efficient init system that offers features such as:

- Parallelization of startup processes

- Automatic dependency management

- Improved logging and debugging capabilities

Overall, the switch from SysVinit to systemd in RHEL 7 provides a more efficient, scalable, and reliable init system for managing system services and startup processes.

Why RHEL switched to systemd ?

Parallel service initialization : Unlikes the SysV init program, systemd is capable of launching services in parallel. The init program, by contrast, launches them one by one.

Faster process communication : The parallel capabilities of systemd carry over to inter-process communication. systemd is able to offer parallel access to sockets and system bus, significantly reducing process wait times for communication resources.

Automatic restart : If a service crashes, systemd can detect that and attempt to restart it. Most of the times, a simple restart is all that is needed for an application to begin functioning again, unless there are more fundamental issues.

RHEL 7/8/9 booting process is faster than RHEL6 due to parallel service initialization .

Key Differences of RHEL 6 and RHEL 7/8/9 Commands Usage Below

Task Name	RHEL 6 Command	RHEL 7/8/9 Command
Service Management	service <service> start/stop	systemctl start/stop <service>
	chkconfig <service> on/off	systemctl enable/disable <service>
	chkconfig --del <service_name>	systemctl mask <service>
	chkconfig --add <service_name>	systemctl unmask <service>
	chkconfig --list	systemctl list-unit-files
System Boot and Runlevel	runlevel	systemctl get-default
	init <runlevel>	systemctl isolate <target>
	N/A	systemctl list-units --type=target
Log Management	tail /var/log/messages	journalctl -u <service>
	N/A	journalctl --since "today"
		journalctl -u sshd --since="2024-12-01"
	N/A	journalctl -xe
Network Management	service network start/stop	systemctl start/stop NetworkManager
	ifconfig	ip addr
	netstat -tulnp	ss -tulnp
Firewall Management	iptables commands	firewalld with firewall-cmd
	service iptables start/stop	systemctl start/stop firewalld
	iptables-save	firewall-cmd --permanent --list-all
Package Management	yum install <package>	yum install <package> (backed by DNF)                                                                 dnf install <package> -> RH8/9
Disk and Filesystem Management	fdisk	parted or gdisk for GPT disks
	mount	mount
User Session Management	who, w	who, w
	last	last
Shutdown and Reboot	shutdown -h now	systemctl poweroff
	reboot	systemctl reboot
	halt	systemctl halt

Runlevels and Targets Details

RHEL 6 Runlevel	RHEL 6 description	RHEL 7/8/9 target name	RHEL 7/8/9 description
0	Halt  (shutdown the system)	poweroff.target	Powers off the system.
1	Single-user mode  (maintenance mode)	rescue.target	Single-user mode for troubleshooting (similar to runlevel 1).
2	Multi-user mode  (without networking)	emergency.target	Minimal environment for system recovery.
3	Full multi-user mode (text mode)	multi-user.target	Multi-user system without a GUI (similar to runlevel 3).
4	Unused  (available for customization)	Unused	NA
5	Full multi-user mode  (with graphical interface)	graphical.target	Multi-user system with a graphical user interface (GUI).
6	Reboot (restart the system)	reboot.target	Reboots the system.
		default.target	The default target set for the system (symlink to another target).

Run level/Targets	RHEL 6 Command	RHEL 7/8/9 Command
Check default runlevel	cat /etc/inittab	systemctl get-default
Check current runlevel	runlevel	systemctl is-active <target> ; shows active or not
Change runlevel / Target	sudo init <runlevel>	sudo systemctl isolate <target>
Set Default	vi /etc/inittab => id:3:initdefault: <== change required levels	sudo systemctl set-default <target>