Some of the most popular Linux system initialization daemons are:

• init
• systemd
• upstart 

The system initialization daemon will use PID 1. In this example, systemd has PID 1.

[john.doe@server1 ]# ps -e
 PID  TTY        TIME  CMD
   1  ?      00:00:02  systemd
   2  ?      00:00:00  kthreadd
   3  ?      00:00:01  ksoftirqd/0
   . . .

service vs. systemctl

Probably the most obvious different between init and systemd is that init uses the services command to start, stop, and restart services, and systemd uses the systemctl command to start, stop, and restart services. For example, to restart the httpd service on an init system, the following command would be used.

[user1@server1 ]# service httpd restart

To restart the httpd service on an init sytem, the following command would be used.

[user1@server1 ]# systemctl restart httpd

chkconfig vs. systemctl

Another noteworthy difference is that init uses the chkconfig command to configure a service to be enabled or disabled on boot, and systemd uses the systemctl command. For example, to ensure the httpd service is turned on on an init system, the following command would be used:

[user1@server1 ]# chkconfig httpd on

To enable the httpd service on a systemd system, the following command would be used.

[user1@server1 ]# systemctl enable httpd

Runlevel vs. systemctl

Another important different is that init uses the /etc/inittab file to set the runlevel of the system.

With systemd, you use the following command to determine if the system is in text-only mode or graphical mode.

multi-user.target is text-only.

[root@server1]# systemctl get-default
multi-user.target

graphical.target is graphical mode.

[root@server1]# systemctl get-default
graphical.target

The following command can be used to set the runlevel to either multi-user.target or graphical.target.

[root@server1]# systemctl set-default multi-user.target
[root@server1]# systemctl set-default graphical.target