On quite a few Linux distributions these days, the init system has been switched to systemd. This is true of Debian and CentOS starting with Version 8 and 7, respectively, but other distributions such as Fedora, Ubuntu, Arch Linux, and others have switched as well. Although some administrators prefer sysvinit, which was the previous dominant init system, systemd offers quite a few advancements over older systems.

With systemd, commands you would use to start processes are now different, though the majority of the older commands still work (for now). With sysvinit on a Debian 7 system, you would use the following command to restart Samba:

/etc/init.d/samba restart

However, with systemd, we now use systemctl to start, stop, or restart a process:

# systemctl restart samba

The sysvinit style of managing processes was the same in CentOS and Debian, and it is still the same now. At the time of this writing, both have switched to systemd. But the older /etc/init...