Init

source
The first program that is called which was compiled with the standard C library after the kernel boots. Usually it is locatted in sbin/init but it can also be in other places as a simple shell script.

Init Process

It is a process of the system, with PID=1, being the parent to all processes.

• inittab file:
  Used to define the initial runlevel (software configuration to allow only a selected group of processes to exist). Both GRUB and LILO can change runlevel at boot time.

• init.d:
  This is a directory that can be used to start/stop individual daemons (background processes).

• /etc/rc.d:
  Contains scripts to used to control the starting, stopping and restarting of daemons. scripts starting with K will run first and kill the associated processes, while those starting with S will start them

• chkconfig:
  Command used to configure services on/off and add/remove them from runlevels.

systemd Process

It another process of the system, with PID=1, being the parent to all processes. It is becoming more prevalent and is mostly compatible with init.

• /etc/systemd/system/:
  Everything is in this directory, with lots of symbolic links to startup files.
  Main Commands: systemctl and journalctl.

• Targets instead of Runlevels Runlevels may still exist as symbolic links to new targets

• Works on units
  Are different services, sockets, targets, devices, mount, snapshots. Mosly services.

• systemclt used to enable or disable units.