Init System Identification

Identify the init system in use.

Commands

ps -p 1	Checks the PID 1 process. The name can indicate the init system (systemd, init, etc.).
ps -p 1 -o comm=	PID 1 CMD field only.
systemctl	Presence indicates systemd. If command runs, your system uses systemd.
systemctl --version	If present, usually indicates systemd.
stat /proc/1/exe	Displays info about the init process. Follow the symlink to see if it points to systemd, upstart, etc.
stat /sbin/init	Symbolic link to Upstart's process.
/proc/1/comm	Reads the process name of PID 1 directly from the kernel.
ls /sbin/init	Checks what /sbin/init points to. It can be a symlink to the actual init system like systemd or upstart.
ls /etc/init.d	Presence of scripts in /etc/init.d often suggests sysvinit or *Upstart.
ls /etc/init	Presence of upstart config files means it's Upstart.
initctl	If present, Upstart is likely.
lsb_release -a	Displays Linux distribution information, sometimes including init system type.
man init	Can reveal Init System.

Checks for a systemd-specific directory:

test -d /run/systemd/system && echo "systemd"

Shows location of init files (SysV, SystemD, Upstart, etc):

sudo ls -latr /proc/1/exe

Reading CMD Field for PID 1: systemd: systemd sysvinit: init Upstart: /sbin/init (symbolic link to Upstart's process)

SysV

Boot Process

BIOS

MBR - Finds and executes GRUB

GRUB - Select OS to run, loads kernel

Kernel - Once kernel connects to filesystem it moves to init phase

Init - kernel reads script /sbin/init. This script kicks off services. It looks in /etc/inittab for the initdefault entry (initdefault:3). Once it finds this level, it looks for the run scripts which are stored in /etc/rc.d OR /etc.

The scripts look like this:

/etc/rc.d/rc3.d OR /etc/rc3.d

Run - Once these scripts are done, the system is running.

Boot Levels

0 – Halt

1 – Single User

2 – Multi User (Without NFS or networking features)

3 – Multi User (Typically the default. Can have GUI, but most systems use run level 5 for that.)

4 – User Defined (Basically means if you want to create your own run level you can.)

5 – X11: A graphical protocol that allows you to have a GUI.

6 – Reboot

Systemd

Milestones (Runlevels)

poweroff.target SysV's Halt - 0

rescue.target recovery shell, single user-mode - 1

multiuser.target SysV's 2-4. Allows you to turn features off an of without restart

graphical.target GUI - 5

reboot.target Reboot - 6

runlevel - shows current runlevel.