systemd
What is an init system?
Running Programs Automatically on Your Tiny Computer: systemd: Writing and Enabling a Service
Raspberry Pi Boot Process
Clarification about the boot process (systemd / init)
How To Use Systemctl to Manage Systemd Services and Units
How To Configure a Linux Service to Start Automatically After a Crash or Reboot – Part 2: Reference
How does systemd use /etc/init.d scripts?

Before: SysVinit (System V init, or just init)
Now:    systemd

- In systemd, there is no concept of runlevels.
- These are replaced by targets.
- Targets can and do depend on each other.
  basic.target => multi-user.target => graphical.target
- Under SysVinit, scripts run strictly in order

runlevel                                             => systemd target
0: halt
1: single-user
2: multi-user
3: multi-user with networking                        => multi-user.target
4: undefined (user defined)
5: multi-user with display manager (graphical login) => graphical.target
6: reboot

basic.target
network.target
multi-user.target
graphical.target

init.scope
system.slice
-.mount
-.slice

$ sudo apt-get update
$ sudo apt-get dist-upgrade
$ sudo apt-get install graphviz
$ sudo apt-get install tree

$ systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.

graphical.target @33.104s
└─multi-user.target @33.103s
  └─getty.target @33.103s
    └─getty@tty1.service @33.102s
      └─systemd-user-sessions.service @33.030s +16ms
        └─network.target @33.023s
          └─dhcpcd.service @2.326s +30.692s
            └─basic.target @2.161s
              └─sockets.target @2.161s
                └─triggerhappy.socket @2.160s
                  └─sysinit.target @2.155s
                    └─systemd-timesyncd.service @1.930s +224ms
                      └─systemd-tmpfiles-setup.service @1.864s +55ms
                        └─local-fs.target @1.854s
                          └─boot.mount @1.836s +16ms
                            └─systemd-fsck@dev-disk-by\x2dpartuuid-fc38305d\x2d01.service @1.492s +336ms
                              └─dev-disk-by\x2dpartuuid-fc38305d\x2d01.device @1.489s

$ systemd-analyze dot | dot -Tsvg > systemd.svg
   Color legend: black     = Requires
                 dark blue = Requisite
                 dark grey = Wants
                 red       = Conflicts
                 green     = After

$ pstree
systemd-+-agetty
        |-avahi-daemon---avahi-daemon
        |-bluetoothd
        |-cron
        |-dbus-daemon
        |-dhcpcd
        |-hciattach
        |-login---bash
        |-rsyslogd-+-{in:imklog}
        |          |-{in:imuxsock}
        |          `-{rs:main Q:Reg}
        |-sshd---sshd---sshd---bash---pstree
        |-systemd---(sd-pam)
        |-systemd-journal
        |-systemd-logind
        |-systemd-timesyn---{sd-resolve}
        |-systemd-udevd
        |-thd
        `-wpa_supplicant

Global:
/etc/systemd/system
/lib/systemd/system/

User:
/etc/systemd/user,
/run/systemd/user
/lib/systemd/user
/usr/lib/systemd/user

$ systemctl status

$ systemctl
or 
$ systemctl list-units

$ systemctl list-unit-files

Service units
Target units

unit
slice
scope
service

services (.service)
mount points (.mount)
devices (.device)
sockets (.socket)
(.automount)
(.swap)
(.target)
(.path)
(.timer)
(.slice)
(.scope) 

$ sudo systemctl start myscript.service
$ sudo systemctl stop myscript.service
$ sudo systemctl enable myscript.service

autologin@.service
bluetooth.target.wants
dbus-org.bluez.service -> /lib/systemd/system/bluetooth.service
dbus-org.freedesktop.Avahi.service -> /lib/systemd/system/avahi-daemon.service
dhcpcd.service.d
dhcpcd5.service -> /lib/systemd/system/dhcpcd.service
getty.target.wants
getty@tty1.service.d
halt.target.wants
multi-user.target.wants
network-online.target.wants
poweroff.target.wants
rc-local.service.d
reboot.target.wants
remote-fs.target.wants
sockets.target.wants
sshd.service -> /lib/systemd/system/ssh.service
sysinit.target.wants
syslog.service -> /lib/systemd/system/rsyslog.service
timers.target.wants