The basic and introductory information about Systemd can be found on
the manual page systemd-start.
The sections [Unit] and [Install] concerning all unit
files are covered by our manual page systemd unit
file.
Now the function of the systemd.target unit will be
explained in more detail, which is similar to the commonly known
runlevels.
The different runlevels that are booted or switched to are described by systemd as target units. They have the extension “.target”.
The old sysvinit commands are still supported. (For this a quote from
man systemd: “… is provided for compatibility reasons and
because it is easier to type.”)
| target unit | description |
|---|---|
| emergency.target | launches into an emergency shell on the main console. It is the most minimal version of a system boot to get an interactive shell. This unit can be used to guide the boot process step by step. |
| rescue.target | starts the base system (including system mounts) and an emergency shell. Compared to multi-user.target, this target could be considered as single-user.target. |
| multi-user.target | starts a multi-user system with a working network, without graphics server X. This unit is used when you want to stop X or not to boot into it. This unit is used in special cases (when X itself or the desktop environment are upgraded) to perform a system upgrade (dist-upgrade). |
| graphical.target | is the unit for multi-user mode with network capability and a running X window system. |
| default.target | is the default unit that systemd launches at system startup. In siduction this is a symlink to graphical.target (except for the noX variant). |
A look at the documentation man SYSTEMD.SPECIAL(7) is
mandatory to understand the relationships of the different target
units.
There are three special features to be considered for the target units:
The use on the kernel command line during the boot process
In order to get into the edit mode in the boot manager Grub, you must
press the e key when the boot selection
appears. Then append the desired target to the kernel command line with
the following syntax: “systemd.unit=xxxxx.target”. The table lists the
kernel commands and their still valid numeric equivalents.
| target unit | kernel command | kernel command old |
|---|---|---|
| emergency.target | systemd.unit=emergency.target | - |
| rescue.target | systemd.unit=rescue.target | 1 |
| multi-user.target | systemd.unit=multi-user.target | 3 |
| graphical.target | systemd.unit=graphical.target | 5 |
The old runlevels 2 and 4 refer to multi-user.target
The use in the terminal during a running session
Provided you are in a running graphical session, you can switch to the
virtual terminal tty3 with the key combination
CTRL+ALT+F3.
Here you log in as root user. The following table lists
the terminal commands, where the expression “isolate” ensures
that all services not requested by the target unit are terminated.
| target unit | terminal command | init command alt |
|---|---|---|
| emergency.target | systemctl isolate emergency.target | - |
| rescue.target | systemctl isolate rescue.target | init 1 |
| multi-user.target | systemctl isolate multi-user.target | init 3 |
| graphical.target | systemctl isolate graphical.target | init 5 |
Target units that should not be called directly
A number of target units are used to group intermediate steps with
dependencies during the boot process or the .target change. The
following list shows three frequently used commands that should
not be called with the syntax “isolate xxxxx.target”.
| target | terminal command | init command alt |
|---|---|---|
| halt | systemctl halt | - |
| poweroff | systemctl poweroff | init 0 |
| reboot | systemctl reboot | init 6 |
“halt”, “poweroff”, and “reboot” fetch several units in the correct order to terminate the system in an orderly fashion and to reboot if necessary.
man systemd.target