19 systemd understanding targets | SysV runlevels systemd targets | emergency gives less than rescue

The emergency.target has no corresponding sysvinit runlevel, and would just boot your machine to a shell with really nothing started.

rescue.target is like the old single user or runlevel 1 from sysvinit.
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Runlevels/targets

Systemd has a concept of targets which serve a similar purpose as runlevels but act a little different. Each target is named instead of numbered and is intended to serve a specific purpose. Some targets are implemented by inheriting all of the services of another target and adding additional services to it. There are systemd targets that mimic the common sysvinit runlevels so you can still switch targets using the familiar telinit RUNLEVEL command. The runlevels that are assigned a specific purpose on vanilla Fedora installs; 0, 1, 3, 5, and 6; have a 1:1 mapping with a specific systemd target. Unfortunately, there's no good way to do the same for the user-defined runlevels like 2 and 4. If you make use of those it is suggested that you make a new named systemd target as /etc/systemd/system/$YOURTARGET that takes one of the existing runlevels as a base (you can look at /lib/systemd/system/graphical.target as an example), make a directory /etc/systemd/system/$YOURTARGET.wants, and then symlink the additional services that you want to enable into that directory. (The service unit files that you symlink live in /lib/systemd/system).

Sysvinit Runlevel	Systemd Target	Notes
0	runlevel0.target, poweroff.target	Halt the system.
1, s, single	runlevel1.target, rescue.target	Single user mode.
2, 4	runlevel2.target, runlevel4.target, multi-user.target	User-defined/Site-specific runlevels. By default, identical to 3.
3	runlevel3.target, multi-user.target	Multi-user, non-graphical. Users can usually login via multiple consoles or via the network.
5	runlevel5.target, graphical.target	Multi-user, graphical. Usually has all the services of runlevel 3 plus a graphical login.
6	runlevel6.target, reboot.target	Reboot
emergency	emergency.target	Emergency shell

Changing runlevels:

Sysvinit Command	Systemd Command	Notes
telinit 3	systemctl isolate multi-user.target (OR systemctl isolate runlevel3.target OR telinit 3)	Change to multi-user run level.
sed s/^id:.*:initdefault:/id:3:initdefault:/	ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target	Set to use multi-user runlevel on next reboot.

Kernel Options:

The above systemd targets can be used when booting. At the GRUB menu, edit the selection to add "systemd.unit=target" (without the double-quotation marks) as a kernel option where targetis one of the above. (For example, "rescue.target".)

Tip: the ".target" extention is optional. The "systemd.unit=rescue" kernel option works the same as "systemd.unit=rescue.target".

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Runlevels were an ugly concept in early versions of Linux implemented by the SysV init system. I always preferred the BSD approach which does not have the concept, just simple start everything or stop everything.

1. runlevel0.target -> poweroff.target
2. runlevel1.target -> rescue.target
3. runlevel2.target -> multi-user.target
4. runlevel3.target -> multi-user.target
5. runlevel4.target -> multi-user.target
6. runlevel5.target -> graphical.target
7. runlevel6.target -> reboot.target

• Runlevel 3 is the normal operating mode that allows multiple users and normal network communication. All processes configured to run are started as well.
• When a shutdown command is issued, the runlevel is switched to 0. At this runlevel, files are closed and processes are stopped before the machine is halted.
• Rebooting a Linux system uses runlevel 6. It is similar to a runlevel 0 combined with a runlevel 3 (or 1 or 2).
• Runlevel 1 is single-user mode. This is also known as maintenance mode as generally only the root user can logon from a directly-attached console. Usually only as few as processes as are absolutely necessary are started. This is used when doing major installations or OS upgrades.
• Runlevel 2 is multiuser mode, but without networking. I usually only see this as the intermediate step after booting before entering runlevel 3.

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UNIT files is anything that can be started by systemd

specific type of unit file is systemd target

- systemd targets define the state your system enters
- equivalent to runlevels in previous versions of RHEL
poweroff.target
rescue.target
mutliuser.target
graphical.target
reboot.target
- use emeregency.target to get into a minimal troubleshooting mode

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Sysvinit Command	Systemd Command	Notes
service frobozz start	systemctl start frobozz	Used to start a service (not reboot persistent)
service frobozz stop	systemctl stop frobozz	Used to stop a service (not reboot persistent)
service frobozz restart	systemctl restart frobozz	Used to stop and then start a service
service frobozz reload	systemctl reload frobozz	When supported, reloads the config file without interrupting pending operations.
service frobozz condrestart	systemctl condrestart frobozz	Restarts if the service is already running.
service frobozz status	systemctl status frobozz	Tells whether a service is currently running.
ls /etc/rc.d/init.d/	systemctl (or) systemctl list-unit-files --type=service (or) ls /lib/systemd/system/*.service /etc/systemd/system/*.service	Used to list the services that can be started or stopped Used to list all the services and other units
chkconfig frobozz on	systemctl enable frobozz	Turn the service on, for start at next boot, or other trigger.
chkconfig frobozz off	systemctl disable frobozz	Turn the service off for the next reboot, or any other trigger.
chkconfig frobozz	systemctl is-enabled frobozz	Used to check whether a service is configured to start or not in the current environment.
chkconfig --list	systemctl list-unit-files --type=service (or) ls /etc/systemd/system/*.wants/	Print a table of services that lists which runlevels each is configured on or off
chkconfig --list | grep 5:on	systemctl list-dependencies graphical.target	Print a table of services that will be started when booting into graphical mode
chkconfig frobozz --list	ls /etc/systemd/system/*.wants/frobozz.service	Used to list what levels this service is configured on or off
chkconfig frobozz --add	systemctl daemon-reload	Used when you create a new service file or modify any configuration