Some of the benefits of systemd over the traditional System V init facility include:
- systemd never loses initial log messages
- systemd can respawn daemons as needed
- systemd records runtime data (i.e., captures stdout/stderr of processes)
- systemd doesn't lose daemon context during runtime
- systemd can kill all components of a service cleanly
Transition of sysv init scripts to systemd
- In earlier Red Hat releases before RHEL 7 runlevels were used to identify a set of services that would start or stop when that runlevel was requested.
- Now starting with Red Hat Enterprise Linux 7 runlevel concept is remove and is replaced with "targets" to group together sets of services that are started or stopped. Systemd has replaced sysVinit as the default service manager.
- Some of the sysVinit commands have been symlinked to their RHEL 7 counterparts, however they may eventually be deprecated in favor of the standard systemd commands in the future
- A target can also include other targets (for example, the multi-user target includes an nfs target).
- There are systemd targets that align with the earlier runlevels.
The following list shows how systemd targets align with traditional runlevels:
Traditional runlevel New target name Symbolically linked to...
Runlevel 0 | runlevel0.target -> poweroff.target
Runlevel 1 | runlevel1.target -> rescue.target
Runlevel 2 | runlevel2.target -> multi-user.target
Runlevel 3 | runlevel3.target -> multi-user.target
Runlevel 4 | runlevel4.target -> multi-user.target
Runlevel 5 | runlevel5.target -> graphical.target
Runlevel 6 | runlevel6.target -> reboot.target
Comparison of SysV Runlevels with systemd targets
|
Runlevel
|
Target Units
|
Description
|
|
0
|
runlevel0.target, poweroff.target
|
Shut down and power off the system.
|
|
1
|
runlevel1.target, rescue.target
|
Set up a rescue shell.
|
|
2
|
runlevel2.target, multi-user.target
|
Set up a non-graphical multi-user system.
|
|
3
|
runlevel3.target, multi-user.target
|
Set up a non-graphical multi-user system.
|
|
4
|
runlevel4.target, multi-user.target
|
Set up a non-graphical multi-user system.
|
|
5
|
runlevel5.target, graphical.target
|
Set up a graphical multi-user system.
|
|
6
|
runlevel6.target, reboot.target
|
Shut down and reboot the system.
|
View and change default target (runlevel)
The default runlevel (previously set in the /etc/inittab file) is now replaced by a default target.
The location of the default target is /etc/systemd/system/default.target.
To view the existing default target use the below syntax:
[root@golinuxhub ~]# systemctl get-default
graphical.target
We can also validate the same by checking the symlink of the default.target
[root@golinuxhub ~]# ls -l /etc/systemd/system/default.target
lrwxrwxrwx. 1 root root 36 Aug 20 12:58 /etc/systemd/system/default.target -> /lib/systemd/system/graphical.target
So as you see since the default.target is linked to graphical.target hence the same is set as default.
To set the default target to a different target level using the below syntax:
[root@golinuxhub ~]# systemctl set-default multi-user.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/multi-user.target.
Now validate the same
[root@golinuxhub ~]# ls -l /etc/systemd/system/default.target
lrwxrwxrwx. 1 root root 41 Dec 24 23:31 /etc/systemd/system/default.target -> /usr/lib/systemd/system/multi-user.target
If the server is in rescue mode or in a chrooted environment, the default target can be set with the following command syntax:
# ln -sf /lib/systemd/system/<desired>.target /etc/systemd/system/default.target
Change the current target (runlevel)
In RHEL 6 this was done using telinit runlevel
In RHEL 7 you must use below syntax
# systemctl isolate name.target
For example
[root@golinuxhub ~]# systemctl isolate rescue.target
PolicyKit daemon disconnected from the bus.
We are no longer a registered authentication agent.
Since I switched to runlevel 1 I got disconnected from my termincal and if you see below screen I have logged in to the rescue mode
Changing to Rescue Mode
You can also use below command to switch to rescue mode
[root@golinuxhub ~]# systemctl rescue
PolicyKit daemon disconnected from the bus.
We are no longer a registered authentication agent.
Broadcast message from root@golinuxhub.lab on pts/1 (Sun 2017-12-24 23:47:08 IST):
The system is going down to rescue mode NOW!
Viewing the Current Target
To list all currently loaded target units, run the following command:
[root@golinuxhub ~]# systemctl list-units --type target
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Encrypted Volumes
getty.target loaded active active Login Prompts
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Encrypted Volumes
getty.target loaded active active Login Prompts
graphical.target loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network-online.target loaded active active Network is Online
network.target loaded active active Network
nfs-client.target loaded active active NFS client services
nss-user-lookup.target loaded active active User and Group Name Lookups
paths.target loaded active active Paths
remote-fs-pre.target loaded active active Remote File Systems (Pre)
remote-fs.target loaded active active Remote File Systems
slices.target loaded active active Slices
sockets.target loaded active active Sockets
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
timers.target loaded active active Timers
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
The above command shows only the "ACTIVE" units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option:
[root@golinuxhub ~]# systemctl list-units --type target --all
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Encrypted Volumes
emergency.target loaded inactive dead Emergency Mode
final.target loaded inactive dead Final Step
getty.target loaded active active Login Prompts
graphical.target loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network-online.target loaded active active Network is Online
network-pre.target loaded inactive dead Network (Pre)
network.target loaded active active Network
nfs-client.target loaded active active NFS client services
nss-lookup.target loaded inactive dead Host and Network Name Lookups
nss-user-lookup.target loaded active active User and Group Name Lookups
paths.target loaded active active Paths
remote-fs-pre.target loaded active active Remote File Systems (Pre)
remote-fs.target loaded active active Remote File Systems
rescue.target loaded inactive dead Rescue Mode
shutdown.target loaded inactive dead Shutdown
slices.target loaded active active Slices
sockets.target loaded active active Sockets
sound.target loaded inactive dead Sound Card
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
● syslog.target not-found inactive dead syslog.target
time-sync.target loaded inactive dead System Time Synchronized
timers.target loaded active active Timers
umount.target loaded inactive dead Unmount All Filesystems
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
29 loaded units listed.
To show all installed unit files use 'systemctl list-unit-files'.
Location of services
- Before systemd, services were stored as scripts in the /etc/init.d directory, then linked to different runlevel directories (such as /etc/rc3.d, /etc/rc5.d, and so on).
- Services with systemd are named something.service, such as firewalld.service, and are stored in /lib/systemd/system and /etc/systemd/system directories
- Think of the /lib files as being more permanent and the /etc files as the place you can modify configurations as needed.
- When you enable a service in RHEL 7, the service file is linked to a file in the /etc/systemd/system/multi-user.target.wants directory
systemctl enable kdump.service a symbolic link is created from /etc/systemd/system/multi-user.target.wants/kdump.service that points to /usr/lib/systemd/system/kdump.service as you see below
Created symlink from /etc/systemd/system/multi-user.target.wants/kdump.service to /usr/lib/systemd/system/kdump.service.
Also, the older System V init scripts were actual shell scripts. The systemd files tasked to do the same job are more like .ini files that contain the information needed to launch a service.
Configuration files
- The /etc/inittab file was used by the init process in RHEL 6 and earlier to point to the initialization files (such as /etc/rc.sysinit) and runlevel service directories (such as /etc/rc5.d) needed to start up the system.
- In RHEL 7, services can be modified by adding files to the /etc/systemd directory to override the permanent service files in the /usr/lib/systemd directories.
Reference:
Overview of systemd