Previous versions of CentOS/RHEL Linux use SysV init run levels. These run levels provided the ability to use systems for different purposes and only start the services needed for a specific purpose, at a specific run level. In RHEL 7, run levels have been replaced with systemd target units. Target units have a .target extension and similar to run levels, target units allow you to start a system with only the services that are required for a specific purpose.
RHEL 7 is distributed with a set of predefined targets that are similar to run levels in previous versions of RedHat Linux. The following command returns the absolute pathname of these predefined systemd run level target units:
# find / -name "runlevel*.target" /usr/lib/systemd/system/runlevel5.target /usr/lib/systemd/system/runlevel0.target /usr/lib/systemd/system/runlevel6.target /usr/lib/systemd/system/runlevel1.target /usr/lib/systemd/system/runlevel2.target /usr/lib/systemd/system/runlevel3.target /usr/lib/systemd/system/runlevel4.target
Comparision of SysV Run Levels and Target Units
| Run Level | Target Units | Description |
|---|---|---|
| 0 | runlevel0.target, poweroff.target | Shut down and power off |
| 1 | runlevel1.target, rescue.target | Set up a rescue shell |
| 2,3,4 | runlevel[234].target, multi- user.target | Set up a nongraphical multi-user shell |
| 5 | runlevel5.target, graphical.target | Set up a graphical multi-user shell |
| 6 | runlevel6.target, reboot.target | Shut down and reboot the system |
Each runlevel[0123456].target file is a symbolic link to the system-start target equivalents. For example:
# cd /usr/lib/systemd/system # ls -l runlevel* lrwxrwxrwx. 1 root root 15 Sep 23 19:52 runlevel0.target -> poweroff.target lrwxrwxrwx. 1 root root 13 Sep 23 19:52 runlevel1.target -> rescue.target lrwxrwxrwx. 1 root root 17 Sep 23 19:52 runlevel2.target -> multi-user.target lrwxrwxrwx. 1 root root 17 Sep 23 19:52 runlevel3.target -> multi-user.target lrwxrwxrwx. 1 root root 17 Sep 23 19:52 runlevel4.target -> multi-user.target lrwxrwxrwx. 1 root root 16 Sep 23 19:52 runlevel5.target -> graphical.target lrwxrwxrwx. 1 root root 13 Sep 23 19:52 runlevel6.target -> reboot.target
1. View default/current target unit
Use the following command to view which target unit is used by default:
# systemctl get-default graphical.target
The graphical.target target unit indicates that the system is running in a graphical, multi- user state. This is similar to run level 5 in a SysV init system. You can verify this using the old command runlevel :
# runlevel N 5
The default target unit is represented by the /etc/systemd/system/default.target file. This file is a symbolic link to the current default target unit. For example :
# ls -lrt /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 36 Sep 23 20:01 /etc/systemd/system/default.target -> /lib/systemd/system/graphical.target
2. list the currently active targets
To view the currently active target units on your system use below command.
# 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 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. SUB = The low-level unit activation state, values depend on unit type. 19 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
3. Change default target unit
Use the following command to change the default target unit (for example, to change the default to the multi-user.target unit):
# 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.
Notice that the default.target symbolic link has changed, and is now pointing to the multi-user.target unit:
# ls -lrt /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 41 Sep 24 11:58 /etc/systemd/system/default.target -> /usr/lib/systemd/system/multi-user.target
4. Change currently active system target unit
The set-default command does not change the current state of the system. To change the currently active system target (for example, to change the currently active system target to multi-user.target):
# systemctl isolate multi-user.target
This command is similar to using telinit [runlevel] to change the current run level. This telinit command still exists but is only included for compatibility reasons.
You can also use the following command to enter the default target unit:
# systemctl default
This is equivalent to the following command:
# systemctl isolate default.target
CentOS / RHEL 7 : How to change runlevels (targets) with systemd
CentOS / RHEL 7 : Beginners guide to systemd service units
CentOS / RHEL 7 : Beginners guide to systemd