Beginners Guide to systemd Targets (Runlevels)
What is a systemd Target
In SysVinit systems, you had a defined but configurable set of runlevels numbered from 0 to 6. A runlevel is essentially a set of capabilities or running services that you can predefine and set the system to boot to so you have a predictable set of services.
In systemd, targets are the new runlevels. For example, the default.target file is typically a symbolic link to another target file such as the graphical.target file. Targets directly correspond to the SysVinit runlevels in that you can match them up as shown below:
| Runlevel | Target File | What it does? |
|---|---|---|
| 0 | poweroff.target | Shuts down and power offs the system |
| 1 | rescue.traget | Configures a rescue shell session |
| 2 | multi-user.target | Sets the system to non-graphical multi-user system, typically with no network services |
| 3 | multi-user.target | Sets the system to non-graphical multi-user system with network services |
| 4 | multi-user.target | Sets the system to non-graphical multi-user system with network services |
| 5 | graphical.target | Sets the system to graphical multi-user system with network services |
| 6 | reboot.target | Shuts down and boots the system again |
Selecting a systemd Target
A **systemd **target is a set of **systemd **units that should be started to reach a desired state. Important targets are listed in the following table.
| TARGET | PURPOSE |
|---|---|
| graphical.target | System supports multiple users, graphical and text-based logins. |
| multi-user.target | System supports multiple users, text-based logins only |
| rescue.target | sulogin prompt, basic system initialization completed |
| emergency.targe | sulogin prompt, initramfs pivot complete and system root mounted on / read-only. |
It is possible for a target to be a part of another target; for example, the **graphical.target **includes multi-user.target, which in turn depends on basic.target and others. These dependencies can be viewed from the command line with the following command:
# systemctl list-dependencies graphical.target | grep target
An overview of all available targets can be viewed with:
# systemctl list-units --type=target --all
An overview of all targets installed on disk can be viewed with:
# systemctl list-unit-files --type=target
Selecting a target at runtime
On a running system, administrators can choose to switch to a different target using the systemctl isolate command; for example,** systemctl isolate multi-user.target.**
Note: Not all targets can be isolated. Only targets that have **AllowIsolate=yes **set in their unit files can be isolated; for example, the graphical.target target can be isolated, but the cryptsetup.target target cannot
Setting a default target
When the system starts, and control is passed over to **systemd **from the **initramfs, systemd **will try to activate the **default.target **target. Normally the **default.target **target will be a symbolic link (in /etc/systemd/system/) to either graphical.target or multiuser.target.
The** systemctl **tool provides two commands to manage the link: get-default and setdefault.
# systemctl get-default
multi-user.target
# systemctl set-default graphical.target
rm '/etc/systemd/system/default.target'
ln -s '/usr/lib/systemd/system/graphical.target' '/etc/systemd/system/default.target'
# systemctl get-default
graphical.target
Selecting a different target at boot time
To select a different target at boot time, a special option can be appended to the kernel command line from the boot loader: systemd.unit=. For example, to boot the system into a rescue shell, pass the following option at the interactive boot loader menu:
# systemd.unit=rescue.target
To use this method of selecting a different target, use the following procedure for Red Hat Enterprise Linux 7 systems:
- (Re)boot the system.
- Interrupt the boot loader menu countdown by pressing any key.
- Move the cursor to the entry to be started.
- Press e to edit the current entry.
- Move the cursor to the line that starts with** linux16.** This is the kernel command line.
- Append systemd.unit=desired.target.
- Press **Ctrl+x **to boot with these changes.