Targets: the boot goal of the system

Why targets?

At boot, systemd must know how far to go: start only the basics for a repair, start all network services for a server, or also start a graphical login for a desktop. A target is a named goal that pulls in a set of units. Booting means "reach the default target".

Older RHEL used numbered run levels (3 for text, 5 for graphical). The names runlevel3.target and runlevel5.target still exist as aliases of multi-user and graphical, but use the new names.

Checking and changing the default

$ systemctl get-default
graphical.target
# systemctl set-default multi-user.target
Removed "/etc/systemd/system/default.target".
Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/multi-user.target.
# systemctl isolate multi-user.target     # switch NOW (stops the desktop)
# systemctl isolate graphical.target      # and back (only if a desktop is installed)

Not every target can be isolated; only those meant to be a full system state (they have AllowIsolate=yes), such as multi-user, graphical, rescue and emergency.

Choosing a target for one boot

Sometimes you need a different target only once, for example because the server hangs during normal boot. At the GRUB boot menu, press e to edit the entry, go to the line that starts with linux, add systemd.unit=rescue.target (or emergency.target) at the end, and press Ctrl+X to boot. This change is not saved; the next boot is normal again. You will use this in the boot and recovery module.

linux ($root)/vmlinuz-5.14.0-427.13.1.el9_4.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet systemd.unit=emergency.target

Rescue versus emergency

• rescue.target mounts all local file systems from /etc/fstab and starts a few basic services. Use it to fix most problems, like a broken service or a forgotten setting.
• emergency.target mounts only the root file system, read-only. Use it when rescue cannot even start, typically because a line in /etc/fstab points at a missing disk. Remount root read-write first: mount -o remount,rw /.
• Both ask for the root password. Without it you need the password-reset method from the boot module.

Common mistakes

• Using isolate when the task says "make the system boot into text mode". isolate is temporary; set-default is permanent.
• Trying to edit files in emergency mode and getting "Read-only file system". Run mount -o remount,rw / first.
• Setting graphical.target on a server with no desktop packages. It still boots, but the target cannot be fully reached; use multi-user.target for servers.