Chapter 14. Booting into a target system state

As a system administrator, you can control the boot process of your system, and define the state you want your system to boot into. This is called a systemd target, and it is a set of systemd units that your system starts to reach a certain level of functionality. While working with systemd targets, you can view the default target, select a target at runtime, change the default boot target, boot into emergency or rescue target.

14.1. Target unit files

Targets in systemd are groups of related units that act as synchronization points during the start of your system. Target unit files, which end with the .target file extension, represent the systemd targets. The purpose of target units is to group together various systemd units through a chain of dependencies.

Consider the following examples:

  • The graphical.target unit for starting a graphical session, starts system services such as the GNOME Display Manager (gdm.service) or Accounts Service (accounts-daemon.service), and also activates the multi-user.target unit.
  • Similarly, the multi-user.target unit starts other essential system services such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and activates another target unit named basic.target.

You can set the following systemd targets as default or current targets:

Table 14.1. Common systemd targets

rescue

unit target that pulls in the base system and spawns a rescue shell

multi-user

unit target for setting up a multi-user system

graphical

unit target for setting up a graphical login screen

emergency

unit target that starts an emergency shell on the main console

Additional resources

  • systemd.special(7) man page
  • systemd.target(5) man page

14.2. Changing the default target to boot into

When a system starts, systemd activates the default.target symbolic link, which points to the true target unit. You can find the currently selected default target unit in the /etc/systemd/system/default.target file. Each target represents a certain level of functionality and is used for grouping other units. Additionally, target units serve as synchronization points during boot. You can change the default target your system boots into. When you set a default target unit, the current target remains unchanged until the next reboot.

Prerequisites

  • Root access

Procedure

  1. Determine the current default target unit systemd uses to start the system: 

    # systemctl get-default
graphical.target

  2. List the currently loaded targets: 

    # systemctl list-units --type target

  3. Configure the system to use a different target unit by default: 

    # systemctl set-default <name>.target

    Replace <name> with the name of the target unit you want to use by default. 

    Example:
# 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

  4. Verify the default target unit: 

    # systemctl get-default
multi-user.target

  5. Apply the changes by rebooting: 

    # reboot

Additional resources

  • systemctl(1) man page
  • systemd.special(7) man page
  • bootup(7) man page

14.3. Changing the current target

On a running system, you can change the target unit in the current boot without reboot. If you switch to a different target, systemd starts all services and their dependencies that this target requires, and stops all services that the new target does not enable. Isolating a different target affects only the current boot.

Procedure

  1. Optional: Determine the current target: 

    # systemctl get-default
graphical.target

  2. Optional: Display the list of targets you can select: 

    # systemctl list-units --type target
    Note

    You can only isolate targets that have the AllowIsolate=yes option set in the unit files.

  3. Change to a different target unit in the current boot: 

    # systemctl isolate <name>.target

    Replace <name> with the name of the target unit you want to use in the current boot. 

    Example:
# systemctl isolate multi-user.target

    This command starts the target unit named multi-user and all dependent units, and immediately stops all other unit.

Additional resources

  • systemctl(1) man page

14.4. Booting to rescue mode

You can boot to the rescue mode that provides a single-user environment for troubleshooting or repair if the system cannot get to a later target, and the regular booting process fails. In rescue mode, the system attempts to mount all local file systems and start certain important system services, but it does not activate network interfaces.

Prerequisites

  • Root access

Procedure

  • To enter the rescue mode, change the current target in the current session: 

    # systemctl rescue

Broadcast message from root@localhost on pts/0 (Fri 2023-03-24 18:23:15 CEST):

The system is going down to rescue mode NOW!
    Note

    This command is similar to systemctl isolate rescue.target, but it also sends an informative message to all users that are currently logged into the system.

    To prevent systemd from sending a message, enter the following command with the --no-wall command-line option: 

    # systemctl --no-wall rescue

Troubleshooting steps

If your system is not able to enter the rescue mode, you can boot to emergency mode, which provides the most minimal environment possible. In emergency mode, the system mounts the root file system only for reading, does not attempt to mount any other local file systems, does not activate network interfaces, and only starts a few essential services.

14.5. Troubleshooting the boot process

As a system administrator, you can select a non-default target at boot time to troubleshoot the boot process. Changing the target at boot time affects only a single boot. You can boot to emergency mode, which provides the most minimal environment possible.

Procedure

  1. Reboot the system, and interrupt the boot loader menu countdown by pressing any key except the Enter key, which would initiate a normal boot.
  2. Move the cursor to the kernel entry that you want to start.
  3. Press the E key to edit the current entry.

  4. Move to the end of the line that starts with linux and press Ctrl+E to jump to the end of the line: 

    linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\
kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet

  5. To choose an alternate boot target, append the systemd.unit= parameter to the end of the line that starts with linux: 

    linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\
kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet systemd.unit=<name>.target

    Replace <name> with the name of the target unit you want to use. For example, systemd.unit=emergency.target

  6. Press Ctrl+X to boot with these settings.