RHCSA Rapid Track

Objectives

After completing this section, you should be able to:

Describing the Red Hat Enterprise Linux 8 Boot Process

Modern computer systems are complex combinations of hardware and software. Starting from an undefined, powered-down state to a running system with a login prompt requires a large number of pieces of hardware and software to work together. The following list gives a high-level overview of the tasks involved for a physical x86_64 system booting Red Hat Enterprise Linux 8. The list for x86_64 virtual machines is roughly the same, but the hypervisor handles some of the hardware-specific steps in software.

Rebooting and Shutting Down

To power off or reboot a running system from the command line, you can use the systemctl command.

systemctl poweroff stops all running services, unmounts all file systems (or remounts them read-only when they cannot be unmounted), and then powers down the system.

systemctl reboot stops all running services, unmounts all file systems, and then reboots the system.

You can also use the shorter version of these commands, poweroff and reboot, which are symbolic links to their systemctl equivalents.

Selecting a Systemd Target

A systemd target is a set of systemd units that the system should start to reach a desired state. The following table lists the most important targets.

A target can be a part of another target. For example, the graphical.target includes multi-user.target, which in turn depends on basic.target and others. You can view these dependencies with the following command.

[user@host ~]$ systemctl list-dependencies graphical.target | grep target
graphical.target
* └─multi-user.target
*   ├─basic.target
*   │ ├─paths.target
*   │ ├─slices.target
*   │ ├─sockets.target
*   │ ├─sysinit.target
*   │ │ ├─cryptsetup.target
*   │ │ ├─local-fs.target
*   │ │ └─swap.target
...output omitted...

To list the available targets, use the following command.

[user@host ~]$ systemctl list-units --type=target --all
  UNIT                      LOAD      ACTIVE   SUB    DESCRIPTION
  ---------------------------------------------------------------------------
  basic.target              loaded    active   active Basic System
  cryptsetup.target         loaded    active   active Local Encrypted Volumes
  emergency.target          loaded    inactive dead   Emergency Mode
  getty-pre.target          loaded    inactive dead   Login Prompts (Pre)
  getty.target              loaded    active   active Login Prompts
  graphical.target          loaded    inactive dead   Graphical Interface
...output omitted...

Selecting a Target at Runtime

On a running system, administrators can switch to a different target using the systemctl isolate command.

[root@host ~]# systemctl isolate multi-user.target

Isolating a target stops all services not required by that target (and its dependencies), and starts any required services not yet started.

Not all targets can be isolated. You can only isolate targets that have AllowIsolate=yes set in their unit files. For example, you can isolate the graphical target, but not the cryptsetup target.

[user@host ~]$ systemctl cat graphical.target
# /usr/lib/systemd/system/graphical.target
...output omitted...
[Unit]
Description=Graphical Interface
Documentation=man:systemd.special(7)
Requires=multi-user.target
Wants=display-manager.service
Conflicts=rescue.service rescue.target
After=multi-user.target rescue.service rescue.target display-manager.service
AllowIsolate=yes
[user@host ~]$ systemctl cat cryptsetup.target
# /usr/lib/systemd/system/cryptsetup.target
...output omitted...
[Unit]
Description=Local Encrypted Volumes
Documentation=man:systemd.special(7)

Setting a Default Target

When the system starts, systemd activates the default.target target. Normally the default target in /etc/systemd/system/ is a symbolic link to either graphical.target or multi-user.target. Instead of editing this symbolic link by hand, the systemctl command provides two subcommands to manage this link: get-default and set-default.

[root@host ~]# systemctl get-default
multi-user.target
[root@host ~]# systemctl set-default graphical.target
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target -> /usr/lib/systemd/system/graphical.target.
[root@host ~]# systemctl get-default
graphical.target

Selecting a Different Target at Boot Time

To select a different target at boot time, append the systemd.unit=target.target option to the kernel command line from the boot loader.

For example, to boot the system into a rescue shell where you can change the system configuration with almost no services running, append the following option to the kernel command line from the boot loader.

systemd.unit=rescue.target

This configuration change only affects a single boot, making it a useful tool for troubleshooting the boot process.

To use this method of selecting a different target, use the following procedure: