Objectives

After completing this section, students should be able to describe and influence the Red Hat Enterprise Linux boot process.

The Red Hat Enterprise Linux 7 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 (graphical) 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 7. The list for x86_64 virtual machines is roughly the same, but some of the hardware-specific steps are handled in software by the hypervisor.

Boot, reboot, and shut down

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

systemctl poweroff will stop all running services, unmount all file systems (or remount them read-only when they cannot be unmounted), and then power down the system.

systemctl reboot will stop all running services, unmount all file systems, and then reboot the system.

For the ease of backward compatibility, the poweroff and reboot commands still exist, but in Red Hat Enterprise Linux 7 they are symbolic links to the systemctl tool.

Selecting a systemd target

A systemd target is a set of systemd units that should be started to reach a desired state. The most important of these targets are listed in the following table.

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:

[root@serverX ~]# systemctl list-dependencies graphical.target | grep target

An overview of all available targets can be viewed with:

[root@serverX ~]# systemctl list-units --type=target --all

An overview of all targets installed on disk can be viewed with:

[root@serverX ~]# systemctl list-unit-files --type=target --all

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:

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

Isolating a target will stop all services not required by that target (and its dependencies), and start any required services that have not yet been started.

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 multi-user.target.

Instead of editing this symbolic link by hand, the systemctl tool comes with two commands to manage this link: get-default and set-default.

[root@serverX ~]# systemctl get-default
multi-user.target
[root@serverX ~]# systemctl set-default graphical.target
rm '/etc/systemd/system/default.target'
ln -s '/usr/lib/systemd/system/graphical.target' '/etc/systemd/system/default.target'
[root@serverX ~]# 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 where configuration changes can be made without (almost) any service running, the following can be appended from the interactive boot loader menu before starting:

systemd.unit=rescue.target

This configuration change will only affect 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 for Red Hat Enterprise Linux 7 systems: