Provided by: systemd_245.4-4ubuntu3.24_amd64 
NAME
bootup - System bootup process
DESCRIPTION
A number of different components are involved in the boot of a Linux system. Immediately
after power-up, the system firmware will do minimal hardware initialization, and hand
control over to a boot loader (e.g. systemd-boot(7) or GRUB[1]) stored on a persistent
storage device. This boot loader will then invoke an OS kernel from disk (or the network).
On systems using EFI or other types of firmware, this firmware may also load the kernel
directly.
The kernel (optionally) mounts an in-memory file system, often generated by dracut(8),
which looks for the root file system. Nowadays this is usually implemented as an initramfs
— a compressed archive which is extracted when the kernel boots up into a lightweight
in-memory file system based on tmpfs, but in the past normal file systems using an
in-memory block device (ramdisk) were used, and the name "initrd" is still used to
describe both concepts. It's the boot loader or the firmware that loads both the kernel
and initrd/initramfs images into memory, but the kernel which interprets it as a file
system. systemd(1) may be used to manage services in the initrd, similarly to the real
system.
After the root file system is found and mounted, the initrd hands over control to the
host's system manager (such as systemd(1)) stored in the root file system, which is then
responsible for probing all remaining hardware, mounting all necessary file systems and
spawning all configured services.
On shutdown, the system manager stops all services, unmounts all file systems (detaching
the storage technologies backing them), and then (optionally) jumps back into the initrd
code which unmounts/detaches the root file system and the storage it resides on. As a last
step, the system is powered down.
Additional information about the system boot process may be found in boot(7).
SYSTEM MANAGER BOOTUP
At boot, the system manager on the OS image is responsible for initializing the required
file systems, services and drivers that are necessary for operation of the system. On
systemd(1) systems, this process is split up in various discrete steps which are exposed
as target units. (See systemd.target(5) for detailed information about target units.) The
boot-up process is highly parallelized so that the order in which specific target units
are reached is not deterministic, but still adheres to a limited amount of ordering
structure.
When systemd starts up the system, it will activate all units that are dependencies of
default.target (as well as recursively all dependencies of these dependencies). Usually,
default.target is simply an alias of graphical.target or multi-user.target, depending on
whether the system is configured for a graphical UI or only for a text console. To enforce
minimal ordering between the units pulled in, a number of well-known target units are
available, as listed on systemd.special(7).
The following chart is a structural overview of these well-known units and their position
in the boot-up logic. The arrows describe which units are pulled in and ordered before
which other units. Units near the top are started before units nearer to the bottom of the
chart.
cryptsetup-pre.target
|
(various low-level v
API VFS mounts: (various cryptsetup devices...)
mqueue, configfs, | |
debugfs, ...) v |
| cryptsetup.target |
| (various swap | | remote-fs-pre.target
| devices...) | | | |
| | | | | v
| v local-fs-pre.target | | | (network file systems)
| swap.target | | v v |
| | v | remote-cryptsetup.target |
| | (various low-level (various mounts and | | |
| | services: udevd, fsck services...) | | remote-fs.target
| | tmpfiles, random | | | /
| | seed, sysctl, ...) v | | /
| | | local-fs.target | | /
| | | | | | /
\____|______|_______________ ______|___________/ | /
\ / | /
v | /
sysinit.target | /
| | /
______________________/|\_____________________ | /
/ | | | \ | /
| | | | | | /
v v | v | | /
(various (various | (various | |/
timers...) paths...) | sockets...) | |
| | | | | |
v v | v | |
timers.target paths.target | sockets.target | |
| | | | v |
v \_______ | _____/ rescue.service |
\|/ | |
v v |
basic.target rescue.target |
| |
________v____________________ |
/ | \ |
| | | |
v v v |
display- (various system (various system |
manager.service services services) |
| required for | |
| graphical UIs) v v
| | multi-user.target
emergency.service | | |
| \_____________ | _____________/
v \|/
emergency.target v
graphical.target
Target units that are commonly used as boot targets are emphasized. These units are good
choices as goal targets, for example by passing them to the systemd.unit= kernel command
line option (see systemd(1)) or by symlinking default.target to them.
timers.target is pulled-in by basic.target asynchronously. This allows timers units to
depend on services which become only available later in boot.
USER MANAGER STARTUP
The system manager starts the user@uid.service unit for each user, which launches a
separate unprivileged instance of systemd for each user — the user manager. Similarly to
the system manager, the user manager starts units which are pulled in by default.target.
The following chart is a structural overview of the well-known user units. For
non-graphical sessions, default.target is used. Whenever the user logs into a graphical
session, the login manager will start the graphical-session.target target that is used to
pull in units required for the grahpical session. A number of targets (shown on the right
side) are started when specific hardware is available to the user.
(various (various (various
timers...) paths...) sockets...) (sound devices)
| | | |
v v v v
timers.target paths.target sockets.target sound.target
| | |
\______________ _|_________________/ (bluetooth devices)
\ / |
V v
basic.target bluetooth.target
|
__________/ \_______ (smartcard devices)
/ \ |
| | v
| v smartcard.target
v graphical-session-pre.target
(various user services) | (printers)
| v |
| (services for the graphical sesion) v
| | printer.target
v v
default.target graphical-session.target
BOOTUP IN THE INITIAL RAM DISK (INITRD)
The initial RAM disk implementation (initrd) can be set up using systemd as well. In this
case, boot up inside the initrd follows the following structure.
systemd detects that it is run within an initrd by checking for the file
/etc/initrd-release. The default target in the initrd is initrd.target. The bootup process
begins identical to the system manager bootup (see above) until it reaches basic.target.
From there, systemd approaches the special target initrd.target. Before any file systems
are mounted, it must be determined whether the system will resume from hibernation or
proceed with normal boot. This is accomplished by systemd-hibernate-resume@.service which
must be finished before local-fs-pre.target, so no filesystems can be mounted before the
check is complete. When the root device becomes available, initrd-root-device.target is
reached. If the root device can be mounted at /sysroot, the sysroot.mount unit becomes
active and initrd-root-fs.target is reached. The service initrd-parse-etc.service scans
/sysroot/etc/fstab for a possible /usr mount point and additional entries marked with the
x-initrd.mount option. All entries found are mounted below /sysroot, and initrd-fs.target
is reached. The service initrd-cleanup.service isolates to the initrd-switch-root.target,
where cleanup services can run. As the very last step, the initrd-switch-root.service is
activated, which will cause the system to switch its root to /sysroot.
: (beginning identical to above)
:
v
basic.target
| emergency.service
______________________/| |
/ | v
| initrd-root-device.target emergency.target
| |
| v
| sysroot.mount
| |
| v
| initrd-root-fs.target
| |
| v
v initrd-parse-etc.service
(custom initrd |
services...) v
| (sysroot-usr.mount and
| various mounts marked
| with fstab option
| x-initrd.mount...)
| |
| v
| initrd-fs.target
\______________________ |
\|
v
initrd.target
|
v
initrd-cleanup.service
isolates to
initrd-switch-root.target
|
v
______________________/|
/ v
| initrd-udevadm-cleanup-db.service
v |
(custom initrd |
services...) |
\______________________ |
\|
v
initrd-switch-root.target
|
v
initrd-switch-root.service
|
v
Transition to Host OS
SYSTEM MANAGER SHUTDOWN
System shutdown with systemd also consists of various target units with some minimal
ordering structure applied:
(conflicts with (conflicts with
all system all file system
services) mounts, swaps,
| cryptsetup
| devices, ...)
| |
v v
shutdown.target umount.target
| |
\_______ ______/
\ /
v
(various low-level
services)
|
v
final.target
|
_____________________________________/ \_________________________________
/ | | \
| | | |
v v v v
systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
| | | |
v v v v
reboot.target poweroff.target halt.target kexec.target
Commonly used system shutdown targets are emphasized.
Note that systemd-halt.service(8), systemd-reboot.service, systemd-poweroff.service and
systemd-kexec.service will transition the system and server manager (PID 1) into the
second phase of system shutdown (implemented in the systemd-shutdown binary), which will
unmount any remaining file systems, kill any remaining processes and release any other
remaining resources, in a simple and robust fashion, without taking any service or unit
concept into account anymore. At that point, regular applications and resources are
generally terminated and released already, the second phase hence operates only as safety
net for everything that couldn't be stopped or released for some reason during the
primary, unit-based shutdown phase described above.
SEE ALSO
systemd(1), boot(7), systemd.special(7), systemd.target(5), systemd-halt.service(8),
dracut(8)
NOTES
1. GRUB
https://www.gnu.org/software/grub/