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NAME | DESCRIPTION | SYSTEM MANAGER BOOTUP | USER MANAGER STARTUP | BOOTUP IN THE INITIAL RAM DISK (INITRD) | SYSTEM MANAGER SHUTDOWN | SEE ALSO | NOTES | COLOPHON |
BOOTUP(7) bootup BOOTUP(7)
bootup - System bootup process
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).
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.
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 graphical 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 session) v
| | printer.target
v v
default.target graphical-session.target
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 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.
systemd(1), boot(7), systemd.special(7), systemd.target(5),
systemd-halt.service(8), dracut(8)
1. GRUB
https://www.gnu.org/software/grub/
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
systemd 246 BOOTUP(7)
Pages that refer to this page: init(1) , systemd(1) , org.freedesktop.systemd1(5) , system.conf.d(5) , systemd-system.conf(5) , systemd-user.conf(5) , user.conf.d(5) , 30-systemd-environment-d-generator(7) , boot(7) , dracut.bootup(7) , systemd.directives(7) , systemd.index(7) , systemd.special(7) , systemd-gpt-auto-generator(8) , systemd-halt.service(8) , systemd-kexec.service(8) , systemd-poweroff.service(8) , systemd-reboot.service(8) , systemd-shutdown(8)