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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | DESCRIPTION | SYSTEM MANAGER BOOTUP | USER MANAGER STARTUP | BOOTUP IN THE INITRD | SYSTEM MANAGER SHUTDOWN | SEE ALSO | NOTES | COLOPHON |
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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, which
looks for the root file system. Nowadays this is implemented as
an "initramfs" — a compressed CPIO archive that the kernel
extracts into a tmpfs. 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 veritysetup-pre.target
|
(various low-level v
API VFS mounts: (various cryptsetup/veritysetup 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 | remote-veritysetup.target |
| | services: udevd, fsck services...) | | |
| | tmpfiles, random | | | remote-fs.target
| | 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
Systemd can be used in the initrd as well. It detects the initrd
environment by checking for the /etc/initrd-release file. The
default target in the initrd is initrd.target. The bootup process
is identical to the system manager bootup until the target
basic.target. After that, systemd executes the special target
initrd.target. Before any file systems are mounted, the manager
will determine whether the system shall 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/
| veritysetup
| devices, ...)
| |
v v
shutdown.target umount.target
| |
\_______ ______/
\ /
v
(various low-level
services)
|
v
final.target
|
___________________________/ \_________________________________
/ | | | \
| | | | |
v | | | |
systemd-reboot.service | | | |
| v | | |
| systemd-poweroff.service | | |
v | v | |
reboot.target | systemd-halt.service | |
v | v |
poweroff.target | systemd-kexec.service |
v | |
halt.target | systemd-soft-reboot.service
v |
kexec.target |
v
soft-reboot.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), systemd-soft-reboot.service(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 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) 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 257~devel BOOTUP(7)
Pages that refer to this page: systemd(1), org.freedesktop.systemd1(5), systemd-system.conf(5), boot(7), dracut.bootup(7), systemd.directives(7), systemd.index(7), systemd.special(7), kernel-install(8), systemd-gpt-auto-generator(8), systemd-poweroff.service(8), systemd-soft-reboot.service(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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