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HTML rendering created 2025-09-06 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 | THE EXITRD | 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
non-busy file systems (detaching the storage technologies backing
them), and then (optionally) jumps into the exitrd. The exitrd is
backed by tmpfs and unmounts/detaches the remaining file systems,
including the real root. 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
could not be stopped or released for some reason during the
primary, unit-based shutdown phase described above.
The "exitrd" is a concept symmetrical to the initrd. When the
system manager is shutting down and /run/initramfs/shutdown
exists, it will switch root to /run/initramfs/ and execute
/shutdown. This program runs from the tmpfs mounted on /run/, so
it can unmount the old root file system and perform additional
steps, for example dismantle complex storage or perform additional
logging about the shutdown.
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 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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
[email protected]
systemd 258~rc2 BOOTUP(7)
Pages that refer to this page: systemd(1), org.freedesktop.systemd1(5), systemd-system.conf(5), boot(7), dracut.bootup(7), nfs.systemd(7), systemd.directives(7), systemd.index(7), systemd.special(7), kernel-install(8), systemd-debug-generator(8), systemd-gpt-auto-generator(8), systemd-poweroff.service(8), systemd-soft-reboot.service(8)
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HTML rendering created 2025-09-06 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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