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<refentry id="bootup">
<refpurpose>System bootup process</refpurpose>
<para>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.
<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
<ulink url="">GRUB</ulink>) 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.</para>
<para>The kernel (optionally) mounts an in-memory file system, often generated by
<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may
be used to manage services in the initrd, similarly to the real system.</para>
<para>After the root file system is found and mounted, the initrd hands over control to the host's system
manager (such as
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) 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.</para>
<para>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.</para>
<para>Additional information about the system boot process may be
found in
<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
<title>System Manager Bootup</title>
<para>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
systems, this process is split up in various discrete steps which
are exposed as target units. (See
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.</para>
<para>When systemd starts up the system, it will activate all
units that are dependencies of <filename></filename>
(as well as recursively all dependencies of these dependencies).
Usually, <filename></filename> is simply an alias of
<filename></filename> or
<filename></filename>, 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
<para>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.</para>
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(various low-level v
API VFS mounts: (various cryptsetup/veritysetup devices...)
mqueue, configfs, | |
debugfs, ...) v |
| |
| (various swap | |
| devices...) | | | |
| | | | | v
| v | | | (network file systems)
| | | v v |
| | v | |
| | (various low-level (various mounts and | |
| | services: udevd, fsck services...) | |
| | tmpfiles, random | | | /
| | seed, sysctl, ...) v | | /
| | | | | /
| | | | | | /
\____|______|_______________ ______|___________/ | /
\ / | /
v | / | /
| | /
______________________/|\_____________________ | /
/ | | | \ | /
| | | | | | /
v v | v | | /
(various (various | (various | |/
timers...) paths...) | sockets...) | |
| | | | | |
v v | v | | | | |
| | | | v |
v \_______ | _____/ rescue.service |
\|/ | |
v v | <emphasis></emphasis> |
| |
________v____________________ |
/ | \ |
| | | |
v v v |
display- (various system (various system |
manager.service services services) |
| required for | |
| graphical UIs) v v
| | <emphasis></emphasis>
emergency.service | | |
| \_____________ | _____________/
v \|/
<emphasis></emphasis> v
<para>Target units that are commonly used as boot targets are
<emphasis>emphasized</emphasis>. These units are good choices as
goal targets, for example by passing them to the
<varname>systemd.unit=</varname> kernel command line option (see
or by symlinking <filename></filename> to them.
<para><filename></filename> is pulled-in by
<filename></filename> asynchronously. This allows
timers units to depend on services which become only available
later in boot.</para>
<title>User manager startup</title>
<para>The system manager starts the <filename>user@<replaceable>uid</replaceable>.service</filename> unit
for each user, which launches a separate unprivileged instance of <command>systemd</command> for each
user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
in by <filename></filename>. The following chart is a structural overview of the well-known
user units. For non-graphical sessions, <filename></filename> is used. Whenever the user
logs into a graphical session, the login manager will start the
<filename></filename> 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.</para>
(various (various (various
timers...) paths...) sockets...) (sound devices)
| | | |
v v v v
| | |
\______________ _|_________________/ (bluetooth devices)
\ / |
V v
__________/ \_______ (smartcard devices)
/ \ |
| | v
| v
(various user services) | (printers)
| v |
| (services for the graphical session) v
| |
v v
<title>Bootup in the Initial RAM Disk (initrd)</title>
<para>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.</para>
<para>systemd detects that it is run within an initrd by checking
for the file <filename>/etc/initrd-release</filename>.
The default target in the initrd is
<filename></filename>. The bootup process begins
identical to the system manager bootup (see above) until it
reaches <filename></filename>. From there, systemd
approaches the special target <filename></filename>.
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 <filename>systemd-hibernate-resume@.service</filename>
which must be finished before <filename></filename>,
so no filesystems can be mounted before the check is complete.
When the root device becomes available,
<filename></filename> is reached.
If the root device can be mounted at
<filename>/sysroot</filename>, the
<filename>sysroot.mount</filename> unit becomes active and
<filename></filename> is reached. The service
<filename>initrd-parse-etc.service</filename> scans
<filename>/sysroot/etc/fstab</filename> for a possible
<filename>/usr/</filename> mount point and additional entries
marked with the <emphasis>x-initrd.mount</emphasis> option. All
entries found are mounted below <filename>/sysroot</filename>, and
<filename></filename> is reached. The service
<filename>initrd-cleanup.service</filename> isolates to the
<filename></filename>, where cleanup
services can run. As the very last step, the
<filename>initrd-switch-root.service</filename> is activated,
which will cause the system to switch its root to
<programlisting> : (beginning identical to above)
| emergency.service
______________________/| |
/ | v
| <emphasis></emphasis>
| |
| v
| sysroot.mount
| |
| v
| |
| v
v initrd-parse-etc.service
(custom initrd |
services...) v
| (sysroot-usr.mount and
| various mounts marked
| with fstab option
| x-initrd.mount...)
| |
| v
\______________________ |
isolates to
/ v
| initrd-udevadm-cleanup-db.service
v |
(custom initrd |
services...) |
\______________________ |
Transition to Host OS</programlisting>
<title>System Manager Shutdown</title>
<para>System shutdown with systemd also consists of various target
units with some minimal ordering structure applied:</para>
<programlisting> (conflicts with (conflicts with
all system all file system
services) mounts, swaps,
| cryptsetup/
| veritysetup
| devices, ...)
| |
v v
| |
\_______ ______/
\ /
(various low-level
_____________________________________/ \_________________________________
/ | | \
| | | |
v v v v
systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
| | | |
v v v v
<emphasis></emphasis> <emphasis></emphasis> <emphasis></emphasis> <emphasis></emphasis></programlisting>
<para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
<para>Note that
<filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
<filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> 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.</para>
<title>See Also</title>
<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>