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<refentry id="bootup">

<refentryinfo>

<title>bootup</title>

<productname>systemd</productname>

<authorgroup>

<author>

<contrib>Developer</contrib>

<firstname>Lennart</firstname>

<surname>Poettering</surname>

<email>lennart@poettering.net</email>

</author>

</authorgroup>

</refentryinfo>

<refmeta>

<refentrytitle>bootup</refentrytitle>

<manvolnum>7</manvolnum>

</refmeta>

<refnamediv>

<refname>bootup</refname>

<refpurpose>System bootup process</refpurpose>

</refnamediv>

<refsect1>

<title>Description</title>

<para>A number of different components are involved in the

system boot. Immediately after power-up, the system

BIOS will do minimal hardware initialization, and hand

control over to a boot loader stored on a persistent

storage device. This boot loader will then invoke an

OS kernel from disk (or the network). In the Linux

case this kernel now (optionally) extracts and

executes an initial RAM disk image (initrd) such as

<citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>

which looks for the root file system. After the root

file system is found and mounted the initrd hands over

control to the system manager (such as

<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)

stored on the OS image 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 last step the system is powered down.</para>

<para>Additional information about the system boot

process may be found in

<citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

</refsect1>

<refsect1>

<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

<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>

systems this process is split up in various discrete

steps which are exposed as target units. (See

<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>

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>default.target</filename> (as well as

recursively all dependencies of these

dependencies). Usually

<filename>default.target</filename> is simply an alias

of <filename>graphical.target</filename> or

<filename>multi-user.target</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

<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

<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>

<programlisting>local-fs-pre.target

|

v

(various mounts and (various swap (various cryptsetup

fsck services...) devices...) devices...) (various low-level (various low-level

| | | services: udevd, API VFS mounts:

v v v tmpfiles, random mqueue, configfs,

local-fs.target swap.target cryptsetup.target seed, sysctl, ...) debugfs, ...)

| | | | |

\__________________|_________________ | ___________________|____________________/

\|/

v

sysinit.target

|

_________________/|\___________________

/ | \

| | |

v | v

(various | rescue.service

sockets...) | |

| | v

v | <emphasis>rescue.target</emphasis>

sockets.target |

| |

\_________________ |

\|

v

basic.target

|

__________________________________/| emergency.service

/ | | |

| | | v

v v v <emphasis>emergency.target</emphasis>

display- (various system (various system

manager.service services services)

| required for |

| graphical UIs) v

| | <emphasis>multi-user.target</emphasis>

| | |

\_______________ | _________________/

\|/

v

<emphasis>graphical.target</emphasis></programlisting>

<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

<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)

or by symlinking <filename>default.target</filename>

to them.</para>

</refsect1>

<refsect1>

<title>Systemd in the Initrd</title>

<para>If the initrd creation tool used the services provided

by systemd, the default target in the initrd is the

initrd-fs.target. The process is the same as above until the basic.target is reached.

Systemd now continues to the initrd.target. If the root device could be mounted

on /sysroot, the sysroot.mount unit is active and the initrd-root-fs.target is reached.

initrd-parse-etc.service scans /sysroot/etc/fstab for the /usr mountpoint and for entries

marked with the <emphasis>x-initrd.mount</emphasis> option set. If these mountpoint are

mounted in /sysroot, the initrd-fs.target is reached.

The initrd-cleanup.service isolates to the initrd-switch-root.target,

where cleanup services can run. At the very last end

initrd-switch-root.service is activated, which will cause

the system to switch root to /sysroot.

</para>

<programlisting>

(same as above)

:

:

v

basic.target

| emergency.service

______________________/| |

/ | v

| sysroot.mount <emphasis>emergency.target</emphasis>

| |

| v

| initrd-root-fs.target

| |

| 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

______________________/|

/ |

| initrd-udevadm-cleanup-db.service

| |

(custom initrd services) |

| |

\______________________ |

\|

v

initrd-switch-root.target

|

v

initrd-switch-root.service

|

v

switch-root

</programlisting>

</refsect1>

<refsect1>

<title>System Manager Shutdown</title>

<para>System shutdown 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

| 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

<emphasis>reboot.target</emphasis> <emphasis>poweroff.target</emphasis> <emphasis>halt.target</emphasis> <emphasis>kexec.target</emphasis></programlisting>

<para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>

</refsect1>

<refsect1>

<title>See Also</title>

<para>

<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,

<citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,

<citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>

</para>

</refsect1>

</refentry>