bootup(7) — Linux manual page


BOOTUP(7)                          bootup                          BOOTUP(7)

NAME         top

       bootup - System bootup process

DESCRIPTION         top

       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


       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 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

       When systemd starts up the system, it will activate all units that
       are dependencies of (as well as recursively all
       dependencies of these dependencies). Usually, is
       simply an alias of or, 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

       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

           (various low-level                                v
            API VFS mounts:                 (various cryptsetup 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          |
                                         |                                 |
                                 ________v____________________             |
                                /              |              \            |
                                |              |              |            |
                                v              v              v            |
                            display-    (various system   (various system  |
                        manager.service     services        services)      |
                                |         required for        |            |
                                |        graphical UIs)       v            v
                                |              |  
           emergency.service    |              |              |
                   |            \_____________ | _____________/
                   v                          \|/

       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 to them. is pulled-in by 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 The
       following chart is a structural overview of the well-known user
       units. For non-graphical sessions, is used. Whenever
       the user logs into a graphical session, the login manager will start
       the 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

               (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


       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 The bootup process begins identical to the system
       manager bootup (see above) until it reaches From there,
       systemd approaches the special 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, so no filesystems can be mounted
       before the check is complete. When the root device becomes available, is reached. If the root device can be
       mounted at /sysroot, the sysroot.mount unit becomes active and 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 is reached. The service initrd-cleanup.service
       isolates to the, 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

                                                          : (beginning identical to above)
                                                          |                                 emergency.service
                                   ______________________/|                                         |
                                  /                       |                                         v
                                  |                       |
                                  |                       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


       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
                                                    |                |
                                                    \_______   ______/
                                                            \ /
                                                    (various low-level
                       _____________________________________/ \_________________________________
                      /                         |                        |                      \
                      |                         |                        |                      |
                      v                         v                        v                      v
           systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
                      |                         |                        |                      |
                      v                         v                        v                      v

       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.

SEE ALSO         top

       systemd(1), boot(7), systemd.special(7),,
       systemd-halt.service(8), dracut(8)

NOTES         top

        1. GRUB

COLOPHON         top

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       report for this manual page, see
       ⟨⟩.  This
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systemd 247                                                        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)