bootup(7) — Linux manual page

NAME | DESCRIPTION | SYSTEM MANAGER BOOTUP | USER MANAGER STARTUP | BOOTUP IN THE INITIAL RAM DISK (INITRD) | SYSTEM MANAGER SHUTDOWN | SEE ALSO | NOTES | COLOPHON

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 boot(7).

SYSTEM MANAGER BOOTUP         top

       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
                                                             |
           (various low-level                                v
            API VFS mounts:                 (various cryptsetup 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  |             |              |
            |    |   services: udevd,    fsck services...)   |             |    remote-fs.target
            |    |   tmpfiles, random            |           |             |             /
            |    |   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.

USER MANAGER STARTUP         top

       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

BOOTUP IN THE INITIAL RAM DISK (INITRD)         top

       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
       initrd.target. The bootup process begins identical to the system
       manager bootup (see above) until it reaches basic.target. From
       there, systemd approaches the special target initrd.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
       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 MANAGER SHUTDOWN         top

       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
                                             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
               reboot.target             poweroff.target            halt.target           kexec.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.

SEE ALSO         top

       systemd(1), boot(7), systemd.special(7), systemd.target(5),
       systemd-halt.service(8), dracut(8)

NOTES         top

        1. GRUB
           https://www.gnu.org/software/grub/

COLOPHON         top

       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 2020-12-18.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2020-12-18.)  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 247                                                    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)systemd-gpt-auto-generator(8)systemd-halt.service(8)