BOOTUP(7)                          bootup                          BOOTUP(7)

NAME         top

       bootup - System bootup process

DESCRIPTION         top

       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
       (optionally) extracts and executes an initial RAM disk image
       (initrd), such as generated by dracut(8), which looks for the root
       file system (possibly using systemd(1) for this). 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 on the OS image,
       which is then responsible for probing all remaining hardware,
       mounting all necessary file systems and spawning all configured

       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 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,
       seed, sysctl, ...)      debugfs, ...)
                    |                  |                  |                    |                    |
                    \__________________|_________________ | ___________________|____________________/
                    /                  |                  |                    |                    \
                    |                  |                  |                    |                    |
                    v                  v                  |                    v                    v
                (various           (various               |                (various          rescue.service
               timers...)          paths...)              |               sockets...)               |
                    |                  |                  |                    |                    v
                    v                  v                  |                    v    
                    |                  |                  |                    |
                    v                  \_________________ | ___________________/
                     ____________________________________/|                                 emergency.service
                    /                  |                  |                                         |
                    |                  |                  |                                         v
                    v                  v                  v                       
                display-        (various system    (various system
            manager.service         services           services)
                    |             required for            |
                    |            graphical UIs)           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 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.

       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.

SEE ALSO         top

       systemd(1), boot(7), systemd.special(7),, dracut(8)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
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systemd 234                                                        BOOTUP(7)

Pages that refer to this page: systemd(1)boot(7)dracut.bootup(7)systemd.directives(7)systemd.index(7)systemd.special(7)