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       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 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 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.

           (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                  |                    v              rescue.target
              timers.target      paths.target             |             sockets.target
                    |                  |                  |                    |
                    v                  \_________________ | ___________________/
                     ____________________________________/|                                 emergency.service
                    /                  |                  |                                         |
                    |                  |                  |                                         v
                    v                  v                  v                                 emergency.target
                display-        (various system    (various system
            manager.service         services           services)
                    |             required for            |
                    |            graphical UIs)           v
                    |                  |           multi-user.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


       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 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. 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)

                            services...)                  v
                                  |            (sysroot-usr.mount and
                                  |             various mounts marked
                                  |               with fstab option
                                  |              x-initrd.mount...)
                                  |                       |
                                  |                       v
                                  |                initrd-fs.target
                                  \______________________ |
                                                     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
                                             shutdown.target    umount.target
                                                    |                |
                                                    \_______   ______/
                                                            \ /

       Commonly used system shutdown targets are emphasized.


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

systemd 219 BOOTUP(7)

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