bootup - System bootup process


   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 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 (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 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             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


   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 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 /sysroot.

                                                      : (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.


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


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