BOOTUP(7)                                               BOOTUP(7)

     NAME
          bootup - System bootup process

     DESCRIPTION
          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
          m[blue]GRUBm[][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
          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

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     BOOTUP(7)                                               BOOTUP(7)

          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      |          |

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     BOOTUP(7)                                               BOOTUP(7)

                    |              |        |      |               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
          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)

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     BOOTUP(7)                                               BOOTUP(7)

                                      \ /                                   |
                                       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)
          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
                                      ______________________/|                                         |

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     BOOTUP(7)                                               BOOTUP(7)

                                     /                       |                                         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
          System shutdown with systemd also consists of various target
          units with some minimal ordering structure applied:

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     BOOTUP(7)                                               BOOTUP(7)

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

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

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