Provided by: systemd_242-7ubuntu3_amd64 bug

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 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

       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.

           local-fs-pre.target
                    |
                    v
           (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,
             local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
                    |                  |                  |                    |                    |
                    \__________________|_________________ | ___________________|____________________/
                                                         \|/
                                                          v
                                                   sysinit.target
                                                          |
                     ____________________________________/|\________________________________________
                    /                  |                  |                    |                    \
                    |                  |                  |                    |                    |
                    v                  v                  |                    v                    v
                (various           (various               |                (various          rescue.service
               timers...)          paths...)              |               sockets...)               |
                    |                  |                  |                    |                    v
                    v                  v                  |                    v              rescue.target
              timers.target      paths.target             |             sockets.target
                    |                  |                  |                    |
                    v                  \_________________ | ___________________/
                                                         \|/
                                                          v
                                                    basic.target
                                                          |
                     ____________________________________/|                                 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
                    |                  |                  |
                    \_________________ | _________________/
                                      \|/
                                       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.

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.

       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, initd-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

       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

       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/